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Document and Entity Information - USD ($) $ in Millions	12 Months Ended
Document and Entity Information - USD ($) $ in Millions	Dec. 31, 2018	Mar. 06, 2019	Jun. 30, 2018
Document and Entity Information [Abstract]
Entity Registrant Name	Enphase Energy, Inc.
Entity Central Index Key	0001463101
Document Type	10-K
Document Period End Date	Dec. 31, 2018
Amendment Flag	false
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	Q4
Current Fiscal Year End	--12-31
Entity Filer Category	Accelerated Filer
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Emerging Growth Company	false
Entity Small Business	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		108,519,598
Entity Public Float			$ 588.9

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 106,237	$ 29,144
Accounts receivable, net of allowances of $2,138 and $2,378 at December 31, 2018 and December 31, 2017, respectively	78,938	65,346
Inventory	16,267	25,999
Prepaid expenses and other assets	20,860	9,957
Total current assets	222,302	130,446
Property and equipment, net	20,998	26,483
Intangible assets, net	35,306	515
Goodwill	24,783	3,664
Other assets	36,548	8,039
Total assets	339,937	169,147
Current liabilities:
Accounts payable	48,794	28,747
Accrued liabilities	29,010	22,447
Deferred revenues, current	33,119	15,691
Warranty obligations, current (includes $4,288 and $2,240 measured at fair value at December 31, 2018 and December 31, 2017, respectively)	8,083	7,427
Debt, current	28,155	17,429
Total current liabilities	147,161	91,741
Long-term liabilities:
Deferred revenues, noncurrent	76,911	29,941
Warranty obligations, noncurrent (includes $7,469 and $7,551 measured at fair value at December 31, 2018 and December 31, 2017, respectively)	23,211	22,389
Other liabilities	3,250	1,880
Debt, noncurrent	81,628	32,322
Total liabilities	332,161	178,273
Commitments and contingent liabilities (Note 12)
Stockholders’ equity (deficit):
Preferred stock, $0.00001 par value, 10,000 shares authorized; none issued and outstanding	0	0
Common stock, $0.00001 par value, 150,000 shares and 125,000 shares authorized; and 107,035 and 85,914 shares issued and outstanding at December 31, 2018 and December 31, 2017, respectively	1	1
Additional paid-in capital	353,335	287,256
Accumulated deficit	(346,302)	(295,727)
Accumulated other comprehensive income (loss)	742	(656)
Total stockholders’ equity (deficit)	7,776	(9,126)
Total liabilities and stockholders’ equity (deficit)	$ 339,937	$ 169,147

CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Allowances, accounts receivable	$ 2,138	$ 2,378
Warranty obligations, current at fair value	4,288	2,240
Warranty obligations, non-current at fair value	$ 7,469	$ 7,551
Preferred stock, par value (in usd per share)	$ 0.00001	$ 0.00001
Preferred stock, shares authorized (in shares)	10,000,000	10,000,000
Preferred stock, shares issued (in shares)	0	0
Preferred stock, shares outstanding (in shares)	0	0
Common stock, par value (in usd per share)	$ 0.00001	$ 0.00001
Common stock, shares authorized (in shares)	150,000,000	125,000,000
Common stock, shares issued (in shares)	107,035,000	85,914,000
Common stock, shares outstanding (in shares)	107,035,000	85,914,000

CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Statement [Abstract]
Net revenues	$ 92,289	$ 78,002	$ 75,896	$ 69,972	$ 79,674	$ 77,038	$ 74,704	$ 54,751	$ 316,159	$ 286,166	$ 322,591
Cost of revenues	64,124	52,738	53,195	51,657	60,685	60,577	61,157	47,703	221,714	230,123	264,583
Gross profit	28,165	25,264	22,701	18,315	18,989	16,461	13,547	7,048	94,445	56,043	58,008
Operating expenses:
Research and development	7,340	8,165	9,462	7,620	8,208	7,397	7,947	9,605	32,587	33,157	50,703
Sales and marketing	6,617	7,375	6,828	6,227	4,940	5,453	6,274	6,458	27,047	23,126	38,810
General and administrative	7,664	7,510	6,969	6,943	5,983	5,441	4,964	5,833	29,086	22,221	27,418
Restructuring charges	1,541	2,588	0	0	1,991	4,071	3,609	7,247	4,129	16,917	3,777
Total operating expenses	23,162	25,638	23,259	20,790	21,122	22,362	22,794	29,143	92,849	95,421	120,708
Income (loss) from operations	5,003	(374)	(558)	(2,475)	(2,133)	(5,901)	(9,247)	(22,095)	1,596	(39,378)	(62,700)
Other expense, net
Interest expense									(9,635)	(7,936)	(2,773)
Other income (expense)									(2,190)	1,973	(514)
Total other expense, net	(3,718)	(2,848)	(2,841)	(2,418)	(1,755)	(1,137)	(1,992)	(1,079)	(11,825)	(5,963)	(3,287)
Loss before income taxes	1,285	(3,222)	(3,399)	(4,893)	(3,888)	(7,038)	(11,239)	(23,174)	(10,229)	(45,341)	(65,987)
Income tax benefit (provision)	(576)	(248)	(339)	(235)	948	184	(854)	(131)	(1,398)	149	(1,475)
Net loss	$ 709	$ (3,470)	$ (3,738)	$ (5,128)	$ (2,940)	$ (6,854)	$ (12,093)	$ (23,305)	$ (11,627)	$ (45,192)	$ (67,462)
Net loss per share:
Basic and diluted (in usd per share)					$ (0.03)	$ (0.08)	$ (0.14)	$ (0.30)	$ (0.12)	$ (0.54)	$ (1.34)
Shares used in per share calculation:
Basic and diluted (in shares)									99,619	82,939	50,519

CONSOLIDATED STATEMENTS OF COMPREHENSIVE LOSS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Statement of Comprehensive Income [Abstract]
Net loss	$ (11,627)	$ (45,192)	$ (67,462)
Other comprehensive income (loss):
Foreign currency translation adjustments	1,398	(364)	(82)
Comprehensive loss	$ (10,229)	$ (45,556)	$ (67,544)

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References + Details

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash flows from operating activities:
Net loss	$ (11,627)	$ (45,192)	$ (67,462)
Adjustments to reconcile net loss to net cash provided by (used in) operating activities:
Depreciation and amortization	9,667	9,004	10,638
Provision for doubtful accounts	711	476	3,097
Asset impairment	1,601	1,681	3,190
Gain on business divestiture	0	0	(640)
Amortization of debt issuance costs	2,701	1,673	145
Stock-based compensation	11,432	6,727	10,326
Deferred income tax expense (benefit)	123	(1,394)	651
Changes in operating assets and liabilities:
Accounts receivable	(13,515)	(4,803)	(18,017)
Inventory	9,732	5,961	8,840
Prepaid expenses and other assets	(3,130)	(1,227)	(4,759)
Intangible assets	(10,000)	0	0
Accounts payable, accrued and other liabilities	23,082	(5,078)	8,897
Warranty obligations	1,478	(1,598)	867
Deferred revenues	(6,123)	5,328	11,274
Net cash provided by (used in) operating activities	16,132	(28,442)	(32,953)
Cash flows from investing activities:
Purchases of property and equipment	(4,151)	(4,121)	(12,167)
Purchases of intangible assets	0	0	(678)
Business divestitures	0	0	1,050
Acquisition	(15,000)	0	0
Net cash used in investing activities	(19,151)	(4,121)	(11,795)
Cash flows from financing activities:
Proceeds from issuance of common stock, net of issuance costs	19,766	26,425	16,142
Proceeds from debt, net of issuance costs	68,024	26,442	23,989
Proceeds from borrowings under revolving credit facility	0	0	10,000
Principal payments on debt	(9,976)	0	0
Payments under revolving credit facility	0	(10,100)	(16,900)
Proceeds from issuance of common stock under employee stock plans	2,800	530	1,144
Net cash provided by financing activities	80,614	43,297	34,375
Effect of exchange rate changes on cash	(502)	646	(315)
Net increase (decrease) in cash and cash equivalents	77,093	11,380	(10,688)
Cash and cash equivalents—Beginning of period	29,144	17,764	28,452
Cash and cash equivalents—End of period	106,237	29,144	17,764
Supplemental cash flow disclosure:
Cash paid for interest	6,343	5,816	2,704
Cash paid for income taxes	775	909	1,146
Supplemental disclosures of non-cash investing and financing activities:
Acquisition funded by issuance of common stock	19,219	0	0
Offering and loan costs included in accrued liabilities	0	0	518
Purchases of fixed assets included in accounts payable	$ 895	$ 551	$ 700

CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional Paid-In Capital	Accumulated Deficit	Accumulated Other Comprehensive Income (Loss)
BALANCE, Beginning at Dec. 31, 2015	$ 41,449	$ 0	$ 224,732	$ (183,073)	$ (210)
BALANCE, Beginning (shares) at Dec. 31, 2015		45,821
Issuance of common stock under employee stock plans	1,145	$ 1	1,144
Issuance of common stock under employee stock plans (shares)		1,498
Issuance of common stock, net of offering costs	15,924		15,924
Issuance of common stock, net of offering costs (shares)		14,950
Stock-based compensation	10,326		10,326
Net loss	(67,462)			(67,462)
Foreign currency translation adjustments	(82)				(82)
BALANCE, Ending at Dec. 31, 2016	1,300	$ 1	252,126	(250,535)	(292)
BALANCE, Ending (shares) at Dec. 31, 2016		62,269
Issuance of common stock under employee stock plans	531		531
Issuance of common stock under employee stock plans (shares)		1,752
Issuance of common stock, net of offering costs	26,425		26,425
Issuance of common stock, net of offering costs (shares)		21,893
Issuance of warrants	1,447		1,447
Stock-based compensation	6,727		6,727
Net loss	(45,192)			(45,192)
Foreign currency translation adjustments	(364)				(364)
BALANCE, Ending at Dec. 31, 2017	(9,126)	$ 1	287,256	(295,727)	(656)
BALANCE, Ending (shares) at Dec. 31, 2017		85,914
Issuance of common stock under employee stock plans	2,806		2,806
Issuance of common stock under employee stock plans (shares)		3,185
Issuance of common stock, net of offering costs	19,766		19,766
Issuance of common stock, net of offering costs (shares)		9,524
Issuance of common stock related to acquisition	$ 32,319		32,319
Issuance of common stock related to acquisition (shares)		7,500
Issuance of warrants (shares)	912
Stock-based compensation	$ 11,188		11,188
Net loss	(11,627)			(11,627)
Foreign currency translation adjustments	1,398				1,398
BALANCE, Ending at Dec. 31, 2018	$ 7,776	$ 1	$ 353,335	$ (346,302)	$ 742
BALANCE, Ending (shares) at Dec. 31, 2018		107,035

DESCRIPTION OF BUSINESS AND BASIS OF PRESENTATION	12 Months Ended
DESCRIPTION OF BUSINESS AND BASIS OF PRESENTATION	Dec. 31, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
DESCRIPTION OF BUSINESS AND BASIS OF PRESENTATION	DESCRIPTION OF BUSINESS AND BASIS OF PRESENTATION Description of Business Enphase Energy, Inc. (the “Company”) is a global energy technology company, delivering smart, easy-to-use solutions that connect solar generation, storage and communications management on one intelligent platform. The Company revolutionized solar with its microinverter technology and produce the world’s only truly integrated solar-plus-storage solution. Basis of Presentation and Consolidation The accompanying consolidated financial statements are prepared in accordance with accounting principles generally accepted in the United States (“U.S. GAAP”). The consolidated financial statements include the accounts of the Company and its wholly-owned subsidiaries. All intercompany balances and transactions have been eliminated in consolidation. Use of Estimates The preparation of the Company’s consolidated financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of income and expenses during the reporting period. These estimates are based on information available as of the date of the financial statements; therefore, actual results could differ materially from management’s estimates using different assumptions or under different conditions.

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES

12 Months Ended

Dec. 31, 2018

Accounting Policies [Abstract]

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES

Revenue Recognition

The Company generates revenue from sales of its solutions, which include microinverter units and related accessories, an Envoy communications gateway, the cloud-based Enlighten monitoring service, and AC Battery storage solutions to distributors, large installers, original equipment manufacturers (“OEMs”) and strategic partners.

On January 1, 2018, the Company adopted Accounting Standards Codification (“ASC”) No. 606, “Revenue Recognition” (“ASC 606” or “Topic 606”) and applied the modified retrospective method to all contracts that were not completed as of January 1, 2018. The most significant impacts upon adoption of Topic 606 were how the Company accounts for revenue related to its Envoy™ communications device and related Enphase Enlighten Software™, or Enlighten, service and the timing of when certain sales incentives are recognized. The full consideration for these products represents a single performance obligation and is deferred and recognized over the estimated service period.

Revenues are recognized when control of the promised goods or services are transferred to the Company’s customers in an amount that reflects the consideration that is expected to be received in exchange for those goods or services. The Company generates all of its revenues from contracts with its customers. A description of principal activities from which the Company generates revenues follows.

•

Products Delivered at a Point in Time. The Company sells its products to customers in accordance with the terms of the related customer contracts. The Company generates revenues from sales of its solutions, which include microinverter units and related accessories, an Envoy communications gateway and Enlighten service, communications accessories and AC Battery™ storage solutions to distributors, large installers, OEMs and strategic partners. Microinverter units, microinverter accessories, and AC Battery storage solutions are delivered to customers at a point in time, and the Company recognizes revenue for these products when the Company transfers control of the product to the customer, which is generally upon shipment.

•

Products Delivered Over Time. The sale of an Envoy communications gateway includes the Company’s Enlighten cloud-based monitoring service. The full consideration for these products represents a single performance obligation and is deferred at the sale date and recognized over the estimated service period. The Company also sells certain communication accessories that are delivered over time. The revenue from these products is recognized over the related service period.

The Company previously sold its Envoy communications device to certain customers under a long-term financing arrangement. Under this financing arrangement, the Company nets the unbilled receivables against deferred revenue.

The Company records certain contra revenue promotions as variable consideration and recognizes these promotions at the time the related revenue is recorded.

The Company records upfront contract acquisition costs, such as sales commissions, to be capitalized and amortized over the estimated life of the asset. For contracts that have a duration of less than one year, the Company follows the Topic 606 practical expedient and expenses these costs when incurred. Commissions related to the Company’s sale of monitoring hardware and service are capitalized and amortized over the period of the associated revenue.

See Note 3. “Revenue Recognition,” for additional information related to revenue recognition.

Cost of Revenues

The Company includes the following in cost of revenues: product costs, warranty, manufacturing personnel and logistics costs, freight costs, inventory write-downs, hosting services costs related to the Company’s Enlighten service offering, and depreciation and amortization of manufacturing test equipment. A description of principal activities from which the Company recognizes cost of revenue is as follows.


•	Products Delivered at a Point in Time. Cost of revenue from these products is recognized when the Company transfers control of the product to the customer, which is generally upon shipment.


•	Products Delivered Over Time. Cost of revenue from these products is recognized over the related service period.

Cash and Cash Equivalents

The Company considers all highly liquid investments, such as certificates of deposit and money market instruments with maturities of three months or less at the time of acquisition to be cash equivalents. For all periods presented, its cash balances consist of amounts held in non-interest-bearing deposits and money market accounts.

Fair Value of Financial Instruments

The carrying amounts of the Company’s cash and cash equivalents, accounts receivable, accounts payable and accrued liabilities approximate fair value because of the short maturity of those instruments.

Accounts Receivables and Contract Assets

Allowance for Doubtful Accounts

The Company maintains allowances for doubtful accounts for uncollectible accounts receivable. Management estimates anticipated losses from doubtful accounts based on days past due, collection history and the financial health of customers. The following table sets forth activities in the allowance for doubtful accounts for the periods indicated (in thousands):



	December 31,
	2018			2017			2016
Balance, at beginning of year	$	2,378		$	2,921		$	1,808
Net charges to expenses	711			476			3,097
Write-offs, net of recoveries	(951		)	(1,019		)	(1,984		)
Balance, at end of year	$	2,138		$	2,378		$	2,921

Inventory

Inventory is valued at the lower of cost or market. Market is current replacement cost (by purchase or by reproduction, dependent on the type of inventory). In cases where market exceeds net realizable value (i.e., estimated selling price less reasonably predictable costs of completion and disposal), inventories are stated at net realizable value. Market is not considered to be less than net realizable value reduced by an allowance for an approximately normal profit margin. The Company determines cost on a first-in first-out basis. Management assesses the valuation on a quarterly basis and writes down the value for any excess and obsolete inventory based upon expected demand, anticipated sales price, effect of new product introductions, product obsolescence, customer concentrations, product merchantability and other factors. Inventory write-downs are equal to the difference between the cost of inventories and market.

Long-Lived Assets

Property and equipment are stated at cost less accumulated depreciation. Cost includes amounts paid to acquire or construct the asset as well as any expenditure that substantially adds to the value of or significantly extends the useful life of an existing asset. Repair and maintenance costs are expensed as incurred. Depreciation and amortization are computed using the straight-line method over the estimated useful lives of the assets, which range from 3 to 10 years. Leasehold improvements are amortized over the shorter of the lease term or expected useful life of the improvements.

Internally used software, whether purchased or developed, is capitalized at cost and amortized on a straight-line basis over its estimated useful life. Costs associated with internally developed software are expensed until the point at which the project has reached the development stage. Subsequent additions, modifications or upgrades to internal-use software are capitalized only to the extent that they provide additional functionality. Software maintenance and training costs are expensed in the period in which they are incurred. The capitalization of software requires judgment in determining when a project has reached the development stage and the period over which the Company expects to benefit from the use of that software.

Property, plant and equipment, including capitalized software costs, are reviewed for impairment whenever events or changes in circumstances indicate that the carrying amount of an asset (asset group) may not be recoverable. An impairment loss would be recognized when the carrying amount of an asset exceeds the estimated undiscounted future cash flows expected to result from the use of the asset and its eventual disposition. The amount of the impairment loss to be recorded is calculated by the excess of the asset’s carrying value over its fair value. Fair value is generally determined using a discounted cash flow analysis. The Company recorded asset impairment charges for specific assets that were no longer in use of approximately $1.6 million, $0.8 million and $2.6 million for the years ended 2018, 2017 and 2016, respectively. The fair value of the remaining assets is in excess of the carrying value.

Business Combinations

Assets acquired and liabilities assumed as part of a business acquisition are generally recorded at their fair value at the date of acquisition. The excess of purchase price over the fair value of assets acquired and liabilities assumed is recorded as goodwill. Determining fair value of identifiable assets, particularly intangibles, and liabilities acquired also requires the Company to make estimates, which are based on all available information and in some cases assumptions with respect to the timing and amount of future revenues and expenses associated with an asset. Accounting for business acquisitions requires the Company to make judgments as to whether a purchase transaction is a multiple element contract, meaning that it includes other transaction components. This judgment and determination affect the amount of consideration paid that is allocable to assets and liabilities acquired in the business purchase transaction.

Goodwill

Goodwill results from the purchase consideration paid in excess of the fair value of the net assets recorded in connection with a business acquisition. Goodwill is not amortized but is assessed for potential impairment at least annually during the fourth quarter of each fiscal year or between annual tests if an event occurs or circumstances change that would indicate the carrying amount may be impaired. Goodwill is tested at the reporting unit level, which the Company has determined to be the same as the entity as a whole (entity level). Once goodwill has been assigned to a reporting unit, it is no longer associated with a particular acquisition; therefore, all of the activities within a reporting unit, whether acquired or organically grown, are available to support the goodwill value. Based on management’s goodwill impairment tests, there was no impairment of goodwill in any of the years presented.

Intangible Assets

Intangible assets include patents and other purchased intangible assets. Intangible assets with finite live are amortized on a straight-line basis, with estimated useful lives ranging from 3 to 9 years. Indefinite-lived intangible assets are tested for impairment annually and are also tested for impairment between annual tests if an event occurs or circumstances change that would indicate that the carrying amount may be impaired. Intangible assets with finite live are tested for impairment whenever events or circumstances indicate that the carrying amount of an asset (asset group) may not be recoverable. An impairment loss is recognized when the carrying amount of an asset exceeds the estimated undiscounted cash flows used in determining the fair value of the asset. The amount of the impairment loss to be recorded is calculated by the excess of the asset’s carrying value over its fair value. Fair value is generally determined using a discounted cash flow analysis. There was no impairment of intangible assets in any of the years presented.

Contract Liabilities

Contract liabilities are recorded as deferred revenue on the accompanying consolidated balance sheets and include payments received in advance of performance obligations under the contract and are realized when the associated revenue is recognized under the contract.

Warranty Obligations

Microinverters Sold Through December 31, 2013

The Company’s warranty accrual provides for the replacement of microinverter units that fail during the product’s warranty term (15 years for first and second generation microinverters and up to 25 years for third and fourth generation microinverters). On a quarterly basis, the Company employs a consistent, systematic and rational methodology to assess the adequacy of its warranty liability. This assessment includes updating all key estimates and assumptions for each generation of product, based on historical results, trends and the most current data available as of the filing date. The key estimates and assumptions used in the warranty liability are thoroughly reviewed by management on a quarterly basis. The key estimates used by the Company to estimate its warranty liability are: (1) the number of units expected to fail over time (i.e., failure rate); (2) the number of failed units expected to result in warranty claims over time (i.e., claim rate); and (3) the per unit cost of replacement units, including outbound shipping and limited labor costs, expected to be incurred to replace failed units over time (i.e., replacement cost).

Estimated Failure Rates — The Company’s Quality and Reliability department has primary responsibility to determine the estimated failure rates for each generation of microinverter. To establish initial failure rate estimates for each generation of microinverter, the Company’s quality engineers use a combination of industry standard Mean Time Between Failure (“MTBF”) estimates for individual components contained in its microinverters, third party data collected on similar equipment deployed in outdoor environments similar to those in which the Company’s microinverters are installed, and rigorous long term reliability and accelerated life cycle testing which simulates the service life of the microinverter in a short period of time. As units are deployed into operating environments, the Company continues to monitor product performance through its Enlighten monitoring platform. It typically takes three to nine months between the date of sale and date of end-user installation. Consequently, the Company’s ability to monitor actual failures of units sold similarly lags by three to nine months. When a microinverter fails and is returned, the Company performs diagnostic root cause failure analysis to understand and isolate the underlying mechanism(s) causing the failure. The Company then uses the results of this analysis (combined with the actual, cumulative performance data collected on those units prior to failure through Enlighten) to draw conclusions with respect to how or if the identified failure mechanism(s) will impact the remaining units deployed in the installed base.

Estimated Claim Rates — Warranty claim rate estimates are based upon assumptions with respect to expected customer behavior over the warranty period. As the vast majority of the Company’s microinverters have been sold to end users for residential applications, the Company believes that warranty claim rates will be affected by changes over time in residential home ownership because the Company expects that subsequent homeowners are less likely to file claims than the homeowners who originally purchase the microinverters.

Estimated Replacement Costs — Three factors are considered in the Company’s analysis of estimated replacement cost: (1) the estimated cost of replacement microinverters; (2) the estimated cost to ship replacement microinverters to end users; and (3) the estimated labor reimbursement expected to be paid to third party installers performing replacement services for the end user. Because the Company’s warranty provides for the replacement of defective microinverters over long periods of time (between 15 and 25 years, depending on the generation of product purchased), the estimated per unit cost of current and future product generations is considered in the estimated replacement cost. Estimated costs to ship replacement units are based on observable, market-based shipping costs paid by the Company to third party freight carriers. The Company has a separate program that allows third-party installers to claim fixed-dollar reimbursements for labor costs they incur to replace failed microinverter units for a limited time from the date of original installation. Included in the Company’s estimated replacement cost is an analysis of the number of fixed-dollar labor reimbursements expected to be claimed by third party installers over the limited offering period.

In addition to the key estimates noted above, the Company also compares actual warranty results to expected results and evaluates any significant differences. Management may make additional adjustments to the warranty provision based on performance trends or other qualitative factors. If actual failure rates, claim rates, or replacement costs differ from the Company’s estimates in future periods, changes to these estimates may be required, resulting in increases or decreases in the Company’s warranty obligations. Such increases or decreases could be material.

Fair Value Option for Microinverters Sold Since January 1, 2014

The Company’s warranty obligations related to microinverters sold since January 1, 2014 provide the Company the right, but not the requirement, to assign its warranty obligations to a third-party. Under ASC 825, “Financial Instruments” (also referred to as “fair value option”), an entity may choose to elect the fair value option for such warranties at the time it first recognizes the eligible item. The Company made an irrevocable election to account for all eligible warranty obligations associated with microinverters sold since January 1, 2014 at fair value. This election was made to reflect the underlying economics of the time value of money for an obligation that will be settled over an extended period of up to 25 years.

