12. Revenue

Disaggregation of revenue

In the following table, revenue is disaggregated by major product line and timing of revenue recognition (in thousands):


Major products/services lines	Three months ended				Six months ended
	June 30, 2018		June 30, 2017		June 30, 2018		June 30, 2017
Sales of fuel cell systems	$	14,633	$	5,582	$	21,207	$	7,400
Sale of hydrogen installations and other infrastructure		5,989		2,978		10,873		3,357
Services performed on fuel cell systems and related infrastructure		7,188		5,049		13,786		10,198
Power Purchase Agreements		5,629		4,945		11,118		9,256
Fuel delivered to customers		6,488		3,986		12,023		7,477
Other		—		64		—		151
Total gross revenue	$	39,927	$	22,604	$	69,007	$	37,839

Contract balances

The following table provides information about receivables, contract assets and contract liabilities from contracts with customers (in thousands):


	June 30, 2018		December 31, 2017
Accounts receivable	$	31,509	$	15,331
Contract assets		6,246		9,316
Contract liabilities		49,088		46,777

The contract assets relate to the Company’s rights to consideration for work completed but not billed. These amounts are included within prepaid expenses and other current assets on the accompanying consolidated balance sheets.

The contract liabilities relate to the advance consideration received from customers for services that will be recognized over time (primarily fuel cell and related infrastructure services). These amounts are included within deferred revenue and finance obligations on the accompanying consolidated interim balance sheets. A portion of the finance obligation balance has restricted cash held in escrow, which will be released over the contract period.

Significant changes in the contract assets and the contract liabilities balances during the period are as follows (in thousands):



Contract assets	