The Company estimates the fair value of warranty obligations by calculating the warranty obligations in the same manner as for sales prior to January 1, 2014 and applying an expected present value technique to that result. The expected present value technique, an income approach, converts future amounts into a single current discounted amount. In addition to the key estimates of failure rates, claim rates and replacement costs, the Company used certain inputs that are unobservable and significant to the overall fair value measurement. Such additional assumptions included compensation comprised of a profit element and risk premium required of a market participant to assume the obligation and a discount rate based on the Company’s credit-adjusted risk-free rate. See Note 9. “Fair Value Measurements,” for additional information.

Warranty obligations initially recorded at fair value at the time of sale will be subsequently re-measured to fair value at each reporting date. In addition, the fair value of the liability will be accreted over the corresponding term of the warranty of up to 25 years using the interest method.

Warranty for Other Products

The Company offers a 5‑year warranty for its Envoy communications gateway and a 10‑year warranty on its AC Battery storage solution. The warranties provide the Company with the right, but not the obligation, to assign its warranty obligations to a third-party. As such, warranties for Envoy and AC Battery storage solution products are accounted for under the fair value method of accounting.

Research and Development Costs

The Company expenses research and development costs as incurred.

Stock-Based Compensation

Share-based payments are required to be recognized in the Company’s consolidated statements of operations based on their fair values and the estimated number of shares expected to vest. The Company measures stock-based compensation expense for all share-based payment awards, including stock options made to employees and directors, based on the estimated fair values on the date of the grant. The fair value of stock options granted is estimated using the Black-Scholes option valuation model. The fair value of restricted stock units granted is determined based on the price of the Company’s common stock on the date of grant. The fair value of non-market‑based performance stock units granted is determined based on the date of grant or when achievement of performance is probable. The fair value of market‑based performance stock units granted is determined using a Monte‑Carlo model based on the date of grant or when achievement of performance is probable.

Stock-based compensation for stock options and restricted stock units (“RSUs”) is recognized on a straight-line basis over the requisite service period. Stock-based compensation for performance stock units (“PSUs”) without market conditions is recognized when the performance condition is probable of being achieved, and then on a graded basis over the requisite service period. Stock-based compensation for PSUs with market conditions is recognized on a straight-line basis over the requisite service period. Additionally, the Company estimates its forfeiture rate annually based on historical experience and revise the estimates of forfeiture in subsequent periods if actual forfeitures differ from those estimates.

Comprehensive Income (Loss)

Comprehensive income (loss) consists of two components, net income (loss) and other comprehensive income (loss). Other comprehensive income (loss) refers to gains and losses that are recorded as an element of stockholders’ equity but are excluded from net income (loss). The Company’s other comprehensive income (loss) consists of foreign currency translation adjustments for all periods presented.

Income Taxes

The Company records income taxes using the asset and liability method, which requires the recognition of deferred tax assets and liabilities for the expected tax consequences of temporary differences between the tax bases of assets and liabilities for financial reporting purposes and amounts recognized for income tax purposes. In estimating future tax consequences, generally all expected future events other than enactments or changes in the tax law or rates are considered. Valuation allowances are provided when necessary to reduce deferred tax assets to the amount expected to be realized.

The Company operates in various tax jurisdictions and is subject to audit by various tax authorities. The Company follows accounting for uncertainty in income taxes which requires that the tax effects of a position be recognized only if it is “more likely than not” to be sustained based solely on its technical merits as of the reporting date. The Company considers many factors when evaluating and estimating its tax positions and tax benefits, which may require periodic adjustments and which may not accurately anticipate actual outcomes.

Recently Adopted Accounting Pronouncements

In May 2014, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update No. (“ASU”) 2014-09, “Revenue from Contracts with Customers,” amending revenue recognition guidance and requiring more detailed disclosures to enable users of financial statements to understand the nature, amount, timing, and uncertainty of revenue and cash flows arising from contracts with customers. The amended guidance, herein referred to as ASC 606 or Topic 606. The Company has adopted Topic 606 effective January 1, 2018, using the modified retrospective transition method. The Company recognized the cumulative effect of applying the new revenue standard as an adjustment to the opening balance of retained earnings on January 1, 2018. The comparative information has not been restated and continues to be reported under the accounting standards in effect for the period presented. See Note 2. “Summary of Significant Accounting Policies,” and Note 3. “Revenue Recognition,” for additional accounting policy and transition disclosures.

In January 2016, the FASB issued ASU 2016-01, “Recognition and Measurement of Financial Assets and Financial Liabilities,” which amends certain aspects of recognition, measurement, presentation, and disclosure of financial instruments. Changes to the current guidance include the accounting for equity investments, the presentation and disclosure requirements for financial instruments, and the assessment of valuation allowance on deferred tax assets related to available-for-sale securities. In addition, ASU 2016-01 establishes an incremental recognition and disclosure requirement related to the presentation of fair value changes of financial liabilities for which the fair value option has been elected. Under this guidance, an entity would be required to separately present in other comprehensive income the portion of the total fair value change attributable to instrument-specific credit risk as opposed to reflecting the entire amount in earnings. The Company’ adoption of ASU 2016-01 in the first quarter of 2018 did not have a material impact on the consolidated financial statements.

In August 2016, the FASB issued ASU 2016-15, “Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Cash Payments.” ASU 2016-15 provides amendments that address eight specific cash flow classification issues for which there exists diversity in practice: Debt prepayment or debt extinguishment costs; settlement of zero-coupon debt instruments or other debt instruments with coupon interest rates that are insignificant in relation to the effective interest rate of the borrowing; contingent consideration payments made after a business combination; proceeds from the settlement of insurance claims; proceeds from the settlement of corporate-owned life insurance policies, including bank-owned life insurance policies; distributions received from equity method investees; beneficial interests in securitization transactions; and separately identifiable cash flows and application of the predominance principle. The Company’s retrospective adoption of ASU 2016-15 in the first quarter of 2018 did not have a material effect on the Company’s consolidated financial statements.

In November 2016, the FASB issued ASU 2016-18, “Statement of Cash Flows,” which requires companies to include amounts generally described as restricted cash and restricted cash equivalents in cash and cash equivalents when reconciling beginning-of-period and end-of-period total amounts shown on the statement of cash flows. The Company’s retrospective adoption of ASU 2016-18 in the first quarter of 2018 did not have a material impact on the consolidated financial statements.

In May 2017, the FASB issued ASU 2017-09, “Compensation - Stock Compensation.” ASU 2017-09 was issued to provide clarity and reduce both 1) diversity in practice and 2) cost and complexity when applying the guidance in Topic 718 to a change in the terms or conditions of a share-based payment award. ASU 2017-09 provides guidance about which changes to the terms or conditions of a share-based payment award require an entity to apply modification accounting under Topic 718. The standard, which was adopted in the first quarter of 2018, did not have a material impact on the consolidated financial statements.

Recently Issued Accounting Pronouncements Not Yet Effective

In February 2016, the FASB issued ASU 2016-02, “Leases” (“ASU 2016-02”). The guidance will require lessees to recognize all leases, with certain exceptions, on their balance sheets, whether operating or financing, while continuing to recognize the expenses on their income statements in a manner similar to current practice. The guidance states that a lessee would recognize a lease liability for the obligation to make lease payments and a right-to-use asset for the right to use the underlying asset for the lease term. The guidance will be effective for the Company beginning January 1, 2019 and early adoption is permitted. The Company will adopt ASU 2016-02 on January 1, 2019 and intends to elect the available practical expedients upon adoption. Upon adoption, the Company expects the consolidated balance sheet to include a right of use asset and a corresponding lease liability related to substantially all of the Company’s lease arrangements. We are currently evaluating the impact ASU 2016-02 will have on the Company’s consolidated financial statements and related disclosures.

In June 2018, the FASB issued ASU 2018-07, “Compensation - Stock Compensation: Improvements to Non-employee Share-Based Payment Accounting.” ASU 2018-07 was issued to provide to provide guidance on share-based payments granted to non-employees in exchange for goods or services used or consumed in an entity’s own operations and supersedes the guidance in ASC 505-50, “Equity-Based Payments to Non-Employees.” ASU 2018-07 aligns much of the guidance on measuring and classifying non-employee awards with that of awards to employees, specifically 1) equity-classified non-employee share-based payment awards are no longer measured at the earlier of the date at which a commitment for performance; 2) aligns accounting for non-employee share-based payment awards with performance conditions with accounting for employee share-based payment awards under Topic 718 by requiring entities to consider the probability of satisfying performance conditions; 3) eliminates the reassessment of classification upon vesting and instead, requires the non-employee share-based payment awards to continue to be subject to Topic 718 unless they are modified after the good has been delivered, the service has been rendered, or any other conditions necessary to earn the right to benefit from the instruments have been satisfied, and the non-employee is no longer providing goods or services, consistent with the treatment of employee awards; and 4) allows entities to make an award-by-award election to use either the expected term (consistent with employee share-based payment awards) or the contractual term for non-employee awards. The guidance is effective for annual periods beginning after December 15, 2018, and interim periods within those annual periods. Early adoption is permitted, including in an interim period for which financial statements have not been issued, but not before an entity adopts ASC 606. The Company adopted ASU 2018-07 on January 1, 2019 on a modified retrospective basis and does not expect the adoption of ASU 2018-07 to have a material impact to accumulated deficit upon adoption.

In August 2018, the FASB issued ASU 2018-15, “Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract,” to reduce diversity in practice in accounting for the costs of implementing cloud computing arrangements that are service contracts. ASU 2018-15 allows entities to apply the guidance in the ASC 350-40, “Intangibles–Goodwill and Other–Internal-Use Software,” to determine which implementation costs are eligible to be capitalized as assets in a cloud computing arrangement that is considered a service contract. ASU 2018-15 is effective for fiscal years beginning after December 15, 2019, and interim periods within those fiscal years. Early adoption is permitted, including adoption in any interim period. Entities have the option to apply the guidance prospectively to all implementation costs incurred after the date of adoption or retrospectively and are required to make certain disclosures in the interim and annual period of adoption. The Company is currently evaluating the impact this update will have on its consolidated financial statements.

REVENUE RECOGNITION

12 Months Ended

Dec. 31, 2018

Revenue from Contract with Customer [Abstract]

REVENUE RECOGNITION

On January 1, 2018, the Company adopted Topic 606 and applied the modified retrospective method to all contracts that were not completed as of January 1, 2018. Results for reporting periods beginning after January 1, 2018 are presented under Topic 606, while prior period amounts are not adjusted and continue to be reported under the accounting standards in effect in the prior period. The Company recorded a net reduction to opening equity of $38.9 million on January 1, 2018 for the cumulative effect of adopting Topic 606. The impact to net revenues from the adoption of Topic 606 for the year ended December 31, 2018 was a net increase of $2.3 million.

Changes in accounting policies as a result of adopting Topic 606 and nature of goods

The most significant impacts upon adoption of Topic 606 were how the Company accounts for revenue related to its Envoy communications device and related Enlighten service and the timing of when certain sales incentives are recognized. Under ASC 605, “Revenue Recognition,” the Company’s Envoy communications device and Enlighten service were considered two units of accounting, and the portion of the consideration related to the hardware was recognized at the time of sale with the remaining consideration deferred and recognized over the estimated service period. Under ASC 606 the full consideration for these products represents a single performance obligation and is deferred and recognized over the estimated service period. This treatment resulted in a gross increase to deferred revenue of $77.5 million, an increase in deferred costs of $43.4 million and a net increase in accumulated deficit of $34.1 million upon adoption of ASC 606.

The Company previously sold its Envoy communications device to certain customers under a long-term financing arrangement. Under ASC 605, this arrangement resulted in the recording of both deferred revenue and unbilled receivables on the Company’s consolidated balance sheets. The Company’s opening entries related to ASC 606 included the netting of approximately $6.4 million of unbilled receivables against deferred revenue. Thus, the $77.5 million increase to deferred revenue noted above was partially offset by a $6.4 million reclassification of unbilled receivables to deferred revenue.

Under ASC 605 the Company recorded certain contra revenue promotions at the later of the date revenue was recognized or the date at which the promotional offer was extended. Under ASC 606 all such contra revenue programs are treated as variable consideration and recognized at the time the related revenue is recorded. This change in timing resulted in an increase in accrued liabilities of approximately $5.6 million and an increase to accumulated deficit of the same amount upon adoption of ASC 606. This change in timing is not expected to have a material impact in subsequent periods.

Topic 606 requires upfront contract acquisition costs, such as sales commissions, to be capitalized and amortized over the estimated life of the asset. For contracts that have a duration of less than one year, the Company follows the Topic 606 practical expedient and expenses these costs when incurred. Commissions related to the Company’s sale of monitoring hardware and service are capitalized and amortized over the period of the associated revenue, which is 6.5 years. This treatment resulted in an increase in deferred costs of approximately $0.8 million and a decrease in accumulated deficit of the same amount upon adoption of ASC 606.

Products Delivered at a Point in Time. The Company sells its products to customers in accordance with the terms of the related customer contracts. The Company generates revenues from sales of its solutions, which include microinverter units and related accessories, an Envoy communications gateway and Enlighten service, communications accessories and AC Battery storage solutions to distributors, large installers, OEMs and strategic partners. Microinverter units, microinverter accessories, and AC Battery storage solutions are delivered to customers at a point in time, and in accordance with Topic 606, the Company recognizes revenue for these products when the Company transfers control of the product to the customer, which is generally upon shipment.

Products Delivered Over Time. The sale of an Envoy communications gateway includes the Company’s Enlighten cloud-based monitoring service. Under Topic 606 the full consideration for these products represents a single performance obligation and is deferred at the sale date and recognized ratably over the estimated service period, which is 6.5 years.

The Company also sells certain communication accessories that are delivered over time. The revenue from these products is recognized over the related service period, which is typically 5 or 12 years.

Disaggregation of Revenue

The Company has one business activity, which is the design, manufacture and sale of solutions for the solar photovoltaic (“PV”) industry. The following table provides information about disaggregated revenue by primary geographical market and timing of revenue recognition for the Company’s single product line (in thousands):



	December 31, 2018
Primary geographical markets:
United States	$	219,600
International	96,559
Total	$	316,159

Timing of revenue recognition:
Products delivered at a point in time	$	270,778
Products and services delivered over time	45,381
Total	$	316,159

Contract Balances

As of December 31, 2018, receivables, and contract assets and contract liabilities from contracts with customers consist of the following (in thousands):



	December 31, 2018
Receivables	$	78,938
Short-term contract assets (Prepaid expenses and other assets)	13,516
Long-term contract assets (Other assets)	34,148
Short-term contract liabilities (Deferred revenues)	33,119
Long-term contract liabilities (Deferred revenues)	76,911

The Company receives payments from customers based upon contractual billing schedules. Accounts receivable are recorded when the right to consideration becomes unconditional. Contract assets include deferred product costs and commissions associated with the deferred revenue and will be amortized along with the associated revenue. The Company had no asset impairment charges related to contract assets in the year ended December 31, 2018. Contract liabilities are recorded as deferred revenue on the accompanying consolidated balance sheets and include payments received in advance of performance obligations under the contract and are realized when the associated revenue is recognized under the contract.

Significant changes in the balances of contract liabilities (deferred revenues) and contract assets (prepaid expenses and other assets) during the period are as follows (in thousands):



Contract Liabilities
Balance on January 1, 2018	$	116,830
Revenue recognized	(45,381		)
Increase due to billings	38,581
Balance as of December 31, 2018	$	110,030



Contract Assets
Balance on January 1, 2018	$	47,862
Amount recognized	(14,964		)
Increase	14,766
Balance as of December 31, 2018	$	47,664

Remaining Performance Obligations

The following table includes estimated revenue expected to be recognized in future periods related to performance obligations that are unsatisfied or partially unsatisfied at the end of the reporting period (in thousands).



Years	December 31, 2018
2019	$	33,119
2020	26,303
2021	20,124
2022	15,084
2023	9,282
Thereafter	6,118
Total	$	110,030

Practical Expedients and Exemptions

The Company generally expenses sales commissions related to products delivered at a point in time when the commissions are incurred because the amortization period would have been less than one year. The Company records these costs as sales and marketing expense. The Company expenses shipping and handling costs as incurred.

Impact of Adoption of Topic 606

In accordance with Topic 606, the disclosure of the impact of adoption to the Company’s consolidated statements of operations and consolidated balance sheets was as follows (in thousands):



	Year Ended December 31, 2018
	As Reported			Adjustments			Without Adoption of Topic 606
Net revenues	$	316,159		$	(2,251	)	$	313,908
Cost of revenues	221,714			(13		)	221,701
Gross profit	94,445			(2,238		)	92,207
Operating expenses:
Research and development	32,587			—			32,587
Sales and marketing	27,047			(117		)	26,930
General and administrative	29,086			—			29,086
Restructuring charges	4,129			—			4,129
Total operating expenses	92,849			(117		)	92,732
Income (loss) from operations	1,596			(2,121		)	(525		)
Other expense, net
Interest expense	(9,635		)	—			(9,635		)
Other expense	(2,190		)	—			(2,190		)
Total other expense, net	(11,825		)	—			(11,825		)
Loss before income taxes	(10,229		)	(2,121		)	(12,350		)
Income tax provision	(1,398		)	—			(1,398		)
Net loss	$	(11,627	)	$	(2,121	)	$	(13,748	)
Net loss per share:
Basic and diluted	$	(0.12	)	$	(0.02	)	$	(0.14	)
Shares used in per share calculation:
Basic and diluted	99,619			—			99,619



	December 31, 2018
	As Reported			Adjustments			Without Adoption of Topic 606
Prepaid expenses and other	$	20,860		$	(10,788	)	$	10,072
Other assets	36,548			(28,766		)	7,782
Accrued liabilities	29,010			(4,257		)	24,753
Deferred revenues, current	33,119			(20,800		)	12,319
Deferred revenues, noncurrent	76,911			(50,044		)	26,867
Accumulated deficit	(346,302		)	35,547			(310,755		)

INVENTORY

12 Months Ended

Dec. 31, 2018

Inventory Disclosure [Abstract]

INVENTORY

Inventory as of December 31, 2018 and 2017 consists of the following (in thousands):



	December 31,
	2018		2017
Raw materials	$	970	$	2,341
Finished goods	15,297		23,658
Total inventory	$	16,267	$	25,999

PROPERTY AND EQUIPMENT, NET

12 Months Ended

Dec. 31, 2018

Property, Plant and Equipment [Abstract]

PROPERTY AND EQUIPMENT, NET

As of December 31, 2018 and 2017, property and equipment consist of the following (in thousands):



	Estimated Useful Life (Years)	December 31,
	Estimated Useful Life (Years)	2018			2017
Equipment and machinery	3-10	$	43,566		$	41,148
Furniture and fixtures	5-10	2,239			2,188
Computer equipment	3-5	2,958			2,627
Capitalized software costs	3-5	12,114			11,749
Leasehold improvements	3-10	8,482			8,537
Construction in process		3,536			4,672
Total		72,895			70,921
Less accumulated depreciation and amortization		(51,897		)	(44,438		)
Property and equipment, net		$	20,998		$	26,483

Depreciation expense for property and equipment for the years ended December 31, 2018, 2017 and 2016 was $8.3 million, $8.6 million and $10.0 million, respectively.

As of December 31, 2018 and 2017, unamortized capitalized software costs were $0.7 million and $1.1 million, respectively.

GOODWILL AND INTANGIBLE ASSETS

12 Months Ended

Dec. 31, 2018

Goodwill and Intangible Assets Disclosure [Abstract]

GOODWILL AND INTANGIBLE ASSETS

The following table presents the details of the Company’s goodwill and purchased intangible assets as of December 31, 2018 and 2017 (in thousands):



	December 31, 2018									December 31, 2017
	Gross		Additions		Accumulated Amortization			Net		Gross		Accumulated Amortization			Net
Goodwill	$	3,664	$	21,119	$	—		$	24,783	$	3,664	$	—		$	3,664

Intangible assets:
Other indefinite-lived intangibles	$	286	$	—	$	—		$	286	$	286	$	—		$	286
Intangible assets with finite live:
Patents and licensed technology	1,665		—		(1,665		)	—		1,665		(1,436		)	229
Developed technology	—		13,100		(909		)	12,191		—		—			—
Customer relationships	—		23,100		(271		)	22,829		—		—			—
Total purchased intangible assets	$	1,951	$	36,200	$	(2,845	)	$	35,306	$	1,951	$	(1,436	)	$	515

In July 2014, the Company purchased certain patents related to system interconnection and photovoltaic AC module construction. The patents were amortized over their legal life of 3 years. In October 2015, the Company licensed certain technology related to application specific integrated circuits (“ASIC”) development for a 3‑year term, which is also its estimated useful life. As of December 31, 2018, these patents were fully amortized.

In August 2018, the Company acquired certain finite-lived intangible assets in its acquisition of SunPower Corporation’s (“SunPower”) microinverter business pursuant to an Asset Purchase Agreement (“APA”), primarily developed technology and customer relationships. See Note 20. “Acquisition,” for additional information related to this acquisition.

Amortization expense related to finite-lived intangible assets for the years ended December 31, 2018, 2017 and 2016 was $1.4 million, $0.4 million and $0.7 million, respectively. Amortization expense of $1.4 million for the year ended December 31, 2018 includes $0.3 million as a reduction to revenue, related to customer relationships.

ACCRUED LIABILITIES

12 Months Ended

Dec. 31, 2018

Payables and Accruals [Abstract]

ACCRUED LIABILITIES

As of December 31, 2018 and 2017 accrued liabilities consists of the following (in thousands):



	December 31,
	2018		2017
Salaries, commissions, incentive compensation and benefits	$	4,107	$	3,608
Customer rebates and sales incentives	8,527		8,988
Freight	7,286		3,853
Other	9,090		5,998
Total accrued liabilities	$	29,010	$	22,447

WARRANTY OBLIGATIONS

12 Months Ended

Dec. 31, 2018

Product Warranties Disclosures [Abstract]

WARRANTY OBLIGATIONS

The Company’s warranty activities during the years ended December 31, 2018, 2017 and 2016 were as follows (in thousands):



	Years Ended December 31,
	2018			2017			2016
Warranty obligations, beginning of period	$	29,816		$	31,414		$	30,547
Accruals for warranties issued during period	3,040			3,797			4,130
Changes in estimates	6,515			(732		)	2,562
Settlements	(8,579		)	(7,037		)	(8,523		)
Increase due to accretion expense	1,989			2,053			1,772
Other	(1,487		)	321			926
Warranty obligations, end of period	31,294			29,816			31,414
Less current portion	(8,083		)	(7,427		)	(8,596		)
Noncurrent	$	23,211		$	22,389		$	22,818

The Company sold approximately 1.0 million first and second generation microinverters from 2008 through mid-2012. The Company has sold approximately 3.9 million third generation microinverters since mid-2012 through mid-2015. The Company has sold approximately 10.7 million of its fourth generation microinverters, which were introduced in mid-2013 and are still being sold. The Company began selling its IQ series microinverters in 2017, sales of which totaled approximately 3.5 million units.

Changes in Estimates

On a quarterly basis, the Company uses the best and most complete underlying information available, following a consistent, systematic and rational methodology to determine its warranty obligations. The Company considers all available evidence to assess the reasonableness of all key assumptions underlying its estimated warranty obligations for each generation of microinverter. The changes in estimates discussed below resulted from consideration of new or additional information becoming available and subsequent developments. Changes in estimates included in the table above were comprised of the following:

2018

In 2018, the Company recorded a $0.9 million increase to warranty expense related to cost increases primarily for backwards compatibility cables, supply constrained inventory components as well as tariffs. The Company also recorded additional warranty expense of $3.3 million based on continuing analysis of field performance data and diagnostic root-cause failure analysis primarily relating to its second and third generation products. In addition, the Company recorded an increase of $2.1 million related to increased estimated claim rates and an increase to warranty expense of $0.2 million for labor reimbursement costs expected to be paid to third party installers performing replacement services. These increases were partially offset by a $1.5 million reduction to warranty expense, presented as “Other” in the table above, related to changes in the discount rates for fair value accounting.

2017

In 2017, primarily in the fourth quarter, the Company recorded the impact of further product-cost reduction initiatives for its sixth generation microinverters, which are backwards compatible with previous microinverter generations and will be used to fulfill future warranty obligations for all microinverter generations in the field. This resulted in a $2.2 million decrease to warranty expense related to estimated future replacement costs. The Company also recorded, primarily in the third quarter, a decrease to warranty expense of $1.9 million for labor reimbursement costs expected to be paid to third party installers performing replacement services for its second‑generation product as a result of a change in its reimbursement policy. In addition, the Company recorded additional warranty expense of $3.9 million based on continuing analysis of field performance data and diagnostic root-cause failure analysis primarily relating to its second‑generation product.

2016

In 2016, primarily in the fourth quarter, the Company recorded the impact of product-cost reduction initiatives for its sixth generation microinverters, which are backwards compatible with previous microinverter generations and will be used to fulfill future warranty obligations for all microinverter generations in the field. This resulted in a $2.1 million decrease to warranty expense related to estimated future replacement costs. This decrease was offset by an increase to warranty expense of $1.5 million for an increase in labor reimbursement costs expected to be paid to third party installers performing replacement services for its second‑generation product. In addition, the Company recorded additional warranty expense of $3.0 million based on continuing analysis of field performance data and diagnostic root-cause failure analysis primarily relating to its second‑generation product.

FAIR VALUE MEASUREMENTS

12 Months Ended

Dec. 31, 2018

Fair Value Disclosures [Abstract]

FAIR VALUE MEASUREMENTS

The accounting guidance defines fair value as the price that would be received from selling an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date. When determining the fair value measurements for assets and liabilities recorded at fair value, the Company considers the principal or most advantageous market in which it would transact and considers assumptions that market participants would use when pricing the asset or liability, such as inherent risk, transfer restrictions, and risk of nonperformance.

The fair value hierarchy requires an entity to maximize the use of observable inputs and minimize the use of unobservable inputs when measuring fair value. An asset’s or liability’s categorization within the fair value hierarchy is based upon the lowest level of input that is significant to the fair value measurement. Three levels of inputs may be used to measure fair value:


•	Level 1—Valuations based on quoted prices in active markets for identical assets or liabilities that the Company is able to access. Since valuations are based on quoted prices that are readily and regularly available in an active market, valuation of such assets or liabilities do not entail a significant degree of judgment.


•	Level 2—Valuations based on one or more quoted prices in markets that are not active or for which all significant inputs are observable, either directly or indirectly.


•	Level 3—Valuations based on inputs that are unobservable and significant to the overall fair value measurement.

The following table presents the Company’s liabilities that were measured at fair value on a recurring basis and its categorization within the fair value hierarchy at December 31, 2018 and 2017 (in thousands):



	Fair Value Hierarchy	Years Ended December 31,
	Fair Value Hierarchy	2018		2017
Warranty obligations	Level 3	$	11,757	$	9,791
Third party option to purchase receivables at a discount	Level 3	—		700

Fair Value Option for Warranty Obligations Related to Microinverters Sold Since January 1, 2014

The Company estimates the fair value of warranty obligations by calculating the warranty obligations in the same manner as for sales prior to January 1, 2014 and applying an expected present value technique to that result. The expected present value technique, an income approach, converts future amounts into a single current discounted amount. In addition to the key estimates of failure rates, claim rates and replacement costs, the Company used certain Level 3 inputs which are unobservable and significant to the overall fair value measurement. Such additional assumptions included a discount rate based on the Company’s credit-adjusted risk-free rate and compensation comprised of a profit element and risk premium required of a market participant to assume the obligation.

The following table provides information regarding changes in nonfinancial liabilities related to the Company’s warranty obligations measured at fair value on a recurring basis using significant unobservable inputs (Level 3) for the periods indicated (in thousands):



	Years Ended December 31,
	2018			2017			2016
Balance at beginning of period	$	9,791		$	10,332		$	6,182
Accruals for warranties issued during period	3,040			3,591			4,091
Changes in estimates	2,455			(4,551		)	(1,616		)
Settlements	(4,030		)	(1,956		)	(1,023		)
Increase due to accretion expense	1,989			2,053			1,772
Other	(1,488		)	322			926
Balance at end of period	$	11,757		$	9,791		$	10,332

Third Party Option to Purchase Receivables at a Discount

The Company entered into an agreement with a third party in the fourth quarter of 2017 to sell certain current and future receivables at a discount. The buyer had the option to purchase certain additional future receivables at various fixed discounts. This option was valued using the discounted cash flow methodology based on the counterparty credit-adjusted-risk-free rate and recorded as a liability. As of December 31, 2018, all purchases relating to this option had been made, and the liability has been relieved. As of December 31, 2017, $0.7 million relating to this option was recorded as a liability with corresponding offset to debt on the Company’s consolidated balance sheets.

Quantitative and Qualitative Information about Level 3 Fair Value Measurements

As of December 31, 2018, the significant unobservable inputs used in the fair value measurement of the Company’s liabilities designated as Level 3 are as follows:



Item Measured at Fair Value	Valuation Technique	Description of Significant Unobservable Input	Percent Used (Weighted-Average)
Warranty obligations for microinverters sold since January 1, 2014	Discounted cash flows	Profit element and risk premium	16.25%
	Discounted cash flows	Credit-adjusted risk-free rate	18.75%

As of December 31, 2017, the significant unobservable inputs used in the fair value measurement of the Company’s liabilities designated as Level 3 are as follows:



Item Measured at Fair Value	Valuation Technique	Description of Significant Unobservable Input	Percent Used (Weighted-Average)
Warranty obligations for microinverters sold since January 1, 2014	Discounted cash flows	Profit element and risk premium	17%
	Discounted cash flows	Credit-adjusted risk-free rate	17%
Third party option to purchase receivables at a discount	Discounted cash flows	Counter party credit-adjusted risk-free rate	4%

Sensitivity of Level 3 Inputs - Warranty Obligations

Each of the significant unobservable inputs is independent of the other. The profit element and risk premium are estimated based on requirements of a third-party participant willing to assume the Company’s warranty obligations. The credit‑adjusted risk‑free rate (“discount rate”) is determined by reference to the Company’s own credit standing at the fair value measurement date. Increasing the profit element and risk premium input by 100 basis points would result in a $0.1 million increase to the liability. Decreasing the profit element and risk premium by 100 basis points would result in a $0.1 million reduction of the liability. Increasing the discount rate by 100 basis points would result in a $0.4 million reduction of the liability. Decreasing the discount rate by 100 basis points would result in a $0.5 million increase to the liability.

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

RESTRUCTURING

12 Months Ended

Dec. 31, 2018

Restructuring and Related Activities [Abstract]

RESTRUCTURING



	Years Ended December 31,
	2018		2017		2016
Redundancy and employee severance and benefit arrangements	$	2,228	$	2,827	$	1,263
Asset impairments	1,601		522		2,575
Consultants engaged in restructuring activities	—		12,100		—
Lease loss reserves	300		1,468		579
Gain on business divestiture	—		—		(640		)
Total restructuring	$	4,129	$	16,917	$	3,777

2018 Plan

In the third quarter of 2018, the Company began implementing restructuring actions (the “2018 Plan”) to lower its operating expenses. The restructuring actions include reorganization of the Company’s global workforce, elimination of certain non-core projects and consolidation of facilities. The Company expects to complete this restructuring in 2019.

The following table presents the details of the Company’s restructuring charges under the 2018 Plan for the periods indicated (in thousands):



	Year Ended December 31, 2018
Redundancy and employee severance and benefit arrangements	$	2,228
Asset impairments	1,636
Lease loss reserves	340
Total restructuring	$	4,204

The following table provides information regarding changes in the Company’s 2018 Plan accrued restructuring balance for the periods indicated (in thousands):



	Redundancy and Employee Severance and Benefits			Lease Loss Reserves and Contractual Obligations			Total
Charges	$	2,228		$	340		$	2,568
Cash payments	(1,336		)	(61		)	(1,397		)
Non-cash settlement and other	12			9			21
Balance as of December 31, 2018	$	904		$	288		$	1,192

2016 Plan

In the third quarter of 2016, the Company began implementing restructuring actions (the “2016 Plan”) to lower its operating expenses. The restructuring actions have included reductions in the Company’s global workforce, the elimination of certain non-core projects, consolidation of office space at the Company’s corporate headquarters and the engagement of management consultants to assist the Company in making organizational and structural changes to improve operational efficiencies and reduce expenses. The Company substantially completed its restructuring activities under the 2016 Plan in 2017.

The following table presents the details of the Company’s restructuring charges under the 2016 Plan for the periods indicated (in thousands):



	Years Ended December 31,
	2018			2017		2016
Employee severance and benefit arrangements	$	—		$	2,827	$	1,263
Asset impairments	(35		)	522		2,575
Consultants engaged in restructuring activities	—			12,100		—
Lease loss reserves and contract termination costs	(40		)	1,468		579
Gain on business divestiture	—			—		(640		)
Total restructuring	$	(75	)	$	16,917	$	3,777

The following table provides information regarding the computation of the Company’s gain on business divestiture included in restructuring and other (in thousands) for the year ended December 31, 2016:



Consideration	$	1,375
Identifiable assets	(979		)
Contingent consideration	244
Gain on business divestiture	$	640

The following table provides information regarding changes in the Company’s 2016 Plan accrued restructuring balance for the periods indicated (in thousands):



	Employee Severance and Benefits			Lease Loss Reserves and Contractual Obligations			Total
Balance as of December 31, 2016	$	198		$	484		$	682
Charges	2,827			13,568			16,395
Cash payments	(2,796		)	(12,958		)	(15,754		)
Balance as of December 31, 2017	229			1,094			1,323
Charges and adjustments	—			(40		)	(40		)
Cash payments and receipts, net	(229		)	537			308
Balance as of December 31, 2018	$	—		$	1,591		$	1,591

DEBT

12 Months Ended

Dec. 31, 2018

Debt Disclosure [Abstract]

DEBT

Long-term debt was comprised of the following at December 31, 2018 and 2017 (in thousands):



	December 31,
	2018			2017
Convertible Notes due 2023	$	65,000		$	—
Less unamortized issuance costs	(2,361		)	—
Carrying amount of Convertible Notes due 2023	62,639			—

Term loan	41,524			50,000
Less unamortized discount and issuance costs	(1,059		)	(2,111		)
Carrying amount of term loan	40,465			47,889

Sale of long-term financing receivable recorded as debt	6,679			2,562
Less value of future purchase option	—			(700		)
Carrying amount of sale of long-term financing receivable recorded as debt	6,679			1,862
Total carrying amount of debt	109,783			49,751
Less current portion term loan	(25,417		)	(15,715		)
Less current portion of long-term financing receivable recorded as debt	(2,738		)	(1,714		)
Long-term debt	$	81,628		$	32,322

Convertible Notes

In August 2018, the Company sold $60.0 million aggregate principal amount of convertible senior notes due 2023 (the “Convertible Notes”) in a private placement. Additionally, an individual who is a member of the Company’s Board of Directors and a stockholder, Thurman John Rodgers, purchased $5.0 million aggregate principal amount of convertible senior notes due 2023 (the “Affiliate Notes” and together with the Convertible Notes, the “Notes”) in a concurrent private placement. The Company received net proceeds of $62.4 million, after deducting the underwriters’ fees of approximately $2.0 million and other issuance costs of approximately $0.6 million.

The Notes are senior, unsecured and bear interest at a rate of 4.0% per year, payable semi-annually on February 1 and August 1 of each year, beginning on February 1, 2019. The Notes will mature on August 1, 2023, unless earlier repurchased by the Company or converted at the option of the holders. The Company may not redeem the Notes prior to the maturity date, and no sinking fund is provided for the Notes.

The Notes are convertible, at a holder’s election, in multiples of $1,000 principal amount, into shares of the Company’s common stock based on the applicable conversion rate. The initial conversion rate for the Notes is 180.0180 shares of common stock per $1,000 principal amount of Notes (which is equivalent to an initial conversion price of approximately $5.5600 per share). The conversion rate and the corresponding conversion price are subject to adjustment upon the occurrence of certain events but will not be adjusted for any accrued and unpaid interest. Holders of the Notes who convert their Notes in connection with a make-whole fundamental change (as defined in the Indenture entered into in connection with the Convertible Notes) are, under certain circumstances, entitled to an increase in the conversion rate. Additionally, in the event of a fundamental change, holders of the Notes may require the Company to repurchase all or a portion of their Notes at a price equal to 100% of the principal amount of Notes, plus any accrued and unpaid interest, including any additional interest to, but excluding, the repurchase date. Holders may convert all or any portion of their Notes at their option at any time prior to the close of business on the business day immediately preceding the maturity date, in multiples of $1,000 principal amount.

As of December 31, 2018, there were unamortized issuance costs and debt discounts of approximately $2.4 million, which were recorded as a direct deduction from the Notes on the consolidated balance sheets.

Term Loan

In July 2016, the Company entered into a Loan and Security Agreement (the “Original Term Loan Agreement”) with lenders that are affiliates of Tennenbaum Capital Partners, LLC (the “Lenders” or “TCP”). Under the agreement, the Lenders committed to advance a term loan in an aggregate principal amount of up to $25.0 million with a maturity date of July 1, 2020. The Company borrowed the entire $25.0 million of term loan commitments on the loan closing date. Monthly payments due through June 30, 2017 were interest only, followed by consecutive equal monthly payments of principal plus accrued interest that were to begin on July 1, 2017 and continue through the maturity date. The term loan provided for an interest rate per annum equal to the higher of (i) 10.25% or (ii) LIBOR plus 9.5625%, subject to a 1.0% reduction if the Company achieves minimum levels of Revenue and EBITDA (each as defined in the Original Term Loan Agreement) for the twelve-consecutive month period ending June 30, 2017 as set forth in the Original Term Loan Agreement. In addition, the Company paid a commitment fee of 3.3% of the loan amount upon closing and a closing fee of 10.0% of the loan amount is payable in four equal installments at each anniversary of the closing date. The Company could elect to prepay the loan by incurring a prepayment fee between 1% and 3% of the principal amount of the term loan depending on the timing and circumstances of prepayment.

In February 2017, the Company entered into an Amended and Restated Loan and Security Agreement (the “Loan Agreement”) that amended and restated the Original Term Loan Agreement. The Loan Agreement provides for a $25.0 million secured term loan to the Company (the “New Term Loan”), which is in addition to the $25.0 million secured term loan borrowed by the Company under the Original Term Loan Agreement (together with the “New Term Loan” the “Term Loans”). The New Term Loan had the same July 1, 2020 maturity date that was applicable to the Original Term Loan Agreement. The New Term Loan was fully drawn at closing, with approximately $10.3 million of the proceeds used to repay existing combined principal and interest due under the Company’s Revolver with Wells Fargo Bank N.A. (“Wells Fargo”). Upon the repayment of loans under the Wells Fargo Revolver, the Wells Fargo Revolving Credit Agreement was terminated. The Company used the remainder of the proceeds from the New Term Loan for general corporate purposes.

Monthly payments under the Term Loans through February 28, 2018 were interest only, followed by consecutive equal monthly payments of principal plus accrued interest beginning on March 1, 2018 and continuing through the maturity date. Interest on the Term Loans was the greater of (a) 10.3125%, and (b) a fluctuating rate of interest per annum equal to the three-month LIBOR Rate (rounded up to the nearest 1/16th of one percent) plus 9.25%. In addition, the Company paid a commitment fee of 3.0% of the New Term Loan amount upon closing and a closing fee of 4.0% of the New Term Loan amount, which is payable with the closing fee under the Original Term Loan Agreement in four equal installments at each anniversary of the closing date of the Original Term Loan Agreement. The Company may elect to prepay the Term Loans by incurring a prepayment fee between 1% and 3% of the principal amount of the Term Loans depending on the timing and circumstances of prepayment.

On February 28, 2018, the Company entered into a Second Amendment to the Term Loans. The Second Amendment decreased the amount of principal repayments required under the Loan Agreement by 50% for the period from March 1, 2018 through December 31, 2018 and provided that the Company shall not prepay any part of the Term Loans during that same period without the Collateral Agent’s prior written consent.

The Term Loans were secured by a first-priority security interest on substantially all assets of the Company; provided, however, that the security interest in the Company’s intellectual property may be released if the Company satisfies certain requirements. The Company’s obligations under the Term Loans were not guaranteed by any of the Company’s existing subsidiaries, nor have any existing subsidiaries of the Company pledged any of their assets to secure the Term Loans.

The Loan Agreement required that (i) at all times from the closing date to and including March 31, 2018, the Company, and any future guarantors, have Unrestricted Cash (as defined in the Loan Agreement) of at least $10.0 million; (ii) at all times from the closing date to and including March 31, 2018, that the aggregate amount of Consolidated Unrestricted Cash, plus the value of Consolidated Receivables, plus the value of Consolidated Inventory (each as defined in the Loan Agreement) divided by the outstanding principal amount of Term Loans, shall equal or exceed 1.5; and (iii) at all times from April 1, 2018 and thereafter, that the aggregate amount of Consolidated Unrestricted Cash, plus the value of Consolidated Receivables, plus the value of Consolidated Inventory divided by the outstanding principal amount of Term Loans, shall equal or exceed 1.75. In addition, the Loan Agreement was subject to customary affirmative and negative covenants including restrictions on creation of liens, dispositions of assets, mergers, changing the nature of its business and dividends and other distributions, in each case subject to certain exceptions. The Loan Agreement also contained certain customary events of default including, but not limited to, failure to pay interest, principal and fees or other amounts when due, material breach of any representation or warranty, covenant defaults, cross defaults to other material indebtedness, events of bankruptcy and the occurrence of a material adverse change (as defined in the agreement) to the Company’s business. The Term Loan Agreement offered TCP customary rights and remedies in any event of default, including the ability to declare all amounts outstanding immediately due and payable.

As of December 31, 2018, the estimated schedule of principal payments due on the Term Loans was as follows (in thousands):



Year Due	Principal Payments
2019	$	25,317
2020	16,207
Total	$	41,524

On January 28, 2019, the Company repaid in full the remaining principal amount of the Term Loans of $39.5 million plus accrued interest and fees.

Warrants

In connection with the New Term Loan, the Company issued to the Lenders warrants to purchase an aggregate of approximately 1.2 million shares of the Company’s Common Stock at an exercise price of $1.05 per share. The warrants have a term of seven years and contain a “cashless exercise” feature that allows the holder to exercise the warrant without a cash payment upon the terms set forth therein.

The Company estimated the fair value of the warrants by using the Black-Scholes approach and the following assumptions: stock price of $1.56 per share; strike price of $1.05 per share; volatility of 85.9%, risk-free rate of 2.23%; dividend yield of 0%; and a term of seven years. The resulting fair value was used to allocate the proceeds from the Term Loan between liability and equity components.

The Company classified the warrants as equity and allocated the proceeds from the New Term Loan and warrants using the relative fair value method. Using this method, the Company allocated $1.4 million to the warrants, which was recorded as equity. This amount represents debt discount that will be amortized to interest expense over the term of the loan. The Lenders converted the warrants into approximately 0.9 million shares of the Company’s common stock in a cashless exercise in the first quarter of 2018.

Sale of Long-Term Financing Receivables

The Company entered into an agreement with a third party in the fourth quarter of 2017 to sell certain current and future receivables at a discount. In December 2017, the third party made an initial purchase of receivables that resulted in net proceeds to the Company of $2.8 million. This transaction was recorded as debt on the accompanying consolidated balance sheets, and the debt balance will be relieved by January 2019 as the underlying receivables are settled. During the year ended December 31, 2018, the third party made three additional purchases of receivables that resulted in total net proceeds to the Company of $5.6 million. These transactions were recorded as debt on the accompanying consolidated balance sheets, and the total associated debt balance will be relieved by September 2021 as the underlying receivables are settled. After the initial purchase, the buyer had the option to purchase certain additional future receivables at various fixed discounts. This option was valued at $0.7 million and was recorded as a liability with a corresponding offset to debt as of December 31, 2017. As of December 31, 2018, all purchases relating to this option had been made, and the liability has been relieved. See Note 9. “Fair Value Measurements,” for additional information.

Revolving Credit Facility

The Company had a $50.0 million revolving credit facility (the “Revolver”) with Wells Fargo Bank, N.A. that was entered into in November 2012. The Revolver was fully repaid and terminated in February 2017.

COMMITMENTS AND CONTINGENCIES

12 Months Ended

Dec. 31, 2018

Commitments and Contingencies Disclosure [Abstract]

COMMITMENTS AND CONTINGENCIES

COMMITMENTS AND CONTINGENT LIABILITIES

Operating Leases

The Company leases office facilities under noncancelable operating leases that expire on various dates through 2028. The terms of the lease agreements generally provide for rental payments on a graduated basis, and certain leases require the Company to pay its portion of executory costs such as taxes, insurance, and operating expenses. The Company recognizes rent expense on a straight-line basis over the lease term.

Rent expense for the years ended December 31, 2018, 2017 and 2016 was $3.2 million, $2.8 million and $3.8 million, respectively.

The Company’s minimum lease payments under noncancelable operating leases, exclusive of executory costs, as of December 31, 2018 are as follows (in thousands):



	Minimum Lease Payments
Fiscal year due:
2019	$	3,738
2020	3,532
2021	3,276
2022	1,810
2023	945
Thereafter	1,252
Total operating lease minimum payments	14,553
Sublease income to be recognized in the future under noncancelable subleases	(922		)
Net operating leases	$	13,631

Purchase Obligations

The Company has contractual obligations related to component inventory that its primary contract manufacturer procures on its behalf in accordance with its production forecast as well as other inventory related purchase commitments. As of December 31, 2018, these purchase obligations totaled approximately $41.1 million.

Contingencies

From time-to-time, the Company may be involved in litigation relating to claims arising out of its operations. The Company is not currently involved in any material legal proceedings; however, the Company may be involved in material legal proceedings in the future. Such matters are subject to uncertainty and there can be no assurance that such legal proceedings will not have a material adverse effect on its business, results of operations, financial position or cash flows.

SALE OF COMMON STOCK	12 Months Ended
SALE OF COMMON STOCK	Dec. 31, 2018
Equity [Abstract]
SALE OF COMMON STOCK	SALE OF COMMON STOCK In February 2018, the Company entered into a Securities Purchase Agreement with an investor pursuant to which the Company, in a private placement, issued and sold to the investor 9.5 million shares of the Company’s common stock at a price per share of $2.10, for gross proceeds of $20.0 million. In January 2017, the Company completed a private placement of securities that resulted in the issuance of approximately 10.8 million shares of common stock and gross proceeds of $10.0 million. In December 2016, the Company entered into an At Market Issuance Sales Agreement (ATM) under which it could sell shares of its common stock up to a gross aggregate offering price of $17.0 million. During the three months ended March 31, 2017 the Company sold approximately 11.1 million shares of common stock under the ATM and received net proceeds of approximately $16.6 million.

STOCK-BASED COMPENSATION

12 Months Ended

Dec. 31, 2018

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

STOCK-BASED COMPENSATION

Description of Equity Incentive Plans

2006 Plan

Under the Company’s 2006 Equity Incentive Plan (the “2006 Plan”), equity awards granted generally vest over a 4‑year period from the date of grant with a contractual term of up to 10 years. As of December 31, 2018, there were 1.6 million shares of options outstanding under the 2006 Plan. No further stock options or other stock awards may be granted under the 2006 Plan.

2011 Plan

Under the 2011 Equity Incentive Plan (the “2011 Plan”), the Company could initially issue up to 2,643,171 shares of its common stock pursuant to stock options, stock appreciation rights (“SARS”), restricted stock awards (“RSA”), RSUs, PSUs, and other forms of equity compensation, or collectively, stock awards, all of which may be granted to employees, including officers, and to non-employee directors and consultants. Options granted under the 2011 Plan before August 1, 2012 generally expire 10 years after the grant date and options granted thereafter generally expire 7 years after the grant date. Equity awards granted under the 2011 Plan generally vest over a 4-year period from the date of grant based on continued employment. The number of shares of the Company’s common stock authorized for issuance under the 2011 Plan automatically increases on each January 1 by 4.5% of the total number of shares of the Company’s common stock outstanding on December 31 of the preceding calendar year, or such lesser number of shares of common stock as determined by the board of directors. As of December 31, 2018, 1,319,960 shares remained available for issuance pursuant to future grants under the 2011 Plan. On January 31, 2019, the shares available for issuance under the 2011 Plan automatically increased by 4,816,556 shares.

2011 Employee Stock Purchase Plan

The 2011 Employee Stock Purchase Plan (“ESPP”) became effective immediately upon the execution and delivery of the underwriting agreement for the Company’s initial public offering on March 29, 2012. The ESPP authorized the issuance of 669,603 shares of the Company’s common stock pursuant to purchase rights granted to employees. The number of shares of common stock reserved for issuance will automatically increase, on each January 1, by a lesser of (i) 330,396 shares of the Company’s common stock or (ii) 1.0% of the total number of shares of the Company’s common stock outstanding on December 31 of the preceding calendar year, as determined by the Company’s board of directors. At the Annual Meeting of Stockholders held on May 18, 2017 the Company’s stockholders approved a one-time amendment to the Company’s ESPP to increase the aggregate number of shares available for purchase by 400,000 shares and to increase the annual automatic minimum increase in shares reserved for issuance from 330,396 to 700,000 shares effective January 1, 2018. As of December 31, 2018, 551,203 shares remained available for future issuance under the ESPP. On January 31, 2019, the shares available for issuance under the ESPP automatically increased by 700,000 shares.

The ESPP is implemented by concurrent offering periods and each offering period may contain up to four interim purchase periods. In general, offering periods consists of the 24-month periods commencing on each May 15 and November 15 of a calendar year.

Generally, all full-time employees, including executive officers, are eligible to participate in the ESPP. The ESPP permits eligible employees to purchase the Company’s common stock through payroll deductions, which may not exceed 15% of the employee’s total compensation subject to certain limits. Stock may be purchased under the plan at a price equal to 85% of the fair market value of the Company’s stock on either the date of purchase or the first day of an offering period, whichever is lower. A two‑year look-back feature in the Company’s ESPP causes an offering period to reset if the fair value of the Company’s common stock on a purchase date is less than that on the initial offering date for that offering period. The reset feature, when triggered, will be accounted for as a modification to the original offering, resulting in additional expense to be recognized over the 24-month period of the new offering. During any calendar year, participants may not purchase shares of common stock having a value greater than $25,000, based on the fair market value per share of the common stock at the beginning of an offering period.

Valuation of Equity Awards

Stock Options

The fair value of each option granted was estimated on the date of grant using the Black-Scholes option-pricing model with the following assumptions:

•

Expected term—The expected term of the option awards represents the period of time between the grant date of the option awards and the date the option awards are either exercised, converted or canceled, including an estimate for those option awards still outstanding. The Company used the simplified method, as permitted by the SEC for companies with a limited history of stock option exercise activity, to determine the expected term for its option grants.


•	Expected volatility—The expected volatility was calculated based on the Company’s historical stock prices, supplemented as necessary with historical volatility of the common stock of several peer companies with characteristics similar to those of the Company.


•	Risk-free interest rate—The risk-free interest rate was based on the U.S. Treasury yield curve in effect at the time of grant and with a maturity that approximated the Company’s expected term.


•	Dividend yield—The dividend yield was based on the Company’s dividend history and the anticipated dividend payout over its expected term.

The following table presents the weighted-average grant date fair value of options granted for the periods presented and the assumptions used to estimate those values using a Black-Scholes option pricing model:



	Years Ended December 31,
	2018			2017			2016
Weighted average grant date fair value	$	2.83		$	0.76		$	1.29
Expected term (in years)	4.0			4.4			4.5
Expected volatility	88.5		%	83.9		%	80.0		%
Annual risk-free rate of return	2.6		%	1.8		%	1.1		%
Dividend yield	—		%	—		%	—		%

Restricted Stock Units

The fair value of the Company’s RSU awards granted is based upon the closing price of the Company’s stock price on the date of grant.

Performance Stock Units

The fair value of the Company’s non-market PSU awards granted was based upon the closing price of the Company’s stock price on the date of grant. The fair value of awards of the Company’s PSU awards containing market conditions was determined using a Monte Carlo simulation model based upon the terms of the conditions, the expected volatility of the underlying security, and other relevant factors.

Stock-based Compensation Expense

Stock-based compensation expense for all stock-based awards expected to vest is measured at fair value on the date of grant and recognized ratably over the requisite service period. The following table summarizes the components of total stock-based compensation expense included in the consolidated statements of operations for the periods presented (in thousands):



	Years Ended December 31,
	2018		2017		2016
Cost of revenues	$	1,071	$	1,072	$	1,188
Research and development	2,940		2,573		3,879
Sales and marketing	3,074		1,157		2,144
General and administrative	4,347		1,925		3,115
Total	$	11,432	$	6,727	$	10,326

The following table summarizes the various types of stock-based compensation expense for the periods presented (in thousands):



	Years Ended December 31,
	2018		2017		2016
Stock options, RSUs, and PSUs	$	10,691	$	5,559	$	8,384
Employee stock purchase plan	741		1,168		1,942
Total	$	11,432	$	6,727	$	10,326

As of December 31, 2018, there was approximately $17.1 million of total unrecognized stock-based compensation expense related to unvested equity awards expected to be recognized over a weighted-average period of 2.5 years.

No income tax benefit has been recognized relating to stock-based compensation expense and no tax benefits have been realized from exercised stock options for the three years ended December 31, 2018.

Equity Awards Activity

Stock Options

The following is a summary of stock option activity for the three years ended December 31, 2018 (in thousands, except per share data):



	Number of Shares Outstanding		Weighted- Average Exercise Price per Share		Weighted- Average Remaining Contractual Term (in years)	Aggregate Intrinsic Value ⁽¹⁾
Outstanding at December 31, 2015	8,170		$	5.36
Granted	2,440		2.12
Exercised	(375	)	0.39			$	729
Canceled	(1,505	)	6.01
Outstanding at December 31, 2016	8,730		4.55
Granted	4,500		1.22
Exercised	(425	)	0.51			544
Canceled	(4,379	)	6.91
Outstanding at December 31, 2017	8,426		1.77
Granted	213		4.43
Exercised	(1,346	)	1.75			5,096
Canceled	(521	)	2.94
Outstanding at December 31, 2018	6,772		1.76		4.2	$	21,547
Vested and expected to vest at December 31, 2018	6,772		1.76		4.2	$	21,547
Exercisable at December 31, 2018	4,645		1.87		3.5	$	14,670

(1)

The intrinsic value of options exercised is based upon the value of the Company’s stock at exercise. The intrinsic value of options outstanding, vested and expected to vest, and exercisable as of December 31, 2018 is based on the closing price of the Company’s stock fair value on December 31, 2018 or the earlier of the last trading day prior to December 31, 2018, if December 31, 2018 is a non-trading day. The Company’s stock fair value used in this computation was $4.73 per share.

The following table summarizes information about stock options outstanding at December 31, 2018:



	Options Outstanding				Options Exercisable
Range of Exercise Prices	Number of Shares (in thousands)	Weighted- Average Remaining Life (in years)	Weighted- Average Exercise Price		Number of Shares (in thousands)	Weighted- Average Exercise Price
$0.27 —– $0.79	1,551	2.9	$	0.49	1,349	$	0.45
$0.79 —– $1.29	1,352	5.9	1.22		433	1.21
$1.31 —– $1.31	1,751	5.3	1.31		1,167	1.31
$1.37 —– $2.61	1,388	3.0	1.87		1,150	1.86
$2.76 —– $12.57	730	3.6	6.32		546	7.15
Total	6,772	4.2	1.76		4,645	1.87

Restricted Stock Units

The following is a summary of RSU activity for the three years ended December 31, 2018 (in thousands, except per share data):



	Number of Shares Outstanding		Weighted- Average Fair Value per Share at Grant Date		Weighted- Average Remaining Contractual Term (in years)	Aggregate Intrinsic Value ⁽¹⁾
Outstanding at December 31, 2015	1,313		$	9.31
Granted	54		1.99
Vested	(464	)	9.06			$	875
Canceled	(297	)	8.32
Outstanding at December 31, 2016	606		9.33
Granted	5,418		1.46
Vested	(885	)	3.81			932
Canceled	(1,634	)	1.90
Outstanding at December 31, 2017	3,505		2.03
Granted	3,152		4.45
Vested	(1,399	)	2.75			6,657
Canceled	(906	)	2.17
Outstanding at December 31, 2018	4,352		3.52		1.4	$	20,586
Expected to vest at December 31, 2018	4,351		3.52		1.4	$	20,581

(1)

The intrinsic value of RSUs vested is based upon the value of the Company’s stock when vested. The intrinsic value of RSUs outstanding and expected to vest as of December 31, 2018 is based on the closing price of the Company’s stock on December 31, 2018 or the earlier of the last trading day prior to December 31, 2018, if December 31, 2018 is a non-trading day. The Company’s stock fair value used in this computation was $4.73 per share.

On April 3, 2017, the Company commenced a Tender Offer (the “Offer”) to exchange out of the money stock options for RSUs. The Offer expired on May 1, 2017. Pursuant to the Offer, the Company accepted elections to exchange options to purchase 2,362,470 shares of common stock and issued replacement awards of RSUs for 733,559 shares of common stock. As the transaction approximated a value-for-value exchange, it did not have a material impact on the Company’s stock-based compensation expense.

Performance Stock Units

The following is a summary of PSU activity for the year ended December 31, 2018 (in thousands, except per share data):



	Number of Shares Outstanding		Weighted- Average Fair Value per Share at Grant Date		Weighted- Average Remaining Contractual Term (in years)	Aggregate Intrinsic Value ⁽¹⁾
Outstanding at December 31, 2017	—
Granted	1,477		$	4.65
Vested	—
Canceled	(147	)
Outstanding at December 31, 2018	1,330		4.66		0.3	$	6,291
Expected to vest at December 31, 2018	1,330		4.66		0.3	$	6,291

(1)

The intrinsic value of PSUs vested is based upon the value of the Company’s stock when vested. The intrinsic value of PSUs outstanding and expected to vest as of December 31, 2018 is based on the closing price of the Company’s stock on December 31, 2018 or the earlier of the last trading day prior to December 31, 2018, if December 31, 2018 is a non-trading day. The Company’s stock fair value used in this computation was $4.73 per share.

Stock-based compensation expense is measured at the grant date based on the fair value of the award. During the second quarter of 2018 the Company issued PSU grants of 1.44 million shares, of which 720 thousand shares include market conditions. Each grantee is granted a target award of PSUs and may earn between 0% and 150% of the target award depending on the Company’s performance against the performance goals. The grant date fair value of PSUs without market conditions is recognized as expense when the performance condition is probable of being achieved, and then on a graded basis over the requisite service period. The grant date fair value of PSUs with market conditions is recognized as expense on a straight-line basis over the requisite service period. The weighted average estimated fair value of the PSUs without market conditions was $4.57 per share, and the weighted average estimated fair value of the PSUs with market conditions, based on the Monte Carlo model, was $4.54 per share.

Employee Stock Purchase Plan

A summary of ESPP activity for the years presented is as follows: (in thousands, except per share data):



	Years Ended December 31,
	2018		2017		2016
Proceeds from common stock issued under ESPP	$	397	$	313	$	999
Shares of common stock issued	439		478		659
Weighted-average price per share	$	0.90	$	0.65	$	1.52

INCOME TAXES

12 Months Ended

Dec. 31, 2018

Income Tax Disclosure [Abstract]

INCOME TAXES

The domestic and foreign components of loss before income taxes consisted of the following (in thousands):



	Years Ended December 31,
	2018			2017			2016
United States	$	(14,322	)	$	(47,882	)	$	(67,631	)
Foreign	4,093			2,541			1,644
Loss before income taxes	$	(10,229	)	$	(45,341	)	$	(65,987	)

The provision for (benefit from) income taxes for the years presented is as follows (in thousands):



	Years Ended December 31,
	2018			2017			2016
Current:
Federal	$	—		$	—		$	—
State	42			21			36
Foreign	1,233			1,224			785
	1,275			1,245			821
Deferred:
Federal	(35		)	(1,092		)	594
State	(21		)	(21		)	59
Foreign	179			(281		)	1
	123			(1,394		)	654
Provision for (benefit from) income taxes	$	1,398		$	(149	)	$	1,475

A reconciliation of the provision for (benefit from) income taxes and the amount computed by applying the statutory federal income tax rate of 21% in 2018 and 34% in 2017 and 2016 to loss before income taxes for the years presented is as follows (in thousands):



	Years Ended December 31,
	2018			2017			2016
Income tax benefit at statutory federal rate	$	(2,148	)	$	(15,416	)	$	(22,435	)
State taxes, net of federal benefit	17			(64		)	63
Change in valuation allowance	8,198			(20,571		)	21,370
Foreign tax rate and tax law differential	313			(133		)	27
Tax credits	(378		)	(382		)	(1,179		)
Stock-based compensation	(953		)	761			1,775
Other permanent items	235			479			776
Other nondeductible/nontaxable items	(5,112		)	930			920
Uncertain tax positions	107			106			158
Tax law changes	—			34,141			—
GILTI	917			—			—
Section 162(m)	202			—			—
Provision for (benefit from) income taxes	$	1,398		$	(149	)	$	1,475

A summary of significant components of the Company’s deferred tax assets and liabilities as of December 31, 2018 and 2017 is as follows (in thousands):



	December 31,
	2018			2017
Deferred tax assets:
Allowances and reserves	$	10,022		$	9,748
Net operating loss and tax credit carryforwards	71,568			66,243
Stock-based compensation	3,662			2,528
Deferred revenue	19,562			7,210
Fixed assets and intangibles	3,836			4,369
Sec. 163(j) interest carryforward	2,064			—
Other	2,084			278
Subtotal	112,798			90,376
Less valuation allowance	(98,631		)	(88,789		)
Total deferred tax assets, net of valuation allowance	14,167			1,587
Deferred tax liabilities:
Goodwill	(1,070		)	(992		)
Unremitted foreign earnings	(16		)	(10		)
Deferred cost of goods sold	(12,655		)	—
Total deferred tax liabilities	(13,741		)	(1,002		)
Net deferred tax asset	$	426		$	585

Accounting for income taxes requires that companies assess whether valuation allowances should be established against their deferred tax assets based on consideration of all available evidence, both positive and negative, using a “more likely than not” standard. This assessment considers, among other matters, the nature, frequency and amount of recent losses, the duration of statutory carryforward periods, and tax planning strategies. In making such judgments, significant weight is given to evidence that can be objectively verified. Due to the history of losses the Company has generated in the U.S. since inception, the Company believes that it is more-likely-than-not that all of its U.S. and state deferred tax assets will not be realized as of December 31, 2018. Therefore, the Company has maintained a full valuation allowance on its U.S. and state deferred tax assets at December 31, 2018; this is an increase of $9.8 million. Should the Company determine that it would be able to realize its deferred tax assets in the foreseeable future, an adjustment to the deferred tax assets may cause a material increase to income in the period such determination is made. Significant management judgment is required in determining the period in which the reversal of a valuation allowance should occur.

On December 22, 2017, H.R. 1 (the “Act”) was enacted and included broad tax reforms. The Act reduced the U.S. corporate tax rate from 35% to 21% effective January 1, 2018. The rate change resulted in a $28.3 million reduction in the Company’s 2017 deferred tax asset balance from 2016. Tax law changes created the ability for federal indefinite lived assets to be generated in the years following 2017. As such, an income tax benefit was recorded to reduce the domestic indefinite lived liability that is now partially offset by indefinite lived deferred tax assets. The Company has elected to treat taxes on Global Intangible Low Tax Income (“GILTI”) as period costs. The Act also imposed a deemed repatriation of foreign earnings previously considered deferred for U.S. tax purposes. The Company had previously recorded a deferred tax liability for such unremitted earnings that were not considered permanently reinvested in the foreign jurisdictions. As of December 31, 2017, the Company has recognized the previously deferred federal liability in accordance with the Act. As of December 31, 2018, the Company continues to include deferred tax liabilities for potential future tax consequences of remitting such earnings, such as foreign withholding and state taxes.

The Company has net operating loss carryforwards for federal and California income tax purposes of approximately $193.4 million and $101.2 million, respectively, as of December 31, 2018. The federal and state net operating loss carryforwards, if not utilized, will expire beginning in 2028.

The Company has approximately $11.8 million of federal research credit and $12.1 million of state research credit carryforwards. The federal credits begin to expire in 2026 and the state credits can be carried forward indefinitely.

Utilization of some of the federal and state net operating loss and credit carryforwards are subject to annual limitations due to the “change in ownership” provisions of the Internal Revenue Code of 1986 and similar state provisions. The Company has completed a Section 382 analysis through December 31, 2018, which indicated no such change has occurred through December 31, 2018.

The accounting for uncertain tax positions prescribes a recognition threshold and measurement attribute for the financial statement recognition and measurement of a tax position taken or expected to be taken in a tax return. The Company is required to recognize in the financial statements the impact of a tax position, if that position is more-likely-than-not of being sustained on audit, based on the technical merits of the position. The Company recorded a net charge for unrecognized tax benefits in 2018 of $0.2 million.

The Company does not have any tax positions for which it is reasonably possible the total amount of gross unrecognized tax benefits will increase or decrease over the next year. The unrecognized tax benefits may increase or change during the next year for items that arise in the ordinary course of business.

A tabular reconciliation of the total amounts of unrecognized tax benefits for the years presented is as follows (in thousands):



	Years Ended December 31,
	2018			2017			2016
Unrecognized tax benefits—at beginning of year	$	6,106		$	6,016		$	5,482
Decreases in balances related to tax positions taken in prior years	—			(135		)	—
Increases in balances related to tax positions taken in current year	329			306			571
Lapses in statutes of limitations	(110		)	(81		)	(37		)
Unrecognized tax benefits—at end of year	$	6,325		$	6,106		$	6,016

The Company includes interest and penalties related to unrecognized tax benefits within the benefit from (provision for) income taxes. As of years ended December 31, 2018 and 2017, the total amount of gross interest and penalties accrued in each year was immaterial. Both the unrecognized tax benefits and the associated interest and penalties that are not expected to result in payment or receipt of cash within one year are classified as other non-current liabilities in the consolidated balance sheets. In connection with tax matters, the Company’s interest and penalty expense recognized in 2018, 2017 and 2016 in the consolidated statement of operations was immaterial.

The Company’s tax returns continue to remain effectively subject to examination by U.S. federal authorities for the years 2006 through 2018 and by California state authorities for the years 2006 through 2018 due to use and carryovers of net operating losses and credits.

CONCENTRATION OF CREDIT RISK AND MAJOR CUSTOMERS	12 Months Ended
CONCENTRATION OF CREDIT RISK AND MAJOR CUSTOMERS	Dec. 31, 2018
Risks and Uncertainties [Abstract]
CONCENTRATION OF CREDIT RISK AND MAJOR CUSTOMERS	CONCENTRATION OF CREDIT RISK AND MAJOR CUSTOMERS The Company is potentially subject to financial instrument concentration of credit risk through its cash and cash equivalents and accounts receivable. The Company places its cash and cash equivalents with high quality institutions and performs periodic evaluations of their relative credit standing. Accounts receivable can be potentially exposed to a concentration of credit risk with its major customers. As of December 31, 2018, amounts due from two customers represented approximately 22% and 13% of the total accounts receivable balance. As of December 31, 2017, amounts due from one customer represented 22% of the total accounts receivable balance. In 2018, one customer accounted for approximately 19% of total net revenues. In 2017, two customers accounted for approximately 15% and 11% of total net revenues. In 2016, one customer accounted for approximately 18% of total net revenues.

NET LOSS PER SHARE

12 Months Ended

Dec. 31, 2018

Earnings Per Share [Abstract]

NET LOSS PER SHARE

Basic net loss per share is computed by dividing net loss by the weighted average number of shares of common stock outstanding during the period. Diluted loss per share is computed in a similar manner, but it also includes the effect of potential common shares outstanding during the period, when dilutive. Potential common shares include outstanding in-the-money stock options, RSUs, PSUs, shares to be purchased under the Company’s ESPP, warrants to purchase common stock, and the Notes. The dilutive effect of potentially dilutive common shares is reflected in diluted earnings per share by application of the treasury stock method. To the extent these potential common shares are antidilutive, they are excluded from the calculation of diluted net loss per share.

The following table presents the computation of basic and diluted net loss per share for the periods presented (in thousands, except per share data):



	Years Ended December 31,
	2018			2017			2016
Numerator:
Net loss	$	(11,627	)	$	(45,192	)	$	(67,462	)

Denominator:
Weighted average common shares outstanding	99,619			82,939			50,519

Net loss per share, basic and diluted	$	(0.12	)	$	(0.54	)	$	(1.34	)

As the Company incurred a net loss for all periods presented, potential dilutive securities from employee stock options, RSUs, PSUs, shares to be purchased under the Company’s ESPP, warrants to purchase common stock, and the Notes have been excluded from the diluted net loss per share computations because the effect of including such shares would have been anti-dilutive.

The following outstanding shares of common stock equivalents were excluded from the calculation of the diluted net loss per share attributable to common stockholders because their effect would have been anti-dilutive (in thousands):



	Years Ended December 31,
	2018	2017	2016
Employee stock options	7,710	8,433	8,981
RSUs and PSUs	5,273	3,029	906
Warrants to purchase common stock	—	1,083	—
Convertible notes	11,701	—	—
Total	24,684	12,545	9,887

SEGMENT AND GEOGRAPHIC INFORMATION

12 Months Ended

Dec. 31, 2018

Segment Reporting [Abstract]

SEGMENT AND GEOGRAPHIC INFORMATION

The Company’s chief operating decision maker is the Chief Executive Officer. The Chief Executive Officer reviews financial information presented on a consolidated basis. The Company has one business activity, which entails the design, development, manufacture and sale of solutions for the solar photovoltaic industry. There are no segment managers who are held accountable for operations, operating results or plans for levels or components below the consolidated unit level. Accordingly, management has determined that the Company has a single operating and reportable segment.

The following tables present net revenues (based on the destination of shipments) and long-lived assets by geographic region as of and for the periods presented (in thousands):

Net Revenues



	Years Ended December 31,
	2018		2017		2016
United States	$	219,600	$	199,565	$	259,080
International	96,559		86,601		63,511
Total	$	316,159	$	286,166	$	322,591

Long-Lived Assets



	December 31,
	2018		2017
United States	$	13,146	$	16,899
China	5,504		7,164
Other	2,348		2,420
Total	$	20,998	$	26,483

RELATED PARTY	12 Months Ended
RELATED PARTY	Dec. 31, 2018
Related Party Transactions [Abstract]
Related Party	RELATED PARTY In August 2018, the Company and SunPower, which at December 31, 2018, holds 7.5 million shares of the Company’s common stock, as well as the ability to appoint a board member to the Company’s board of directors, entered into a Master Supply Agreement (“MSA”). Pursuant to the terms of the MSA, the Company becomes the exclusive supplier of module level power electronics (“MLPEs”) for SunPower’s residential business in U.S. for a period of five years. Revenue recognized under the MSA for the year ended December 31, 2018 was $12.4 million. At December 31, 2018, the Company had account receivables of $10.3 million from SunPower. There are no special terms or payment arrangements under the MSA. See Note 20. “Acquisition,” for additional information related to this MSA. In August 2018, a member of the Company’s board of directors and a stockholder, Thurman John Rodgers, purchased $5.0 million aggregate principal amount of the Notes in a concurrent private placement. As of December 31, 2018, $5.0 million aggregate principal amount of the Notes were outstanding. See Note 11. “Debt,” for additional information related to this purchase.

ACQUISITION

12 Months Ended

Dec. 31, 2018

Business Combinations [Abstract]

ACQUISITION

On August 9, 2018, the Company completed its acquisition of SunPower’s microinverter business pursuant to an APA by which the Company acquired certain assets and liabilities of SunPower relating to the research and development and manufacturing of microinverters. The acquisition was accounted for as a business combination and, accordingly, the total purchase price was allocated to the preliminary net tangible and intangible assets and liabilities based on their preliminary fair values on the acquisition date.

In conjunction with the APA, the Company entered into an MSA with SunPower. Pursuant to the terms of the MSA, the Company becomes the exclusive supplier of MLPEs for SunPower’s residential business in the U.S. for a period of five years. The resulting customer relationship intangible is accounted for as a distinct transaction from the acquired business.

The acquisition date fair value of the consideration transferred was approximately $57.3 million, which consisted of the following (in thousands):



Cash consideration	$	25,000
Common stock issued	32,319
Total	$	57,319

The fair value of the Company’s 7.5 million shares of common stock issued, valued at $32.3 million, was determined based on the closing market price of the Company’s common stock on the acquisition date, less a discount of 14% to 30% (depending on the year) for lack of marketability as the shares issued are subject to a restriction that limits their trade or transfer with a lock-up period of six months and restrictions on the number of shares that can be transferred by SunPower in each six-month period following the lock-up period.

The following table summarizes the preliminary estimated fair values of the assets acquired and liabilities assumed at the acquisition date (in thousands):



Intangible assets	$	36,200
Goodwill	21,119
Net assets acquired	$	57,319

The excess of the consideration paid over the fair values assigned to the assets acquired and liabilities assumed represents the goodwill resulting from the acquisition. The $21.1 million of goodwill recognized is attributable primarily to the benefits the Company expects to derive from enhanced scale and efficiency to better serve its markets. Goodwill is expected to be deductible over the next 15 years for income tax purposes.

The fair values assigned to tangible and identifiable intangible assets acquired are based on management’s estimates and assumptions. The fair values of assets acquired are preliminary and may be subject to change within the measurement period as the fair value assessments are finalized.

The following table shows the fair value of the separately identifiable intangible assets at the time of acquisition and the period over which each intangible asset will be amortized:

The developed technology acquired is embedded in the microinverters that SunPower sells to its customers. The Company already has developed microinverter technology and the Company will supply its microinverters to SunPower through the term of the MSA. The Company does not intend to actively use the developed technology acquired from SunPower but does plan to hold the developed technology to prevent others from using it. Therefore, the Company will account for the developed technology as a defensive intangible asset. The Company expects to realize the benefits of the developed technology over the period of time in which the Company will supply microinverters to SunPower. The Company does expect changes in microinverter technology during the life of the customer relationship with SunPower and expects to benefit from preventing competitors’ access to the technology over a period of six years, therefore, the Company will amortize the value of the developed technology intangible asset over a period of six years.

The SunPower microinverter business’ contributions to revenue and income for the period from the date of acquisition to December 31, 2018 were not material. The MSA was negotiated together with the APA and provides the Company with the exclusive right to supply SunPower with MLPEs for a period of five years, with options for renewals. The exclusivity arrangement extends throughout the term of the MSA, which comprises all of the expected cash flows from the customer relationship intangible asset, and was a condition to, and was an essential part of the acquisition of the microinverter business by the Company. As the fair value ascribed to the customer relationship intangible asset represents payments to a customer, the Company will amortize the value of the customer relationship intangible asset as a reduction to revenue using a pattern of economic benefit method over a useful life of nine years.

The table below shows estimated fair values of the assets acquired funded by cash and issuance of common stock at the acquisition date (in thousands):



	Cash Purchase Price		Issuance of Common Stock		Total Consideration		% of Total Consideration
Developed technology and goodwill	$	15,000	$	19,219	$	34,219	60	%
Customer relationship	10,000		13,100		23,100		40	%
Total consideration	$	25,000	$	32,319	$	57,319	100	%

The Company allocated $10.0 million of the $25.0 million paid of the cash purchase price to cash flows from operating activities and the remaining $15.0 million to cash used in investing activities in the consolidated statements of cash flows for the year ended December 31, 2018. The allocation was based on the valuation of the customer relationship relative to the overall consideration. In addition, the Company disclosed $19.2 million from issuance of common stock and $15.0 million of cash purchase price paid for the developed technology and goodwill as investing activities in the consolidated statements of cash flows for the year ended December 31, 2018.

During 2018, total acquisition-related costs were approximately $0.8 million, which were included in general and administrative expenses.

The Company determined it is impractical to include such pro forma information given the difficulty in obtaining the historical financial information for the SunPower microinverter business as the business was part of SunPower and did not have discrete financial information prior to the acquisition. Inclusion of such information would require the Company to make estimates and assumptions regarding the acquired business historical financial results that the Company believes may ultimately prove inaccurate.

SELECTED UNAUDITED QUARTERLY FINANCIAL INFORMATION

12 Months Ended

Dec. 31, 2018

Quarterly Financial Information Disclosure [Abstract]

SELECTED UNAUDITED QUARTERLY FINANCIAL INFORMATION

The following tables show a summary of the Company’s quarterly financial information for each of the four quarters of 2018 and 2017 (in thousands, except per share data):



	Year Ended December 31, 2018*
	March 31			June 30			September 30			December 31
Net revenues	$	69,972		$	75,896		$	78,002		$	92,289
Cost of revenues	51,657			53,195			52,738			64,124
Gross profit	18,315			22,701			25,264			28,165
Operating expenses:
Research and development	7,620			9,462			8,165			7,340
Sales and marketing	6,227			6,828			7,375			6,617
General and administrative	6,943			6,969			7,510			7,664
Restructuring charges	—			—			2,588			1,541
Total operating expenses	20,790			23,259			25,638			23,162
Income (loss) from operations	(2,475		)	(558		)	(374		)	5,003
Other expense, net	(2,418		)	(2,841		)	(2,848		)	(3,718		)
Income (loss) before income taxes	(4,893		)	(3,399		)	(3,222		)	1,285
Provision for income taxes	(235		)	(339		)	(248		)	(576		)
Net income (loss)	$	(5,128	)	$	(3,738	)	$	(3,470	)	$	709
Net income (loss) per share, basic	$	(0.06	)	$	(0.04	)	$	(0.03	)	$	0.01
Net income (loss) per share, diluted	$	(0.06	)	$	(0.04	)	$	(0.03	)	$	0.01


*	Amounts include the impact of the adoption of Accounting Standards Codification (“ASC”) No. 606, “Revenue Recognition,” on January 1, 2018. See Note 3, “Revenue Recognition” (“ASC 606” or “Topic 606”) of the notes to consolidated financial statements included in Item 8 of this Form 10-K.



	Year Ended December 31, 2017
	March 31			June 30			September 30			December 31
Net revenues	$	54,751		$	74,704		$	77,038		$	79,674
Cost of revenues	47,703			61,157			60,577			60,685
Gross profit	7,048			13,547			16,461			18,989
Operating expenses:
Research and development	9,605			7,947			7,397			8,208
Sales and marketing	6,458			6,274			5,453			4,940
General and administrative	5,833			4,964			5,441			5,983
Restructuring charges	7,247			3,609			4,071			1,991
Total operating expenses	29,143			22,794			22,362			21,122
Loss from operations	(22,095		)	(9,247		)	(5,901		)	(2,133		)
Other expense, net	(1,079		)	(1,992		)	(1,137		)	(1,755		)
Loss before income taxes	(23,174		)	(11,239		)	(7,038		)	(3,888		)
Income tax benefit (provision)	(131		)	(854		)	184			948
Net loss	$	(23,305	)	$	(12,093	)	$	(6,854	)	$	(2,940	)
Net loss per share, basic and diluted	$	(0.30	)	$	(0.14	)	$	(0.08	)	$	(0.03	)

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Policies)

12 Months Ended

Dec. 31, 2018

Accounting Policies [Abstract]

Basis of Presentation and Consolidation

The accompanying consolidated financial statements are prepared in accordance with accounting principles generally accepted in the United States (“U.S. GAAP”). The consolidated financial statements include the accounts of the Company and its wholly-owned subsidiaries. All intercompany balances and transactions have been eliminated in consolidation.

Use of Estimates

The preparation of the Company’s consolidated financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of income and expenses during the reporting period. These estimates are based on information available as of the date of the financial statements; therefore, actual results could differ materially from management’s estimates using different assumptions or under different conditions.

Revenue Recognition and Cost of Revenues

Contract Liabilities

Revenue Recognition

•

The Company records certain contra revenue promotions as variable consideration and recognizes these promotions at the time the related revenue is recorded.

See Note 3. “Revenue Recognition,” for additional information related to revenue recognition.

Cost of Revenues


•	Products Delivered at a Point in Time. Cost of revenue from these products is recognized when the Company transfers control of the product to the customer, which is generally upon shipment.


•	Products Delivered Over Time. Cost of revenue from these products is recognized over the related service period.

Cash and Cash Equivalents

Fair Value of Financial Instruments

Accounts Receivables and Contract Assets

Allowances for Doubtful Accounts

Allowance for Doubtful Accounts

Inventory

Property and Equipment

Capitalized Software Costs

Impairment of Long-Lived Assets

Business Combinations

Goodwill and Intangible Assets

Goodwill

Intangible Assets

Warranty Obligations

Microinverters Sold Through December 31, 2013

Fair Value Option for Microinverters Sold Since January 1, 2014

Warranty for Other Products

Research and Development Costs

The Company expenses research and development costs as incurred.

Stock-Based Compensation

Comprehensive Income (Loss)

Income Taxes

Recently Adopted Accounting Pronouncements and Recently Issued Accounting Pronouncements Not Yet Effective

Recently Adopted Accounting Pronouncements

Recently Issued Accounting Pronouncements Not Yet Effective

Fair Value Measurement


•	Level 1—Valuations based on quoted prices in active markets for identical assets or liabilities that the Company is able to access. Since valuations are based on quoted prices that are readily and regularly available in an active market, valuation of such assets or liabilities do not entail a significant degree of judgment.


•	Level 2—Valuations based on one or more quoted prices in markets that are not active or for which all significant inputs are observable, either directly or indirectly.


•	Level 3—Valuations based on inputs that are unobservable and significant to the overall fair value measurement.

Earnings Per Share

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Tables)

12 Months Ended

Dec. 31, 2018

Accounting Policies [Abstract]

Schedule of Activity in Allowance for Doubtful Accounts

The following table sets forth activities in the allowance for doubtful accounts for the periods indicated (in thousands):



	December 31,
	2018			2017			2016
Balance, at beginning of year	$	2,378		$	2,921		$	1,808
Net charges to expenses	711			476			3,097
Write-offs, net of recoveries	(951		)	(1,019		)	(1,984		)
Balance, at end of year	$	2,138		$	2,378		$	2,921

REVENUE RECOGNITION (Tables)

12 Months Ended

Dec. 31, 2018

Revenue from Contract with Customer [Abstract]

Summary of Revenue Disaggregation

The following table provides information about disaggregated revenue by primary geographical market and timing of revenue recognition for the Company’s single product line (in thousands):



	December 31, 2018
Primary geographical markets:
United States	$	219,600
International	96,559
Total	$	316,159

Timing of revenue recognition:
Products delivered at a point in time	$	270,778
Products and services delivered over time	45,381
Total	$	316,159

Summary of Contract Assets and Contract Liabilities, and Changes in Balances from Contracts with Customers

Significant changes in the balances of contract liabilities (deferred revenues) and contract assets (prepaid expenses and other assets) during the period are as follows (in thousands):



Contract Liabilities
Balance on January 1, 2018	$	116,830
Revenue recognized	(45,381		)
Increase due to billings	38,581
Balance as of December 31, 2018	$	110,030



Contract Assets
Balance on January 1, 2018	$	47,862
Amount recognized	(14,964		)
Increase	14,766
Balance as of December 31, 2018	$	47,664

As of December 31, 2018, receivables, and contract assets and contract liabilities from contracts with customers consist of the following (in thousands):



	December 31, 2018
Receivables	$	78,938
Short-term contract assets (Prepaid expenses and other assets)	13,516
Long-term contract assets (Other assets)	34,148
Short-term contract liabilities (Deferred revenues)	33,119
Long-term contract liabilities (Deferred revenues)	76,911

Summary of Estimated Revenue Expected to be Recognized in Future Periods



Years	December 31, 2018
2019	$	33,119
2020	26,303
2021	20,124
2022	15,084
2023	9,282
Thereafter	6,118
Total	$	110,030

Summary of New Accounting Adoption Impact on Consolidated Statements of Income and Condensed Consolidated Balance Sheets

In accordance with Topic 606, the disclosure of the impact of adoption to the Company’s consolidated statements of operations and consolidated balance sheets was as follows (in thousands):



	Year Ended December 31, 2018
	As Reported			Adjustments			Without Adoption of Topic 606
Net revenues	$	316,159		$	(2,251	)	$	313,908
Cost of revenues	221,714			(13		)	221,701
Gross profit	94,445			(2,238		)	92,207
Operating expenses:
Research and development	32,587			—			32,587
Sales and marketing	27,047			(117		)	26,930
General and administrative	29,086			—			29,086
Restructuring charges	4,129			—			4,129
Total operating expenses	92,849			(117		)	92,732
Income (loss) from operations	1,596			(2,121		)	(525		)
Other expense, net
Interest expense	(9,635		)	—			(9,635		)
Other expense	(2,190		)	—			(2,190		)
Total other expense, net	(11,825		)	—			(11,825		)
Loss before income taxes	(10,229		)	(2,121		)	(12,350		)
Income tax provision	(1,398		)	—			(1,398		)
Net loss	$	(11,627	)	$	(2,121	)	$	(13,748	)
Net loss per share:
Basic and diluted	$	(0.12	)	$	(0.02	)	$	(0.14	)
Shares used in per share calculation:
Basic and diluted	99,619			—			99,619



	December 31, 2018
	As Reported			Adjustments			Without Adoption of Topic 606
Prepaid expenses and other	$	20,860		$	(10,788	)	$	10,072
Other assets	36,548			(28,766		)	7,782
Accrued liabilities	29,010			(4,257		)	24,753
Deferred revenues, current	33,119			(20,800		)	12,319
Deferred revenues, noncurrent	76,911			(50,044		)	26,867
Accumulated deficit	(346,302		)	35,547			(310,755		)

INVENTORY (Tables)

12 Months Ended

Dec. 31, 2018

Inventory Disclosure [Abstract]

Summary of Inventory

Inventory as of December 31, 2018 and 2017 consists of the following (in thousands):



	December 31,
	2018		2017
Raw materials	$	970	$	2,341
Finished goods	15,297		23,658
Total inventory	$	16,267	$	25,999

PROPERTY AND EQUIPMENT, NET (Tables)

12 Months Ended

Dec. 31, 2018

Property, Plant and Equipment [Abstract]

Summary of Property and Equipment

As of December 31, 2018 and 2017, property and equipment consist of the following (in thousands):



	Estimated Useful Life (Years)	December 31,
	Estimated Useful Life (Years)	2018			2017
Equipment and machinery	3-10	$	43,566		$	41,148
Furniture and fixtures	5-10	2,239			2,188
Computer equipment	3-5	2,958			2,627
Capitalized software costs	3-5	12,114			11,749
Leasehold improvements	3-10	8,482			8,537
Construction in process		3,536			4,672
Total		72,895			70,921
Less accumulated depreciation and amortization		(51,897		)	(44,438		)
Property and equipment, net		$	20,998		$	26,483

GOODWILL AND INTANGIBLE ASSETS (Tables)

12 Months Ended

Dec. 31, 2018

Goodwill and Intangible Assets Disclosure [Abstract]

Schedule of Goodwill and Intangible Assets

The following table presents the details of the Company’s goodwill and purchased intangible assets as of December 31, 2018 and 2017 (in thousands):



	December 31, 2018									December 31, 2017
	Gross		Additions		Accumulated Amortization			Net		Gross		Accumulated Amortization			Net
Goodwill	$	3,664	$	21,119	$	—		$	24,783	$	3,664	$	—		$	3,664

Intangible assets:
Other indefinite-lived intangibles	$	286	$	—	$	—		$	286	$	286	$	—		$	286
Intangible assets with finite live:
Patents and licensed technology	1,665		—		(1,665		)	—		1,665		(1,436		)	229
Developed technology	—		13,100		(909		)	12,191		—		—			—
Customer relationships	—		23,100		(271		)	22,829		—		—			—
Total purchased intangible assets	$	1,951	$	36,200	$	(2,845	)	$	35,306	$	1,951	$	(1,436	)	$	515

ACCRUED LIABILITIES (Tables)

12 Months Ended

Dec. 31, 2018

Payables and Accruals [Abstract]

Schedule of Accrued Liabilities

As of December 31, 2018 and 2017 accrued liabilities consists of the following (in thousands):



	December 31,
	2018		2017
Salaries, commissions, incentive compensation and benefits	$	4,107	$	3,608
Customer rebates and sales incentives	8,527		8,988
Freight	7,286		3,853
Other	9,090		5,998
Total accrued liabilities	$	29,010	$	22,447

WARRANTY OBLIGATIONS (Tables)

12 Months Ended

Dec. 31, 2018

Product Warranties Disclosures [Abstract]

Summary of Warranty Activities

The Company’s warranty activities during the years ended December 31, 2018, 2017 and 2016 were as follows (in thousands):



	Years Ended December 31,
	2018			2017			2016
Warranty obligations, beginning of period	$	29,816		$	31,414		$	30,547
Accruals for warranties issued during period	3,040			3,797			4,130
Changes in estimates	6,515			(732		)	2,562
Settlements	(8,579		)	(7,037		)	(8,523		)
Increase due to accretion expense	1,989			2,053			1,772
Other	(1,487		)	321			926
Warranty obligations, end of period	31,294			29,816			31,414
Less current portion	(8,083		)	(7,427		)	(8,596		)
Noncurrent	$	23,211		$	22,389		$	22,818

FAIR VALUE MEASUREMENTS (Tables)

12 Months Ended

Dec. 31, 2018

Fair Value Disclosures [Abstract]

Schedule of Assets and Liabilities Measured at Fair Value on a Recurring Basis



	Fair Value Hierarchy	Years Ended December 31,
	Fair Value Hierarchy	2018		2017
Warranty obligations	Level 3	$	11,757	$	9,791
Third party option to purchase receivables at a discount	Level 3	—		700

Schedule of Changes in Nonfinancial Liabilities Related to Warrant Obligations Measured at Fair Value on a Recurring Basis Using Significant Unobservable Inputs



	Years Ended December 31,
	2018			2017			2016
Balance at beginning of period	$	9,791		$	10,332		$	6,182
Accruals for warranties issued during period	3,040			3,591			4,091
Changes in estimates	2,455			(4,551		)	(1,616		)
Settlements	(4,030		)	(1,956		)	(1,023		)
Increase due to accretion expense	1,989			2,053			1,772
Other	(1,488		)	322			926
Balance at end of period	$	11,757		$	9,791		$	10,332

Summary of Significant Unobservable Inputs used in the Fair Value Measurement of Liabilities Designated as Level 3

As of December 31, 2018, the significant unobservable inputs used in the fair value measurement of the Company’s liabilities designated as Level 3 are as follows:



Item Measured at Fair Value	Valuation Technique	Description of Significant Unobservable Input	Percent Used (Weighted-Average)
Warranty obligations for microinverters sold since January 1, 2014	Discounted cash flows	Profit element and risk premium	16.25%
	Discounted cash flows	Credit-adjusted risk-free rate	18.75%

As of December 31, 2017, the significant unobservable inputs used in the fair value measurement of the Company’s liabilities designated as Level 3 are as follows:



Item Measured at Fair Value	Valuation Technique	Description of Significant Unobservable Input	Percent Used (Weighted-Average)
Warranty obligations for microinverters sold since January 1, 2014	Discounted cash flows	Profit element and risk premium	17%
	Discounted cash flows	Credit-adjusted risk-free rate	17%
Third party option to purchase receivables at a discount	Discounted cash flows	Counter party credit-adjusted risk-free rate	4%

RESTRUCTURING (Tables)

12 Months Ended

Dec. 31, 2018

Restructuring and Related Activities [Abstract]

Restructuring and Related Costs



	Years Ended December 31,
	2018		2017		2016
Redundancy and employee severance and benefit arrangements	$	2,228	$	2,827	$	1,263
Asset impairments	1,601		522		2,575
Consultants engaged in restructuring activities	—		12,100		—
Lease loss reserves	300		1,468		579
Gain on business divestiture	—		—		(640		)
Total restructuring	$	4,129	$	16,917	$	3,777

The following table presents the details of the Company’s restructuring charges under the 2018 Plan for the periods indicated (in thousands):



	Year Ended December 31, 2018
Redundancy and employee severance and benefit arrangements	$	2,228
Asset impairments	1,636
Lease loss reserves	340
Total restructuring	$	4,204

The following table presents the details of the Company’s restructuring charges under the 2016 Plan for the periods indicated (in thousands):



	Years Ended December 31,
	2018			2017		2016
Employee severance and benefit arrangements	$	—		$	2,827	$	1,263
Asset impairments	(35		)	522		2,575
Consultants engaged in restructuring activities	—			12,100		—
Lease loss reserves and contract termination costs	(40		)	1,468		579
Gain on business divestiture	—			—		(640		)
Total restructuring	$	(75	)	$	16,917	$	3,777



Consideration	$	1,375
Identifiable assets	(979		)
Contingent consideration	244
Gain on business divestiture	$	640

Schedule of Restructuring Reserve by Type of Cost

The following table provides information regarding changes in the Company’s 2018 Plan accrued restructuring balance for the periods indicated (in thousands):



	Redundancy and Employee Severance and Benefits			Lease Loss Reserves and Contractual Obligations			Total
Charges	$	2,228		$	340		$	2,568
Cash payments	(1,336		)	(61		)	(1,397		)
Non-cash settlement and other	12			9			21
Balance as of December 31, 2018	$	904		$	288		$	1,192

The following table provides information regarding changes in the Company’s 2016 Plan accrued restructuring balance for the periods indicated (in thousands):



	Employee Severance and Benefits			Lease Loss Reserves and Contractual Obligations			Total
Balance as of December 31, 2016	$	198		$	484		$	682
Charges	2,827			13,568			16,395
Cash payments	(2,796		)	(12,958		)	(15,754		)
Balance as of December 31, 2017	229			1,094			1,323
Charges and adjustments	—			(40		)	(40		)
Cash payments and receipts, net	(229		)	537			308
Balance as of December 31, 2018	$	—		$	1,591		$	1,591

DEBT (Tables)

12 Months Ended

Dec. 31, 2018

Debt Disclosure [Abstract]

Schedule of Debt

Long-term debt was comprised of the following at December 31, 2018 and 2017 (in thousands):



	December 31,
	2018			2017
Convertible Notes due 2023	$	65,000		$	—
Less unamortized issuance costs	(2,361		)	—
Carrying amount of Convertible Notes due 2023	62,639			—

Term loan	41,524			50,000
Less unamortized discount and issuance costs	(1,059		)	(2,111		)
Carrying amount of term loan	40,465			47,889

Sale of long-term financing receivable recorded as debt	6,679			2,562
Less value of future purchase option	—			(700		)
Carrying amount of sale of long-term financing receivable recorded as debt	6,679			1,862
Total carrying amount of debt	109,783			49,751
Less current portion term loan	(25,417		)	(15,715		)
Less current portion of long-term financing receivable recorded as debt	(2,738		)	(1,714		)
Long-term debt	$	81,628		$	32,322

Schedule of Maturities of Long-term Debt

As of December 31, 2018, the estimated schedule of principal payments due on the Term Loans was as follows (in thousands):



Year Due	Principal Payments
2019	$	25,317
2020	16,207
Total	$	41,524

COMMITMENTS AND CONTINGENCIES (Tables)

12 Months Ended

Dec. 31, 2018

Commitments and Contingencies Disclosure [Abstract]

Schedule of Future Minimum Rental Payments for Operating Leases

The Company’s minimum lease payments under noncancelable operating leases, exclusive of executory costs, as of December 31, 2018 are as follows (in thousands):



	Minimum Lease Payments
Fiscal year due:
2019	$	3,738
2020	3,532
2021	3,276
2022	1,810
2023	945
Thereafter	1,252
Total operating lease minimum payments	14,553
Sublease income to be recognized in the future under noncancelable subleases	(922		)
Net operating leases	$	13,631

STOCK-BASED COMPENSATION (Tables)

12 Months Ended

Dec. 31, 2018

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Summary of the Weighted-Average Grant Date Fair Value of Options Granted



	Years Ended December 31,
	2018			2017			2016
Weighted average grant date fair value	$	2.83		$	0.76		$	1.29
Expected term (in years)	4.0			4.4			4.5
Expected volatility	88.5		%	83.9		%	80.0		%
Annual risk-free rate of return	2.6		%	1.8		%	1.1		%
Dividend yield	—		%	—		%	—		%

Summary of the Components of Total Stock-Based Compensation Expense

The following table summarizes the components of total stock-based compensation expense included in the consolidated statements of operations for the periods presented (in thousands):



	Years Ended December 31,
	2018		2017		2016
Cost of revenues	$	1,071	$	1,072	$	1,188
Research and development	2,940		2,573		3,879
Sales and marketing	3,074		1,157		2,144
General and administrative	4,347		1,925		3,115
Total	$	11,432	$	6,727	$	10,326

Summary of Stock-Based Compensation Associated with Each Type of Award

The following table summarizes the various types of stock-based compensation expense for the periods presented (in thousands):



	Years Ended December 31,
	2018		2017		2016
Stock options, RSUs, and PSUs	$	10,691	$	5,559	$	8,384
Employee stock purchase plan	741		1,168		1,942
Total	$	11,432	$	6,727	$	10,326

Summary of Stock Option Activity

The following is a summary of stock option activity for the three years ended December 31, 2018 (in thousands, except per share data):



	Number of Shares Outstanding		Weighted- Average Exercise Price per Share		Weighted- Average Remaining Contractual Term (in years)	Aggregate Intrinsic Value ⁽¹⁾
Outstanding at December 31, 2015	8,170		$	5.36
Granted	2,440		2.12
Exercised	(375	)	0.39			$	729
Canceled	(1,505	)	6.01
Outstanding at December 31, 2016	8,730		4.55
Granted	4,500		1.22
Exercised	(425	)	0.51			544
Canceled	(4,379	)	6.91
Outstanding at December 31, 2017	8,426		1.77
Granted	213		4.43
Exercised	(1,346	)	1.75			5,096
Canceled	(521	)	2.94
Outstanding at December 31, 2018	6,772		1.76		4.2	$	21,547
Vested and expected to vest at December 31, 2018	6,772		1.76		4.2	$	21,547
Exercisable at December 31, 2018	4,645		1.87		3.5	$	14,670

(1)

Summary of Stock Option Outstanding

The following table summarizes information about stock options outstanding at December 31, 2018:



	Options Outstanding				Options Exercisable
Range of Exercise Prices	Number of Shares (in thousands)	Weighted- Average Remaining Life (in years)	Weighted- Average Exercise Price		Number of Shares (in thousands)	Weighted- Average Exercise Price
$0.27 —– $0.79	1,551	2.9	$	0.49	1,349	$	0.45
$0.79 —– $1.29	1,352	5.9	1.22		433	1.21
$1.31 —– $1.31	1,751	5.3	1.31		1,167	1.31
$1.37 —– $2.61	1,388	3.0	1.87		1,150	1.86
$2.76 —– $12.57	730	3.6	6.32		546	7.15
Total	6,772	4.2	1.76		4,645	1.87

Summary of Restricted Stock Unit Activity

The following is a summary of RSU activity for the three years ended December 31, 2018 (in thousands, except per share data):



	Number of Shares Outstanding		Weighted- Average Fair Value per Share at Grant Date		Weighted- Average Remaining Contractual Term (in years)	Aggregate Intrinsic Value ⁽¹⁾
Outstanding at December 31, 2015	1,313		$	9.31
Granted	54		1.99
Vested	(464	)	9.06			$	875
Canceled	(297	)	8.32
Outstanding at December 31, 2016	606		9.33
Granted	5,418		1.46
Vested	(885	)	3.81			932
Canceled	(1,634	)	1.90
Outstanding at December 31, 2017	3,505		2.03
Granted	3,152		4.45
Vested	(1,399	)	2.75			6,657
Canceled	(906	)	2.17
Outstanding at December 31, 2018	4,352		3.52		1.4	$	20,586
Expected to vest at December 31, 2018	4,351		3.52		1.4	$	20,581

(1)

Share-based Compensation, Performance Shares Award Outstanding Activity

The following is a summary of PSU activity for the year ended December 31, 2018 (in thousands, except per share data):



	Number of Shares Outstanding		Weighted- Average Fair Value per Share at Grant Date		Weighted- Average Remaining Contractual Term (in years)	Aggregate Intrinsic Value ⁽¹⁾
Outstanding at December 31, 2017	—
Granted	1,477		$	4.65
Vested	—
Canceled	(147	)
Outstanding at December 31, 2018	1,330		4.66		0.3	$	6,291
Expected to vest at December 31, 2018	1,330		4.66		0.3	$	6,291

(1)

Schedule of Share-based Compensation, Employee Stock Purchase Plan, Activity

A summary of ESPP activity for the years presented is as follows: (in thousands, except per share data):



	Years Ended December 31,
	2018		2017		2016
Proceeds from common stock issued under ESPP	$	397	$	313	$	999
Shares of common stock issued	439		478		659
Weighted-average price per share	$	0.90	$	0.65	$	1.52

INCOME TAXES (Tables)

12 Months Ended

Dec. 31, 2018

Income Tax Disclosure [Abstract]

Schedule of Domestic and Foreign Components of Loss before Provision for Income Taxes

The domestic and foreign components of loss before income taxes consisted of the following (in thousands):



	Years Ended December 31,
	2018			2017			2016
United States	$	(14,322	)	$	(47,882	)	$	(67,631	)
Foreign	4,093			2,541			1,644
Loss before income taxes	$	(10,229	)	$	(45,341	)	$	(65,987	)

Schedule of Provision for Income Taxes

The provision for (benefit from) income taxes for the years presented is as follows (in thousands):



	Years Ended December 31,
	2018			2017			2016
Current:
Federal	$	—		$	—		$	—
State	42			21			36
Foreign	1,233			1,224			785
	1,275			1,245			821
Deferred:
Federal	(35		)	(1,092		)	594
State	(21		)	(21		)	59
Foreign	179			(281		)	1
	123			(1,394		)	654
Provision for (benefit from) income taxes	$	1,398		$	(149	)	$	1,475

Schedule of Effective Income Tax Rate Reconciliation



	Years Ended December 31,
	2018			2017			2016
Income tax benefit at statutory federal rate	$	(2,148	)	$	(15,416	)	$	(22,435	)
State taxes, net of federal benefit	17			(64		)	63
Change in valuation allowance	8,198			(20,571		)	21,370
Foreign tax rate and tax law differential	313			(133		)	27
Tax credits	(378		)	(382		)	(1,179		)
Stock-based compensation	(953		)	761			1,775
Other permanent items	235			479			776
Other nondeductible/nontaxable items	(5,112		)	930			920
Uncertain tax positions	107			106			158
Tax law changes	—			34,141			—
GILTI	917			—			—
Section 162(m)	202			—			—
Provision for (benefit from) income taxes	$	1,398		$	(149	)	$	1,475

Schedule of Deferred Tax Assets and Liabilities

A summary of significant components of the Company’s deferred tax assets and liabilities as of December 31, 2018 and 2017 is as follows (in thousands):



	December 31,
	2018			2017
Deferred tax assets:
Allowances and reserves	$	10,022		$	9,748
Net operating loss and tax credit carryforwards	71,568			66,243
Stock-based compensation	3,662			2,528
Deferred revenue	19,562			7,210
Fixed assets and intangibles	3,836			4,369
Sec. 163(j) interest carryforward	2,064			—
Other	2,084			278
Subtotal	112,798			90,376
Less valuation allowance	(98,631		)	(88,789		)
Total deferred tax assets, net of valuation allowance	14,167			1,587
Deferred tax liabilities:
Goodwill	(1,070		)	(992		)
Unremitted foreign earnings	(16		)	(10		)
Deferred cost of goods sold	(12,655		)	—
Total deferred tax liabilities	(13,741		)	(1,002		)
Net deferred tax asset	$	426		$	585

Schedule of Reconciliation of Total Amounts of Unrecognized Tax Benefits

A tabular reconciliation of the total amounts of unrecognized tax benefits for the years presented is as follows (in thousands):



	Years Ended December 31,
	2018			2017			2016
Unrecognized tax benefits—at beginning of year	$	6,106		$	6,016		$	5,482
Decreases in balances related to tax positions taken in prior years	—			(135		)	—
Increases in balances related to tax positions taken in current year	329			306			571
Lapses in statutes of limitations	(110		)	(81		)	(37		)
Unrecognized tax benefits—at end of year	$	6,325		$	6,106		$	6,016

NET LOSS PER SHARE (Tables)

12 Months Ended

Dec. 31, 2018

Earnings Per Share [Abstract]

Schedule of Computation of Basic and Diluted Net Loss Per Share

The following table presents the computation of basic and diluted net loss per share for the periods presented (in thousands, except per share data):



	Years Ended December 31,
	2018			2017			2016
Numerator:
Net loss	$	(11,627	)	$	(45,192	)	$	(67,462	)

Denominator:
Weighted average common shares outstanding	99,619			82,939			50,519

Net loss per share, basic and diluted	$	(0.12	)	$	(0.54	)	$	(1.34	)

Schedule of Potentially Dilutive Securities Excluded from the Computation of Diluted Net Loss Per Share



	Years Ended December 31,
	2018	2017	2016
Employee stock options	7,710	8,433	8,981
RSUs and PSUs	5,273	3,029	906
Warrants to purchase common stock	—	1,083	—
Convertible notes	11,701	—	—
Total	24,684	12,545	9,887

SEGMENT AND GEOGRAPHIC INFORMATION (Tables)

12 Months Ended

Dec. 31, 2018

Segment Reporting [Abstract]

Summary of Net Revenues and Long-Lived Assets by Geographic Region

The following tables present net revenues (based on the destination of shipments) and long-lived assets by geographic region as of and for the periods presented (in thousands):

Net Revenues



	Years Ended December 31,
	2018		2017		2016
United States	$	219,600	$	199,565	$	259,080
International	96,559		86,601		63,511
Total	$	316,159	$	286,166	$	322,591

Long-Lived Assets



	December 31,
	2018		2017
United States	$	13,146	$	16,899
China	5,504		7,164
Other	2,348		2,420
Total	$	20,998	$	26,483

ACQUISITION (Tables)

12 Months Ended

Dec. 31, 2018

Business Combinations [Abstract]

Summary of consideration transfered for business acquisition

The table below shows estimated fair values of the assets acquired funded by cash and issuance of common stock at the acquisition date (in thousands):



	Cash Purchase Price		Issuance of Common Stock		Total Consideration		% of Total Consideration
Developed technology and goodwill	$	15,000	$	19,219	$	34,219	60	%
Customer relationship	10,000		13,100		23,100		40	%
Total consideration	$	25,000	$	32,319	$	57,319	100	%

The acquisition date fair value of the consideration transferred was approximately $57.3 million, which consisted of the following (in thousands):



Cash consideration	$	25,000
Common stock issued	32,319
Total	$	57,319

Summary of fair values of assets acquired and liabilities assumed

The following table summarizes the preliminary estimated fair values of the assets acquired and liabilities assumed at the acquisition date (in thousands):



Intangible assets	$	36,200
Goodwill	21,119
Net assets acquired	$	57,319

Summary of identifiable intangible assets acquired

The table below shows estimated fair values of the assets acquired funded by cash and issuance of common stock at the acquisition date (in thousands):



	Cash Purchase Price		Issuance of Common Stock		Total Consideration		% of Total Consideration
Developed technology and goodwill	$	15,000	$	19,219	$	34,219	60	%
Customer relationship	10,000		13,100		23,100		40	%
Total consideration	$	25,000	$	32,319	$	57,319	100	%

The following table shows the fair value of the separately identifiable intangible assets at the time of acquisition and the period over which each intangible asset will be amortized:

SELECTED UNAUDITED QUARTERLY FINANCIAL INFORMATION (Tables)

12 Months Ended

Dec. 31, 2018

Quarterly Financial Information Disclosure [Abstract]

Schedule of Quarterly Financial Information

The following tables show a summary of the Company’s quarterly financial information for each of the four quarters of 2018 and 2017 (in thousands, except per share data):



	Year Ended December 31, 2018*
	March 31			June 30			September 30			December 31
Net revenues	$	69,972		$	75,896		$	78,002		$	92,289
Cost of revenues	51,657			53,195			52,738			64,124
Gross profit	18,315			22,701			25,264			28,165
Operating expenses:
Research and development	7,620			9,462			8,165			7,340
Sales and marketing	6,227			6,828			7,375			6,617
General and administrative	6,943			6,969			7,510			7,664
Restructuring charges	—			—			2,588			1,541
Total operating expenses	20,790			23,259			25,638			23,162
Income (loss) from operations	(2,475		)	(558		)	(374		)	5,003
Other expense, net	(2,418		)	(2,841		)	(2,848		)	(3,718		)
Income (loss) before income taxes	(4,893		)	(3,399		)	(3,222		)	1,285
Provision for income taxes	(235		)	(339		)	(248		)	(576		)
Net income (loss)	$	(5,128	)	$	(3,738	)	$	(3,470	)	$	709
Net income (loss) per share, basic	$	(0.06	)	$	(0.04	)	$	(0.03	)	$	0.01
Net income (loss) per share, diluted	$	(0.06	)	$	(0.04	)	$	(0.03	)	$	0.01


*	Amounts include the impact of the adoption of Accounting Standards Codification (“ASC”) No. 606, “Revenue Recognition,” on January 1, 2018. See Note 3, “Revenue Recognition” (“ASC 606” or “Topic 606”) of the notes to consolidated financial statements included in Item 8 of this Form 10-K.



	Year Ended December 31, 2017
	March 31			June 30			September 30			December 31
Net revenues	$	54,751		$	74,704		$	77,038		$	79,674
Cost of revenues	47,703			61,157			60,577			60,685
Gross profit	7,048			13,547			16,461			18,989
Operating expenses:
Research and development	9,605			7,947			7,397			8,208
Sales and marketing	6,458			6,274			5,453			4,940
General and administrative	5,833			4,964			5,441			5,983
Restructuring charges	7,247			3,609			4,071			1,991
Total operating expenses	29,143			22,794			22,362			21,122
Loss from operations	(22,095		)	(9,247		)	(5,901		)	(2,133		)
Other expense, net	(1,079		)	(1,992		)	(1,137		)	(1,755		)
Loss before income taxes	(23,174		)	(11,239		)	(7,038		)	(3,888		)
Income tax benefit (provision)	(131		)	(854		)	184			948
Net loss	$	(23,305	)	$	(12,093	)	$	(6,854	)	$	(2,940	)
Net loss per share, basic and diluted	$	(0.30	)	$	(0.14	)	$	(0.08	)	$	(0.03	)

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES Narrative (Details) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Option, Quantitative Disclosures [Line Items]
Restructuring charges	$ 1,541,000	$ 2,588,000	$ 0	$ 0	$ 1,991,000	$ 4,071,000	$ 3,609,000	$ 7,247,000	$ 4,129,000	$ 16,917,000	$ 3,777,000
Asset impairment									1,601,000	1,681,000	3,190,000
Goodwill, impairment loss									0	0	0
Impairment of intangible assets									$ 0	0	0
First and Second Generation
Fair Value, Option, Quantitative Disclosures [Line Items]
Product warranty, term									15 years
Third and Fourth Generation
Fair Value, Option, Quantitative Disclosures [Line Items]
Product warranty, term									25 years
Envoy Communications Gateway
Fair Value, Option, Quantitative Disclosures [Line Items]
Product warranty, term									5 years
AC Battery Storage Solution
Fair Value, Option, Quantitative Disclosures [Line Items]
Product warranty, term									10 years
Minimum
Fair Value, Option, Quantitative Disclosures [Line Items]
Property, plant and equipment, useful life									3 years
Intangible assets, estimated useful life									3 years
Period failure rate measurement lags product sale									3 months
Maximum
Fair Value, Option, Quantitative Disclosures [Line Items]
Property, plant and equipment, useful life									10 years
Intangible assets, estimated useful life									9 years
Product warranty, term									25 years
Period failure rate measurement lags product sale									9 months
Asset Impairments
Fair Value, Option, Quantitative Disclosures [Line Items]
Restructuring charges									$ 1,601,000	522,000	$ 2,575,000
Asset impairment										$ 800,000

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES Schedule of Activity in Allowance for Doubtful Accounts (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Allowance for Doubtful Accounts Receivable [Roll Forward]
Balance, at beginning of year	$ 2,378	$ 2,921	$ 1,808
Net charges to expenses	711	476	3,097
Write-offs, net of recoveries	(951)	(1,019)	(1,984)
Balance, at end of year	$ 2,138	$ 2,378	$ 2,921

REVENUE RECOGNITION - Narrative (Details)	3 Months Ended								12 Months Ended
REVENUE RECOGNITION - Narrative (Details)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($) accounting_unit	Dec. 31, 2016 USD ($)	Jan. 01, 2018 USD ($)
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Accumulated deficit reduction (increase)	$ (346,302,000)				$ (295,727,000)				$ (346,302,000)	$ (295,727,000)		$ 43,400,000
Net revenues	92,289,000	$ 78,002,000	$ 75,896,000	$ 69,972,000	79,674,000	$ 77,038,000	$ 74,704,000	$ 54,751,000	316,159,000	$ 286,166,000	$ 322,591,000
Number of accounting units (accounting unit) \| accounting_unit										2
Accrued liabilities	$ 29,010,000				$ 22,447,000				29,010,000	$ 22,447,000		5,600,000
Increase in deferred costs												800,000
Contract asset impairment charges									0
Products and services delivered over time
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net revenues									45,381,000
Products delivered at a point in time
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net revenues									$ 270,778,000
Envoy Communications Device and Enlighten Service
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Unbilled revenue												$ 77,500,000
Monitoring Hardware and Service \| Incurred Commissions
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Amortization period for capitalized costs												6 years 6 months
Envoy Communications Gateway \| Products and services delivered over time
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue recognition period	6 years 6 months								6 years 6 months
Communication Accessories \| Minimum \| Products delivered at a point in time
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue recognition period	5 years								5 years
Communication Accessories \| Maximum \| Products delivered at a point in time
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue recognition period	12 years								12 years
Adjustments \| Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Accumulated deficit reduction (increase)	$ 35,547,000								$ 35,547,000			$ (38,900,000)
Net revenues									(2,251,000)
Accrued liabilities	$ (4,257,000)								$ (4,257,000)
Adjustments \| Accounting Standards Update 2014-09 \| Envoy Communications Device and Enlighten Service
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Accumulated deficit reduction (increase)												(34,100,000)
Unbilled revenue												6,400,000
Adjustments \| Accounting Standards Update 2014-09 \| Monitoring Hardware and Service \| Incurred Commissions
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Accumulated deficit reduction (increase)												$ 800,000

REVENUE RECOGNITION - Summary of Disaggregated Revenue by Primary Geographical Market and Timing of Revenue Recognition (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net revenues	$ 92,289	$ 78,002	$ 75,896	$ 69,972	$ 79,674	$ 77,038	$ 74,704	$ 54,751	$ 316,159	$ 286,166	$ 322,591
Products delivered at a point in time
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net revenues									270,778
Products and services delivered over time
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net revenues									45,381
United States
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net revenues									219,600	199,565	259,080
International
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net revenues									$ 96,559	$ 86,601	$ 63,511

REVENUE RECOGNITION - Summary of Contract Assets and Contract Liabilities from Contracts with Customers (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Revenue from Contract with Customer [Abstract]
Receivables	$ 78,938	$ 65,346
Short-term contract assets (Prepaid expenses and other assets)	13,516
Long-term contract assets (Other assets)	34,148
Short-term contract liabilities (Deferred revenues)	33,119	15,691
Long-term contract liabilities (Deferred revenues)	$ 76,911	$ 29,941

REVENUE RECOGNITION - Summary of Significant Changes in the Balances of Contract Liabilities and Assets (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($)
Contract Liabilities
Balance, beginning of period	$ 116,830
Revenue recognized	(45,381)
Increase due to billings	38,581
Balance, end of period	110,030
Contract Assets
Balance, beginning of period	47,862
Amount recognized	(14,964)
Increase	14,766
Balance, end of period	$ 47,664

REVENUE RECOGNITION - Summary of Estimated Revenue Expected to be Recognized in Future Periods (Details) $ in Thousands	Dec. 31, 2018 USD ($)
Revenue from Contract with Customer [Abstract]
Total estimated revenue expected to be recognized in future periods	$ 110,030
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-01-01
Revenue from Contract with Customer [Abstract]
Total estimated revenue expected to be recognized in future periods	$ 33,119
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total estimated revenue expected to be recognized in future periods, expected timing	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2020-01-01
Revenue from Contract with Customer [Abstract]
Total estimated revenue expected to be recognized in future periods	$ 26,303
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total estimated revenue expected to be recognized in future periods, expected timing	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-01-01
Revenue from Contract with Customer [Abstract]
Total estimated revenue expected to be recognized in future periods	$ 20,124
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total estimated revenue expected to be recognized in future periods, expected timing	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2022-01-01
Revenue from Contract with Customer [Abstract]
Total estimated revenue expected to be recognized in future periods	$ 15,084
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total estimated revenue expected to be recognized in future periods, expected timing	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2023-01-01
Revenue from Contract with Customer [Abstract]
Total estimated revenue expected to be recognized in future periods	$ 9,282
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total estimated revenue expected to be recognized in future periods, expected timing	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2024-01-01
Revenue from Contract with Customer [Abstract]
Total estimated revenue expected to be recognized in future periods	$ 6,118
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total estimated revenue expected to be recognized in future periods, expected timing

REVENUE RECOGNITION - Summary of New Accounting Adoption Impact on Consolidated Statements of Income (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net revenues	$ 92,289	$ 78,002	$ 75,896	$ 69,972	$ 79,674	$ 77,038	$ 74,704	$ 54,751	$ 316,159	$ 286,166	$ 322,591
Cost of revenues	64,124	52,738	53,195	51,657	60,685	60,577	61,157	47,703	221,714	230,123	264,583
Gross profit	28,165	25,264	22,701	18,315	18,989	16,461	13,547	7,048	94,445	56,043	58,008
Operating expenses:
Research and development	7,340	8,165	9,462	7,620	8,208	7,397	7,947	9,605	32,587	33,157	50,703
Sales and marketing	6,617	7,375	6,828	6,227	4,940	5,453	6,274	6,458	27,047	23,126	38,810
General and administrative	7,664	7,510	6,969	6,943	5,983	5,441	4,964	5,833	29,086	22,221	27,418
Restructuring charges	1,541	2,588	0	0	1,991	4,071	3,609	7,247	4,129	16,917	3,777
Total operating expenses	23,162	25,638	23,259	20,790	21,122	22,362	22,794	29,143	92,849	95,421	120,708
Income (loss) from operations	5,003	(374)	(558)	(2,475)	(2,133)	(5,901)	(9,247)	(22,095)	1,596	(39,378)	(62,700)
Other expense, net
Interest expense									(9,635)	(7,936)	(2,773)
Other expense									(2,190)	1,973	(514)
Total other expense, net	(3,718)	(2,848)	(2,841)	(2,418)	(1,755)	(1,137)	(1,992)	(1,079)	(11,825)	(5,963)	(3,287)
Loss before income taxes	1,285	(3,222)	(3,399)	(4,893)	(3,888)	(7,038)	(11,239)	(23,174)	(10,229)	(45,341)	(65,987)
Income tax provision	(576)	(248)	(339)	(235)	948	184	(854)	(131)	(1,398)	149	(1,475)
Net loss	$ 709	$ (3,470)	$ (3,738)	$ (5,128)	$ (2,940)	$ (6,854)	$ (12,093)	$ (23,305)	$ (11,627)	$ (45,192)	$ (67,462)
Net loss per share:
Basic and diluted (in usd per share)					$ (0.03)	$ (0.08)	$ (0.14)	$ (0.30)	$ (0.12)	$ (0.54)	$ (1.34)
Shares used in per share calculation:
Basic and diluted (in shares)									99,619	82,939	50,519
Adjustments \| Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net revenues									$ (2,251)
Cost of revenues									(13)
Gross profit									(2,238)
Operating expenses:
Research and development									0
Sales and marketing									(117)
General and administrative									0
Restructuring charges									0
Total operating expenses									(117)
Income (loss) from operations									(2,121)
Other expense, net
Interest expense									0
Other expense									0
Total other expense, net									0
Loss before income taxes									(2,121)
Income tax provision									0
Net loss									$ (2,121)
Net loss per share:
Basic and diluted (in usd per share)									$ (0.02)
Shares used in per share calculation:
Basic and diluted (in shares)									0
Without Adoption of Topic 606
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net revenues									$ 313,908
Cost of revenues									221,701
Gross profit									92,207
Operating expenses:
Research and development									32,587
Sales and marketing									26,930
General and administrative									29,086
Restructuring charges									4,129
Total operating expenses									92,732
Income (loss) from operations									(525)
Other expense, net
Interest expense									(9,635)
Other expense									(2,190)
Total other expense, net									(11,825)
Loss before income taxes									(12,350)
Income tax provision									(1,398)
Net loss									$ (13,748)
Net loss per share:
Basic and diluted (in usd per share)									$ (0.14)
Shares used in per share calculation:
Basic and diluted (in shares)									99,619

REVENUE RECOGNITION - Summary of New Accounting Adoption Impact on Condensed Consolidated Balance Sheets (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Prepaid expenses and other assets	$ 20,860		$ 9,957
Other assets	36,548		8,039
Accrued liabilities	29,010	$ 5,600	22,447
Deferred revenues, current	33,119		15,691
Deferred revenues, noncurrent	76,911		29,941
Accumulated deficit	(346,302)	43,400	$ (295,727)
Adjustments \| Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Prepaid expenses and other assets	(10,788)
Other assets	(28,766)
Accrued liabilities	(4,257)
Deferred revenues, current	(20,800)
Deferred revenues, noncurrent	(50,044)
Accumulated deficit	35,547	$ (38,900)
Without Adoption of Topic 606
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Prepaid expenses and other assets	10,072
Other assets	7,782
Accrued liabilities	24,753
Deferred revenues, current	12,319
Deferred revenues, noncurrent	26,867
Accumulated deficit	$ (310,755)

INVENTORY - Summary of Inventory (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Inventory Disclosure [Abstract]
Raw materials	$ 970	$ 2,341
Finished goods	15,297	23,658
Total inventory	$ 16,267	$ 25,999

PROPERTY AND EQUIPMENT, NET Summary of Property and Equipment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 72,895	$ 70,921
Less accumulated depreciation and amortization	(51,897)	(44,438)
Property and equipment, net	$ 20,998	26,483
Minimum
Property, Plant and Equipment [Line Items]
Estimated Useful Life (Years)	3 years
Maximum
Property, Plant and Equipment [Line Items]
Estimated Useful Life (Years)	10 years
Equipment and machinery
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 43,566	41,148
Equipment and machinery \| Minimum
Property, Plant and Equipment [Line Items]
Estimated Useful Life (Years)	3 years
Equipment and machinery \| Maximum
Property, Plant and Equipment [Line Items]
Estimated Useful Life (Years)	10 years
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 2,239	2,188
Furniture and fixtures \| Minimum
Property, Plant and Equipment [Line Items]
Estimated Useful Life (Years)	5 years
Furniture and fixtures \| Maximum
Property, Plant and Equipment [Line Items]
Estimated Useful Life (Years)	10 years
Computer equipment
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 2,958	2,627
Computer equipment \| Minimum
Property, Plant and Equipment [Line Items]
Estimated Useful Life (Years)	3 years
Computer equipment \| Maximum
Property, Plant and Equipment [Line Items]
Estimated Useful Life (Years)	5 years
Capitalized software costs
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 12,114	11,749
Capitalized software costs \| Minimum
Property, Plant and Equipment [Line Items]
Estimated Useful Life (Years)	3 years
Capitalized software costs \| Maximum
Property, Plant and Equipment [Line Items]
Estimated Useful Life (Years)	5 years
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 8,482	8,537
Leasehold improvements \| Minimum
Property, Plant and Equipment [Line Items]
Estimated Useful Life (Years)	3 years
Leasehold improvements \| Maximum
Property, Plant and Equipment [Line Items]
Estimated Useful Life (Years)	10 years
Construction in process
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 3,536	$ 4,672

PROPERTY AND EQUIPMENT, NET Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Abstract]
Depreciation expense	$ 8.3	$ 8.6	$ 10.0
Unamortized capitalized software costs	$ 0.7	$ 1.1

GOODWILL AND INTANGIBLE ASSETS - Schedule of Goodwill and Intangible Assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Finite-Lived Intangible Assets [Line Items]
Goodwill, Gross	$ 3,664	$ 3,664
Goodwill, Additions	21,119
Goodwill, Net	24,783	3,664
Intangible Assets, Net (Excluding Goodwill) [Abstract]
Other indefinite-lived intangibles, Gross	286	286
Other indefinite-lived intangibles, Acquired	0
Intangible assets with finite live:
Gross	1,951	1,951
Additions	36,200
Accumulated Amortization	(2,845)	(1,436)
Net	35,306	515
Patents and licensed technology
Intangible assets with finite live:
Gross	1,665	1,665
Additions	0
Accumulated Amortization	(1,665)	(1,436)
Net	0	229
Developed technology
Intangible assets with finite live:
Gross	0	0
Additions	13,100
Accumulated Amortization	(909)	0
Net	12,191	0
Customer relationship
Intangible assets with finite live:
Gross	0	0
Additions	23,100
Accumulated Amortization	(271)	0
Net	$ 22,829	$ 0

GOODWILL AND INTANGIBLE ASSETS - Narrative (Details) - USD ($) $ in Millions	1 Months Ended		12 Months Ended
	Oct. 31, 2015	Jul. 31, 2014	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Finite-Lived Intangible Assets [Line Items]
Amortization expense related to intangible assets			$ 1.4	$ 0.4	$ 0.7
Patents
Finite-Lived Intangible Assets [Line Items]
Useful life		3 years
ASIC development
Finite-Lived Intangible Assets [Line Items]
Useful life	3 years
Net Revenue
Finite-Lived Intangible Assets [Line Items]
Amortization expense related to intangible assets			$ 0.3

ACCRUED LIABILITIES - Schedule of Accrued Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
Accrued Liabilities [Abstract]
Salaries, commissions, incentive compensation and benefits	$ 4,107		$ 3,608
Customer rebates and sales incentives	8,527		8,988
Freight	7,286		3,853
Other	9,090		5,998
Total accrued liabilities	$ 29,010	$ 5,600	$ 22,447

WARRANTY OBLIGATIONS - Summary of Warranty Activities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Changes in the Company's product warranty liability
Warranty obligations, beginning of period	$ 29,816	$ 31,414	$ 30,547
Accruals for warranties issued during period	3,040	3,797	4,130
Changes in estimates	6,515	(732)	2,562
Settlements	(8,579)	(7,037)	(8,523)
Increase due to accretion expense	1,989	2,053	1,772
Other	(1,487)	321	926
Warranty obligations, end of period	31,294	29,816	31,414
Less current portion	(8,083)	(7,427)	(8,596)
Noncurrent	$ 23,211	$ 22,389	$ 22,818

WARRANTY OBLIGATIONS - Narrative (Details) microinverter in Millions, $ in Millions	12 Months Ended			24 Months Ended	36 Months Ended	54 Months Ended	66 Months Ended
	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2018 microinverter	Jun. 30, 2015 microinverter	Jun. 30, 2012 microinverter	Dec. 31, 2018 microinverter
Sixth Generation \| Replacement Cost
Product Liability Contingency [Line Items]
Warranty expense due to changes in variables		$ (2.2)	$ (2.1)
Sixth Generation \| Claim Rates
Product Liability Contingency [Line Items]
Warranty expense due to changes in variables	$ 2.1
Sixth Generation \| Discount rate
Product Liability Contingency [Line Items]
Warranty expense due to changes in variables	(1.5)
Second Generation Products
Product Liability Contingency [Line Items]
Number of Units Sold \| microinverter						1.0
Second Generation Products \| Replacement Cost
Product Liability Contingency [Line Items]
Warranty expense due to changes in variables	0.9
Second Generation Products \| Failure Rate
Product Liability Contingency [Line Items]
Warranty expense due to changes in variables	3.3	3.9	3.0
Second Generation Products \| Labor Reimbursement Cost
Product Liability Contingency [Line Items]
Warranty expense due to changes in variables	$ 0.2	$ (1.9)	$ 1.5
Third Generation Products
Product Liability Contingency [Line Items]
Number of Units Sold \| microinverter					3.9
Fourth Generation
Product Liability Contingency [Line Items]
Number of Units Sold \| microinverter							10.7
IQ Series
Product Liability Contingency [Line Items]
Number of Units Sold \| microinverter				3.5

FAIR VALUE MEASUREMENTS - Schedule of Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - Recurring - Level 3 - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Warranty obligations
Liabilities:
Fair value liabilities	$ 11,757	$ 9,791	$ 10,332	$ 6,182
Third party option to purchase receivables at a discount
Liabilities:
Fair value liabilities	$ 0	$ 700

FAIR VALUE MEASUREMENTS - Schedule of Changes in Nonfinancial Liabilities Related to Warrant Obligations Measured at Fair Value on a Recurring Basis Using Significant Unobservable Inputs (Details) - Level 3 - Recurring - Warranty obligations - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period	$ 9,791	$ 10,332	$ 6,182
Accruals for warranties issued during period	3,040	3,591	4,091
Changes in estimates	2,455	(4,551)	(1,616)
Settlements	(4,030)	(1,956)	(1,023)
Increase due to accretion expense	1,989	2,053	1,772
Other	(1,488)	322	926
Balance at end of period	$ 11,757	$ 9,791	$ 10,332

FAIR VALUE MEASUREMENTS - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Increase in liability as a result of decreasing the profit element and risk premium input by 100 basis points	$ 100
Decrease in liability as a result of increasing the profit element and risk premium input by 100 basis points	100
Increase to fair value measurement as a result of 100 basis point decrease	400
Decrease to fair value measurement as a result of 100 basis point increase	500
Third party option to purchase receivables at a discount \| Recurring \| Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value liabilities	$ 0	$ 700

FAIR VALUE MEASUREMENTS - Summary of Significant Unobservable Inputs used in the Fair Value Measurement of Liabilities Designated as Level 3 (Details) - Recurring - Level 3	Dec. 31, 2018	Dec. 31, 2017
Measurement Input, Profit Element and Risk Premium \| Warranty obligations for microinverters sold since January 1, 2014
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warranty obligations for microinverters sold since January 1, 2014	0.1625	0.17
Measurement Input, Entity Credit Risk \| Warranty obligations for microinverters sold since January 1, 2014
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warranty obligations for microinverters sold since January 1, 2014	0.1875	0.17
Measurement Input, Counterparty Credit Risk \| Third party option to purchase receivables at a discount
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Third party option to purchase receivables at a discount		0.04

RESTRUCTURING - Summary of Restructuring Charges (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Restructuring Cost and Reserve [Line Items]
Restructuring charges	$ 1,541	$ 2,588	$ 0	$ 0	$ 1,991	$ 4,071	$ 3,609	$ 7,247	$ 4,129	$ 16,917	$ 3,777
Redundancy and employee severance and benefit arrangements
Restructuring Cost and Reserve [Line Items]
Restructuring charges									2,228	2,827	1,263
Asset impairments
Restructuring Cost and Reserve [Line Items]
Restructuring charges									1,601	522	2,575
Consultants engaged in restructuring activities
Restructuring Cost and Reserve [Line Items]
Restructuring charges									0	12,100	0
Lease loss reserves and contract termination costs
Restructuring Cost and Reserve [Line Items]
Restructuring charges									300	1,468	579
Gain on business divestiture
Restructuring Cost and Reserve [Line Items]
Restructuring charges									0	0	(640)
Restructuring Plan 2018
Restructuring Cost and Reserve [Line Items]
Restructuring charges									4,204
Restructuring Plan 2018 \| Redundancy and employee severance and benefit arrangements
Restructuring Cost and Reserve [Line Items]
Restructuring charges									2,228
Restructuring Plan 2018 \| Asset impairments
Restructuring Cost and Reserve [Line Items]
Restructuring charges									1,636
Restructuring Plan 2018 \| Lease loss reserves and contract termination costs
Restructuring Cost and Reserve [Line Items]
Restructuring charges									340
Restructuring Plan 2016
Restructuring Cost and Reserve [Line Items]
Restructuring charges									(75)	16,917	3,777
Restructuring Plan 2016 \| Redundancy and employee severance and benefit arrangements
Restructuring Cost and Reserve [Line Items]
Restructuring charges									0	2,827	1,263
Restructuring Plan 2016 \| Asset impairments
Restructuring Cost and Reserve [Line Items]
Restructuring charges									(35)	522	2,575
Restructuring Plan 2016 \| Consultants engaged in restructuring activities
Restructuring Cost and Reserve [Line Items]
Restructuring charges									0	12,100	0
Restructuring Plan 2016 \| Lease loss reserves and contract termination costs
Restructuring Cost and Reserve [Line Items]
Restructuring charges									(40)	1,468	579
Restructuring Plan 2016 \| Gain on business divestiture
Restructuring Cost and Reserve [Line Items]
Restructuring charges									$ 0	$ 0	$ (640)

RESTRUCTURING - Gain on Business Divestiture (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Restructuring and Related Activities [Abstract]
Consideration			$ 1,375
Identifiable assets			(979)
Contingent consideration			244
Gain on business divestiture	$ 0	$ 0	$ 640

RESTRUCTURING - Rollforward (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Restructuring Cost and Reserve [Line Items]
Restructuring charges	$ 1,541	$ 2,588	$ 0	$ 0	$ 1,991	$ 4,071	$ 3,609	$ 7,247	$ 4,129	$ 16,917	$ 3,777
Lease Loss Reserves And Contract Termination Costs
Restructuring Cost and Reserve [Line Items]
Restructuring charges									300	1,468	579
Employee Severance and Benefits
Restructuring Cost and Reserve [Line Items]
Restructuring charges									2,228	2,827	1,263
Gain on business divestiture
Restructuring Cost and Reserve [Line Items]
Restructuring charges									0	0	(640)
Restructuring Plan 2018
Restructuring Cost and Reserve [Line Items]
Restructuring charges									4,204
Restructuring Reserve [Roll Forward]
Charges									2,568
Cash payments									(1,397)
Non-cash settlement and other									21
Restructuring reserve, ending	1,192								1,192
Restructuring Plan 2018 \| Lease Loss Reserves And Contract Termination Costs
Restructuring Cost and Reserve [Line Items]
Restructuring charges									340
Restructuring Plan 2018 \| Employee Severance and Benefits
Restructuring Cost and Reserve [Line Items]
Restructuring charges									2,228
Restructuring Reserve [Roll Forward]
Charges									2,228
Cash payments									(1,336)
Non-cash settlement and other									12
Restructuring reserve, ending	904								904
Restructuring Plan 2018 \| Lease Loss Reserves and Contractual Obligations
Restructuring Reserve [Roll Forward]
Charges									340
Cash payments									(61)
Non-cash settlement and other									9
Restructuring reserve, ending	288								288
Restructuring Plan 2016
Restructuring Cost and Reserve [Line Items]
Restructuring charges									(75)	16,917	3,777
Restructuring Reserve [Roll Forward]
Restructuring reserve, beginning				1,323				682	1,323	682
Charges									(40)	16,395
Cash payments										(15,754)
Cash payments and receipts, net									308
Restructuring reserve, ending	1,591				1,323				1,591	1,323	682
Restructuring Plan 2016 \| Lease Loss Reserves And Contract Termination Costs
Restructuring Cost and Reserve [Line Items]
Restructuring charges									(40)	1,468	579
Restructuring Plan 2016 \| Employee Severance and Benefits
Restructuring Cost and Reserve [Line Items]
Restructuring charges									0	2,827	1,263
Restructuring Reserve [Roll Forward]
Restructuring reserve, beginning				229				198	229	198
Charges									0	2,827
Cash payments										(2,796)
Cash payments and receipts, net									(229)
Restructuring reserve, ending	0				229				0	229	198
Restructuring Plan 2016 \| Lease Loss Reserves and Contractual Obligations
Restructuring Reserve [Roll Forward]
Restructuring reserve, beginning				$ 1,094				$ 484	1,094	484
Charges									(40)	13,568
Cash payments										(12,958)
Cash payments and receipts, net									537
Restructuring reserve, ending	$ 1,591				$ 1,094				1,591	1,094	484
Restructuring Plan 2016 \| Gain on business divestiture
Restructuring Cost and Reserve [Line Items]
Restructuring charges									$ 0	$ 0	$ (640)

DEBT - Long term debt (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Total carrying amount of debt	$ 109,783	$ 49,751
Debt, current	(28,155)	(17,429)
Debt, noncurrent	81,628	32,322
Convertible Notes Payable \| Convertible Senior Notes Due 2023
Debt Instrument [Line Items]
Carrying amount of long term debt	65,000	0
Less unamortized discount and issuance costs	(2,361)	0
Total carrying amount of debt	62,639	0
Secured Debt \| Term Loan Agreement, July 2016
Debt Instrument [Line Items]
Carrying amount of long term debt	41,524	50,000
Less unamortized discount and issuance costs	(1,059)	(2,111)
Total carrying amount of debt	40,465	47,889
Debt, current	(25,417)	(15,715)
Financing Receivable \| Financing Receivable Recorded as Debt
Debt Instrument [Line Items]
Carrying amount of long term debt	6,679	2,562
Less value of future purchase option	0	(700)
Total carrying amount of debt	6,679	1,862
Debt, current	$ (2,738)	$ (1,714)

DEBT - Narrative (Details)			1 Months Ended					12 Months Ended
DEBT - Narrative (Details)	Jan. 28, 2019 USD ($)	Feb. 28, 2018 USD ($) $ / shares shares	Aug. 31, 2018 USD ($) $ / shares	Feb. 28, 2018 USD ($) $ / shares shares	Dec. 31, 2017 USD ($)	Feb. 28, 2017 USD ($)	Jul. 31, 2016 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Mar. 31, 2018 shares	Nov. 30, 2012 USD ($)
Short-term Debt [Line Items]
Proceeds from debt, net of issuance costs								$ 68,024,000	$ 26,442,000	$ 23,989,000
Payments under revolving credit facility								0	10,100,000	$ 16,900,000
Warrants issued in connection with debt		$ 1,400,000		$ 1,400,000
Value of future purchase option					$ 700,000				700,000
Measurement Input, Quoted Price \| Cashless Exercise Warrants
Short-term Debt [Line Items]
Warrants and rights, measurement input \| $ / shares		1.05		1.05
Measurement Input, Share Price \| Cashless Exercise Warrants
Short-term Debt [Line Items]
Warrants and rights, measurement input \| $ / shares		1.56		1.56
Measurement Input, Price Volatility \| Cashless Exercise Warrants
Short-term Debt [Line Items]
Warrants and rights, measurement input		0.859		0.859
Measurement Input, Risk Free Interest Rate \| Cashless Exercise Warrants
Short-term Debt [Line Items]
Warrants and rights, measurement input		0.0223		0.0223
Measurement Input, Expected Dividend Rate \| Cashless Exercise Warrants
Short-term Debt [Line Items]
Warrants and rights, measurement input		0.00		0.00
Measurement Input, Expected Term \| Cashless Exercise Warrants
Short-term Debt [Line Items]
Warrants and rights outstanding, term		7 years		7 years
Tennenbaum Capital Partners, LLC \| Secured Debt
Short-term Debt [Line Items]
Proceeds from debt, net of issuance costs							$ 25,000,000
Interest rate during period							10.25%
Commitment fee percentage							3.30%
Closing fee							10.00%
Tennenbaum Capital Partners, LLC \| Secured Debt \| Minimum
Short-term Debt [Line Items]
Prepayment fee							1.00%
Tennenbaum Capital Partners, LLC \| Secured Debt \| Maximum
Short-term Debt [Line Items]
Prepayment fee							3.00%
Tennenbaum Capital Partners, LLC \| Secured Debt \| LIBOR
Short-term Debt [Line Items]
Interest rate during period							9.5625%
Reduction of interest rate during period							1.00%
Amended Term Loan Agreement, February 2017 \| Tennenbaum Capital Partners, LLC \| Secured Debt
Short-term Debt [Line Items]
Debt instrument face amount						$ 25,000,000
Second Amendment To Term Loans \| Secured Debt
Short-term Debt [Line Items]
Percent decrease in principal payments		(50.00%)
Convertible Notes Payable \| Convertible Senior Notes Due 2023
Short-term Debt [Line Items]
Debt instrument face amount			$ 60,000,000
Proceeds from issuance of debt			62,400,000
Debt underwriter fees			2,000,000
Debt issuance costs			$ 600,000
Interest rate			4.00%
Principal amount multiples			$ 1,000
Conversion rate of common stock (in dollars per share)			0.1800180
Debt conversion price (in usd per share) \| $ / shares			$ 5.5600
Redemption price percentage			100.00%
Unamortized issuance costs and debt discounts					0			2,361,000	0
Secured Debt \| Amended Term Loan Agreement, February 2017 \| Closing Date to March 31, 2018
Short-term Debt [Line Items]
Liquidity ratio required				1.5
Secured Debt \| Amended Term Loan Agreement, February 2017 \| April 1, 2018 and Thereafter
Short-term Debt [Line Items]
Liquidity ratio required				1.75
Secured Debt \| Amended Term Loan Agreement, February 2017 \| Tennenbaum Capital Partners, LLC
Short-term Debt [Line Items]
Interest rate during period						10.3125%
Commitment fee percentage						3.00%
Closing fee						4.00%
Unrestricted cash				$ 10,000,000
Number of securities called by warrants (in shares) \| shares		1,220,000		1,220,000							912,067
Class of warrant term year				7 years
Secured Debt \| Amended Term Loan Agreement, February 2017 \| Tennenbaum Capital Partners, LLC \| Measurement Input, Quoted Price
Short-term Debt [Line Items]
Warrants and rights, measurement input \| $ / shares		1.05		1.05
Secured Debt \| Amended Term Loan Agreement, February 2017 \| Tennenbaum Capital Partners, LLC \| Minimum
Short-term Debt [Line Items]
Prepayment fee						1.00%
Secured Debt \| Amended Term Loan Agreement, February 2017 \| Tennenbaum Capital Partners, LLC \| Maximum
Short-term Debt [Line Items]
Prepayment fee						3.00%
Secured Debt \| Amended Term Loan Agreement, February 2017 \| Tennenbaum Capital Partners, LLC \| LIBOR
Short-term Debt [Line Items]
Basis spread on variable rate						9.25%
Secured Debt \| Term Loan Agreement, July 2016
Short-term Debt [Line Items]
Unamortized issuance costs and debt discounts					2,111,000			1,059,000	$ 2,111,000
Secured Debt \| Term Loan Agreement, July 2016 \| Tennenbaum Capital Partners, LLC
Short-term Debt [Line Items]
Debt instrument face amount							$ 25,000,000
Line of Credit \| Wells Fargo Bank \| Revolving Credit Facility
Short-term Debt [Line Items]
Payments under revolving credit facility						$ 10,300,000
Maximum borrowing capacity												$ 50,000,000
Financing Receivable \| Financing Receivable Recorded as Debt
Short-term Debt [Line Items]
Proceeds from debt, net of issuance costs					$ 2,800,000			5,600,000
Thurman John Rodgers \| Convertible Notes Payable \| Convertible Senior Notes Due 2023
Short-term Debt [Line Items]
Debt instrument face amount								$ 5,000,000
Subsequent Event
Short-term Debt [Line Items]
Repayments of debt	$ 39,500,000

DEBT - Maturities (Details) - Secured Debt - Second Amendment To Term Loans $ in Thousands	Dec. 31, 2018 USD ($)
Debt Instrument [Line Items]
2019	$ 25,317
2020	16,207
Total	$ 41,524

COMMITMENTS AND CONTINGENCIES - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Commitments and Contingencies Disclosure [Abstract]
Rent expense	$ 3.2	$ 2.8	$ 3.8
Purchase obligation	$ 41.1

COMMITMENTS AND CONTINGENCIES - Schedule of Minimum Lease Payments Under Noncancelable Operating Leases (Details) $ in Thousands	Dec. 31, 2018 USD ($)
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2019	$ 3,738
2020	3,532
2021	3,276
2022	1,810
2023	945
Thereafter	1,252
Total operating lease minimum payments	14,553
Sublease income to be recognized in the future under noncancelable subleases	(922)
Net operating leases	$ 13,631

SALE OF COMMON STOCK (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended			3 Months Ended	12 Months Ended
	Feb. 28, 2018	Jan. 31, 2017	Dec. 31, 2016	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Subsidiary, Sale of Stock [Line Items]
Proceeds from issuance of common stock					$ 19,766	$ 26,425	$ 16,142
Issuance of common stock, net of offering costs					$ 19,766	$ 26,425	$ 15,924
Private Placement
Subsidiary, Sale of Stock [Line Items]
Number of shares issued (in shares)	9,523,809
Price per share issued (in usd per share)	$ 2.1
Proceeds from issuance of common stock	$ 20,000
Common stock, shares issued (shares)		10,800,000
Gross proceeds from issuance of common stock		$ 10,000
Market Issuance Sales Agreement (ATM)
Subsidiary, Sale of Stock [Line Items]
Proceeds from issuance of common stock				$ 16,600
Common stock, shares issued (shares)				11,100,000
Issuance of common stock, net of offering costs			$ 17,000

STOCK-BASED COMPENSATION Narrative (Details)					3 Months Ended	12 Months Ended
STOCK-BASED COMPENSATION Narrative (Details)	Jan. 31, 2019 shares	May 18, 2017 shares	May 01, 2017 shares	Mar. 29, 2012 shares	Jun. 30, 2018 $ / shares shares	Dec. 31, 2018 USD ($) purchase_period $ / shares shares	Dec. 31, 2017 shares	Dec. 31, 2016 shares	Dec. 31, 2015 shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation, outstanding (in shares)						6,772,000	8,426,000	8,730,000	8,170,000
Total unrecognized compensation cost \| $						$ 17,100,000
Weighted-average recognition period for unrecognized compensation cost						2 years 6 months
Common stock to be purchased in exchange for options (in shares)						521,000	4,379,000	1,505,000
Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance target percentage					0.00%
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance target percentage					150.00%
Stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock to be purchased in exchange for options (in shares)			2,362,470
Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award granted (in shares)					1,440,000	1,477,000
Weighted average estimated fair value of award (in usd per share) \| $ / shares					$ 4.57	$ 4.65
Performance Shares with Market Conditions
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award granted (in shares)					720,000
Weighted average estimated fair value of award (in usd per share) \| $ / shares					$ 4.54
Restricted stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock to be purchased in exchange for options (in shares)			733,559
Two Thousand Six Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation, vesting period						4 years
Share-based compensation, weighted-average remaining contractual term						10 years
Share-based compensation, outstanding (in shares)						1,600,000
Two Thousand and Eleven Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation, vesting period						4 years
Share-based compensation, shares authorized (in shares)						2,643,171
Increase in common stock shares reserved for future issuance percentage						4.50%
Share-based compensation, shares available for grant (in shares)						1,319,960
Employee Stock Purchase Plan (ESPP)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation, shares authorized (in shares)				669,603
Share-based compensation, shares available for grant (in shares)						551,203
Share-based compensation, additional shares authorized (in shares)		400,000
Share-based compensation, Evergreen shares (in shares)		700,000		330,396
Share-based compensation, increase in number of shares authorized				1.00%
Employee stock purchase plan, number of interim purchase periods \| purchase_period						4
General duration of employee stock purchase plan						24 months
Share-based compensation, maximum employee subscription rate						15.00%
Share-based compensation, purchase price of common stock percent to fair market value						85.00%
Share-based compensation, look-back feature (in years)						2 years
IRS limitation for employees right to acquire class common stock under ESPP \| $						$ 25,000
Before August 1, 2012 \| Two Thousand and Eleven Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation, weighted-average remaining contractual term						10 years
After August 1, 2012 \| Two Thousand and Eleven Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation, weighted-average remaining contractual term						7 years
Subsequent Event \| Two Thousand and Eleven Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation, additional shares authorized (in shares)	4,816,556
Subsequent Event \| Employee Stock Purchase Plan (ESPP)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation, Evergreen shares (in shares)	700,000

STOCK-BASED COMPENSATION Summary of the Weighted-Average Grant Date Fair Value of Options Granted (Details) - Stock options - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
The fair value of each option granted during the periods
Weighted average grant date fair value (in usd per share)	$ 2.83	$ 0.76	$ 1.29
Expected term	3 years 12 months	4 years 5 months	4 years 6 months
Expected volatility	88.50%	83.90%	80.00%
Annual risk-free rate of return	2.60%	1.80%	1.10%
Dividend yield	0.00%	0.00%	0.00%

STOCK-BASED COMPENSATION Summary of the Components of Total Stock-Based Compensation Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation expense	$ 11,432	$ 6,727	$ 10,326
Cost of revenues
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation expense	1,071	1,072	1,188
Research and development
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation expense	2,940	2,573	3,879
Sales and marketing
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation expense	3,074	1,157	2,144
General and administrative
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation expense	$ 4,347	$ 1,925	$ 3,115

STOCK-BASED COMPENSATION Summary of Stock-Based Compensation Associated with Each Type of Award (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense	$ 11,432	$ 6,727	$ 10,326
Stock options and RSUs and PSUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense	10,691	5,559	8,384
Employee stock purchase plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense	$ 741	$ 1,168	$ 1,942

STOCK-BASED COMPENSATION Summary of Stock Option Activity (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Number of Shares Outstanding
Outstanding, beginning balance (in shares)	8,426	8,730	8,170
Granted (in shares)	213	4,500	2,440
Exercised (in shares)	(1,346)	(425)	(375)
Canceled (in shares)	(521)	(4,379)	(1,505)
Outstanding, ending balance (in shares)	6,772	8,426	8,730
Shares outstanding, vested and expected to vest (in shares)	6,772
Shares outstanding, exercisable (in shares)	4,645
Weighted- Average Exercise Price per Share
Outstanding, beginning balance (in usd per share)	$ 1.77	$ 4.55	$ 5.36
Granted (in usd per share)	4.43	1.22	2.12
Exercised (in usd per share)	1.75	0.51	0.39
Canceled (in usd per share)	2.94	6.91	6.01
Outstanding, ending balance (in usd per share)	1.76	$ 1.77	$ 4.55
Weighted-average exercise price, vested and expected (in usd per share)	1.76
Weighted-average exercise price, exercisable (in usd per share)	$ 1.87
Weighted-Average Remaining Contractual Term
Outstanding	4 years 3 months
Vested and expected to vest	4 years 3 months
Exercisable	3 years 6 months
Aggregate Intrinsic Value
Exercised	$ 5,096	$ 544	$ 729
Outstanding	21,547
Vested and expected to vest	21,547
Exercisable	$ 14,670
Share Price	$ 4.73

STOCK-BASED COMPENSATION Summary of Stock Options Outstanding (Details) - $ / shares shares in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Option Outstanding - Number of Shares (shares)	6,772
Options Outstanding - Weighted- Average Remaining Life	4 years 3 months
Options Outstanding - Weighted- Average Exercise Price (usd per share)	$ 1.76	$ 1.77	$ 4.55	$ 5.36
Options Exercisable - Number of Shares Exercisable (shares)	4,645
Option Exercisable - Weighted-average Exercise Price (usd per share)	$ 1.87
$0.27 —– $0.79
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Option Outstanding - Number of Shares (shares)	1,551
Options Outstanding - Weighted- Average Remaining Life	2 years 10 months
Options Outstanding - Weighted- Average Exercise Price (usd per share)	$ 0.49
Options Exercisable - Number of Shares Exercisable (shares)	1,349
Option Exercisable - Weighted-average Exercise Price (usd per share)	$ 0.45
$0.79 —– $1.29
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Option Outstanding - Number of Shares (shares)	1,352
Options Outstanding - Weighted- Average Remaining Life	5 years 11 months
Options Outstanding - Weighted- Average Exercise Price (usd per share)	$ 1.22
Options Exercisable - Number of Shares Exercisable (shares)	433
Option Exercisable - Weighted-average Exercise Price (usd per share)	$ 1.21
$1.31 —– $1.31
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Option Outstanding - Number of Shares (shares)	1,751
Options Outstanding - Weighted- Average Remaining Life	5 years 3 months
Options Outstanding - Weighted- Average Exercise Price (usd per share)	$ 1.31
Options Exercisable - Number of Shares Exercisable (shares)	1,167
Option Exercisable - Weighted-average Exercise Price (usd per share)	$ 1.31
$1.37 —– $2.61
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Option Outstanding - Number of Shares (shares)	1,388
Options Outstanding - Weighted- Average Remaining Life	2 years 11 months
Options Outstanding - Weighted- Average Exercise Price (usd per share)	$ 1.87
Options Exercisable - Number of Shares Exercisable (shares)	1,150
Option Exercisable - Weighted-average Exercise Price (usd per share)	$ 1.86
$2.76 —– $12.57
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Option Outstanding - Number of Shares (shares)	730
Options Outstanding - Weighted- Average Remaining Life	3 years 7 months
Options Outstanding - Weighted- Average Exercise Price (usd per share)	$ 6.32
Options Exercisable - Number of Shares Exercisable (shares)	546
Option Exercisable - Weighted-average Exercise Price (usd per share)	$ 7.15

STOCK-BASED COMPENSATION Summary of Restricted Stock Unit Activity (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Weighted-Average Remaining Contractual Term
Outstanding	1 year 5 months
Aggregate Intrinsic Value
Share Price	$ 4.73
Restricted stock units
Number of Shares Outstanding
Outstanding, beginning balance (in shares)	3,505	606	1,313
Granted (in shares)	3,152	5,418	54
Vested (in shares)	(1,399)	(885)	(464)
Canceled (in shares)	(906)	(1,634)	(297)
Outstanding, ending balance (in shares)	4,352	3,505	606
Number of shares outstanding, expected to vest (in shares)	4,351
Weighted Average Fair Value per Share at Grant Date
Outstanding, beginning balance (in usd per share)	$ 2.03	$ 9.33	$ 9.31
Granted (in usd per share)	4.45	1.46	1.99
Vested (in usd per share)	2.75	3.81	9.06
Canceled (in usd per share)	2.17	1.90	8.32
Outstanding, ending balance (in usd per share)	3.52	$ 2.03	$ 9.33
Weighted-Average Fair Value per Share at Grant Date, Expected to vest (in usd per share)	$ 3.52
Weighted-Average Remaining Contractual Term
Expected to vest	1 year 5 months
Aggregate Intrinsic Value
Vested	$ 6,657	$ 932	$ 875
Outstanding	20,586
Expected to vest	$ 20,581

STOCK-BASED COMPENSATION Summary of Performance Stock Unit Activity (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2018
Weighted-Average Remaining Contractual Term
Outstanding		1 year 5 months
Aggregate Intrinsic Value
Share Price		$ 4.73
Performance Shares
Number of Shares Outstanding
Outstanding, beginning balance (in shares)		0
Granted (in shares)	1,440	1,477
Vested (in shares)		0
Canceled (in shares)		(147)
Outstanding, ending balance (in shares)		1,330
Number of shares outstanding, expected to vest (in shares)		1,330
Weighted Average Fair Value per Share at Grant Date
Outstanding, beginning balance (in usd per share)
Granted (in usd per share)	$ 4.57	4.65
Vested (in usd per share)
Canceled (in usd per share)
Outstanding, ending balance (in usd per share)		4.66
Weighted-Average Fair Value per Share at Grant Date, Expected to vest (in usd per share)		$ 4.66
Weighted-Average Remaining Contractual Term
Outstanding		3 months
Expected to vest		3 months
Aggregate Intrinsic Value
Outstanding		$ 6,291
Expected to vest		$ 6,291

STOCK-BASED COMPENSATION Summary of ESPP Activity (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Proceeds from common stock issued under ESPP	$ 397	$ 313	$ 999
Shares of common stock issued (shares)	439	478	659
Weighted-average price per share (usd per share)	$ 0.90	$ 0.65	$ 1.52

INCOME TAXES - Schedule of Domestic and Foreign Components of Loss before Provision for Income Taxes (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
United States									$ (14,322)	$ (47,882)	$ (67,631)
Foreign									4,093	2,541	1,644
Loss before income taxes	$ 1,285	$ (3,222)	$ (3,399)	$ (4,893)	$ (3,888)	$ (7,038)	$ (11,239)	$ (23,174)	$ (10,229)	$ (45,341)	$ (65,987)

INCOME TAXES - Schedule of Provision for Income Taxes (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Current:
Federal									$ 0	$ 0	$ 0
State									42	21	36
Foreign									1,233	1,224	785
Total									1,275	1,245	821
Deferred:
Federal									(35)	(1,092)	594
State									(21)	(21)	59
Foreign									179	(281)	1
Total									123	(1,394)	654
Provision for (benefit from) income taxes	$ 576	$ 248	$ 339	$ 235	$ (948)	$ (184)	$ 854	$ 131	$ 1,398	$ (149)	$ 1,475

INCOME TAXES - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
INCOME TAXES - Narrative (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating Loss Carryforwards [Line Items]
Federal statutory income tax rate percent	21.00%	34.00%	34.00%
Increase in valuation allowance	$ 9.8
Reduction in deferred tax assets		$ 28.3
Increase to unrecognized tax benefits	0.2
Federal
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	193.4
Federal \| Research tax credit carryforward
Operating Loss Carryforwards [Line Items]
Research tax credit carryforwards	11.8
State
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	101.2
State \| Research tax credit carryforward
Operating Loss Carryforwards [Line Items]
Research tax credit carryforwards	$ 12.1

INCOME TAXES - Schedule of Effective Income Tax Rate Reconciliation (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Income tax benefit at statutory federal rate									$ (2,148)	$ (15,416)	$ (22,435)
State taxes, net of federal benefit									17	(64)	63
Change in valuation allowance									8,198	(20,571)	21,370
Foreign tax rate and tax law differential									313	(133)	27
Tax credits									(378)	(382)	(1,179)
Stock-based compensation									(953)	761	1,775
Other permanent items									235	479	776
Other nondeductible/nontaxable items									(5,112)	930	920
Uncertain tax positions									107	106	158
Tax law changes									0	34,141	0
GILTI									917	0	0
Section 162(m)									202	0	0
Provision for (benefit from) income taxes	$ 576	$ 248	$ 339	$ 235	$ (948)	$ (184)	$ 854	$ 131	$ 1,398	$ (149)	$ 1,475

INCOME TAXES - Schedule of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets:
Allowances and reserves	$ 10,022	$ 9,748
Net operating loss and tax credit carryforwards	71,568	66,243
Stock-based compensation	3,662	2,528
Deferred revenue	19,562	7,210
Fixed assets and intangibles	3,836	4,369
Sec. 163(j) interest carryforward	2,064	0
Other	2,084	278
Subtotal	112,798	90,376
Less valuation allowance	(98,631)	(88,789)
Total deferred tax assets, net of valuation allowance	14,167	1,587
Deferred tax liabilities:
Goodwill	(1,070)	(992)
Unremitted foreign earnings	(16)	(10)
Deferred cost of goods sold	(12,655)	0
Total deferred tax liabilities	(13,741)	(1,002)
Net deferred tax asset	$ 426	$ 585

INCOME TAXES - Schedule of Reconciliation of Total Amounts of Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized tax benefits—at beginning of year	$ 6,106	$ 6,016	$ 5,482
Decreases in balances related to tax positions taken in prior years	0	(135)	0
Increases in balances related to tax positions taken in current year	329	306	571
Lapses in statutes of limitations	(110)	(81)	(37)
Unrecognized tax benefits—at end of year	$ 6,325	$ 6,106	$ 6,016

CONCENTRATION OF CREDIT RISK AND MAJOR CUSTOMERS (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Customer accounting for the largest concentration of accounts payable \| Accounts receivable \| Credit concentration risk
Revenue, Major Customer [Line Items]
Percentage of concentration risk	22.00%	22.00%
Customer accounting for the second largest concentration of accounts payable \| Accounts receivable \| Credit concentration risk
Revenue, Major Customer [Line Items]
Percentage of concentration risk	13.00%
Customer accounting for the largest concentration of revenue \| Net revenue \| Customer concentration risk
Revenue, Major Customer [Line Items]
Percentage of concentration risk	19.00%	15.00%	18.00%
Customer accounting for second largest concentration of revenue \| Net revenue \| Customer concentration risk
Revenue, Major Customer [Line Items]
Percentage of concentration risk		11.00%

NET LOSS PER SHARE - Schedule of Computation of Basic and Diluted Net Loss Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Numerator:
Net loss	$ 709	$ (3,470)	$ (3,738)	$ (5,128)	$ (2,940)	$ (6,854)	$ (12,093)	$ (23,305)	$ (11,627)	$ (45,192)	$ (67,462)
Denominator:
Weighted average common shares outstanding (in shares)									99,619	82,939	50,519
Net loss per share, basic and diluted (in usd per share)					$ (0.03)	$ (0.08)	$ (0.14)	$ (0.30)	$ (0.12)	$ (0.54)	$ (1.34)
Calculated under Revenue Guidance in Effect before Topic 606
Numerator:
Net loss									$ (13,748)
Denominator:
Weighted average common shares outstanding (in shares)									99,619
Net loss per share, basic and diluted (in usd per share)									$ (0.14)

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References + Details

NET LOSS PER SHARE - Schedule of Potentially Dilutive Securities Excluded from the Computation of Diluted Net Loss Per Share (Details) - shares shares in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities (in shares)	24,684	12,545	9,887
Convertible Notes Payable
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities (in shares)	11,701	0	0
Employee stock options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities (in shares)	7,710	8,433	8,981
RSUs and PSUs
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities (in shares)	5,273	3,029	906
Warrants to purchase common stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities (in shares)	0	1,083	0

SEGMENT AND GEOGRAPHIC INFORMATION - Summary of Net Revenues by Geographic Region (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenues from External Customers and Long-Lived Assets [Line Items]
Net revenues	$ 92,289	$ 78,002	$ 75,896	$ 69,972	$ 79,674	$ 77,038	$ 74,704	$ 54,751	$ 316,159	$ 286,166	$ 322,591
United States
Revenues from External Customers and Long-Lived Assets [Line Items]
Net revenues									219,600	199,565	259,080
International
Revenues from External Customers and Long-Lived Assets [Line Items]
Net revenues									$ 96,559	$ 86,601	$ 63,511

SEGMENT AND GEOGRAPHIC INFORMATION - Summary of Long-Lived Assets by Geographic Region (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Revenues from External Customers and Long-Lived Assets [Line Items]
Long-lived assets	$ 20,998	$ 26,483
United States
Revenues from External Customers and Long-Lived Assets [Line Items]
Long-lived assets	13,146	16,899
China
Revenues from External Customers and Long-Lived Assets [Line Items]
Long-lived assets	5,504	7,164
Other
Revenues from External Customers and Long-Lived Assets [Line Items]
Long-lived assets	$ 2,348	$ 2,420

RELATED PARTY Narrative (Details) - USD ($) shares in Millions		12 Months Ended
	Aug. 09, 2018	Dec. 31, 2018	Aug. 31, 2018
SunPower’s
Related Party Transaction [Line Items]
Revenue from related parties		$ 12,400,000
Accounts receivable, related parties		10,300,000
Convertible Notes Payable \| Convertible Senior Notes Due 2023
Related Party Transaction [Line Items]
Debt instrument face amount			$ 60,000,000
Convertible Notes Payable \| Thurman John Rodgers \| Convertible Senior Notes Due 2023
Related Party Transaction [Line Items]
Debt instrument face amount		$ 5,000,000
SunPower’s
Related Party Transaction [Line Items]
Common stock issued for business acquisition (in shares)	7.5
Supply agreement, period	5 years

ACQUISITION - Narrative (Details) $ in Thousands, shares in Millions		12 Months Ended
	Aug. 09, 2018 USD ($) shares	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Business Acquisition [Line Items]
Goodwill		$ 24,783	$ 3,664
Cash purchase price allocated in cash flows from investing activities		16,132	(28,442)	$ (32,953)
Payments to acquire businesses		15,000	0	0
Cash purchase price allocated in cash flows from operating activities		(19,151)	(4,121)	(11,795)
Acquisition funded by issuance of common stock		19,219	$ 0	$ 0
SunPower’s
Business Acquisition [Line Items]
Agreement for exclusive supplier rights, period	5 years
Consideration transfered for business acquisition	$ 57,319
Common stock issued for business acquisition (in shares) \| shares	7.5
Issuance of Common Stock	$ 32,319
Limitation for trade or transfer, period	6 months
Limitation for number of shares to be transfered by acquiree, period	6 months
Goodwill	$ 21,119
Cash purchase price allocated in cash flows from investing activities		10,000
Payments to acquire businesses		25,000
Cash purchase price allocated in cash flows from operating activities		15,000
Acquisition funded by issuance of common stock		19,200
Acquisition funded by accrued liabilities		15,000
Total acquisition related costs		$ 800
SunPower’s \| Developed technology
Business Acquisition [Line Items]
Amortization period for acquired intangible assets	6 years
SunPower’s \| Customer relationship
Business Acquisition [Line Items]
Consideration transfered for business acquisition	$ 23,100
Issuance of Common Stock	$ 13,100
Amortization period for acquired intangible assets	9 years
Discount rate \| SunPower’s \| Minimum
Business Acquisition [Line Items]
Equity, measurement input	0.14
Discount rate \| SunPower’s \| Maximum
Business Acquisition [Line Items]
Equity, measurement input	0.3

ACQUISITION - Summary of Consideration Transfered for Business Acquisition (Details) - SunPower’s $ in Thousands	Aug. 09, 2018 USD ($)
Business Acquisition [Line Items]
Cash consideration	$ 25,000
Common stock issued	32,319
Total	$ 57,319

ACQUISITION - Summary of Fair Values of Assets Acquired and Liabilities Assumed (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Aug. 09, 2018	Dec. 31, 2017
Business Acquisition [Line Items]
Goodwill	$ 24,783		$ 3,664
SunPower’s
Business Acquisition [Line Items]
Intangible assets		$ 36,200
Goodwill		21,119
Net assets acquired		$ 57,319

ACQUISITION - Summary of Identifiable Intangible Assets Acquired (Details) - SunPower’s $ in Thousands	Aug. 09, 2018 USD ($)
Business Acquisition [Line Items]
Intangible assets	$ 36,200
Developed technology
Business Acquisition [Line Items]
Intangible assets	$ 13,100
Useful life (in years)	6 years
Customer relationship
Business Acquisition [Line Items]
Intangible assets	$ 23,100
Useful life (in years)	9 years

ACQUISITION - Summary of Allocation of Purchase Price to Intangible Assets and Goodwill (Details) - SunPower’s $ in Thousands	Aug. 09, 2018 USD ($)
Business Acquisition [Line Items]
Cash Purchase Price	$ 25,000
Issuance of Common Stock	32,319
Total	$ 57,319
Concentration Risk, Percentage	100.00%
Developed technology and goodwill
Business Acquisition [Line Items]
Cash Purchase Price	$ 15,000
Issuance of Common Stock	19,219
Total	$ 34,219
Concentration Risk, Percentage	60.00%
Customer relationship
Business Acquisition [Line Items]
Cash Purchase Price	$ 10,000
Issuance of Common Stock	13,100
Total	$ 23,100
Concentration Risk, Percentage	40.00%

SELECTED UNAUDITED QUARTERLY FINANCIAL INFORMATION - Schedule of Quarterly Financial Information (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Net revenues	$ 92,289	$ 78,002	$ 75,896	$ 69,972	$ 79,674	$ 77,038	$ 74,704	$ 54,751	$ 316,159	$ 286,166	$ 322,591
Cost of revenues	64,124	52,738	53,195	51,657	60,685	60,577	61,157	47,703	221,714	230,123	264,583
Gross profit	28,165	25,264	22,701	18,315	18,989	16,461	13,547	7,048	94,445	56,043	58,008
Operating expenses:
Research and development	7,340	8,165	9,462	7,620	8,208	7,397	7,947	9,605	32,587	33,157	50,703
Sales and marketing	6,617	7,375	6,828	6,227	4,940	5,453	6,274	6,458	27,047	23,126	38,810
General and administrative	7,664	7,510	6,969	6,943	5,983	5,441	4,964	5,833	29,086	22,221	27,418
Restructuring charges	1,541	2,588	0	0	1,991	4,071	3,609	7,247	4,129	16,917	3,777
Total operating expenses	23,162	25,638	23,259	20,790	21,122	22,362	22,794	29,143	92,849	95,421	120,708
Income (loss) from operations	5,003	(374)	(558)	(2,475)	(2,133)	(5,901)	(9,247)	(22,095)	1,596	(39,378)	(62,700)
Other expense, net	(3,718)	(2,848)	(2,841)	(2,418)	(1,755)	(1,137)	(1,992)	(1,079)	(11,825)	(5,963)	(3,287)
Loss before income taxes	1,285	(3,222)	(3,399)	(4,893)	(3,888)	(7,038)	(11,239)	(23,174)	(10,229)	(45,341)	(65,987)
Income tax provision	(576)	(248)	(339)	(235)	948	184	(854)	(131)	(1,398)	149	(1,475)
Net loss	$ 709	$ (3,470)	$ (3,738)	$ (5,128)	$ (2,940)	$ (6,854)	$ (12,093)	$ (23,305)	$ (11,627)	$ (45,192)	$ (67,462)
Net income (loss) per share, basic (in usd per share)	$ 0.01	$ (0.03)	$ (0.04)	$ (0.06)
Net income (loss) per share, diluted (in usd per share)	$ 0.01	$ (0.03)	$ (0.04)	$ (0.06)
Net loss per share, basic and diluted (in usd per share)					$ (0.03)	$ (0.08)	$ (0.14)	$ (0.30)	$ (0.12)	$ (0.54)	$ (1.34)

Label	Element	Value
Cumulative Effect of New Accounting Principle in Period of Adoption	us-gaap_CumulativeEffectOfNewAccountingPrincipleInPeriodOfAdoption	$ (38,948,000)
Retained Earnings [Member]
Cumulative Effect of New Accounting Principle in Period of Adoption	us-gaap_CumulativeEffectOfNewAccountingPrincipleInPeriodOfAdoption	$ (38,948,000)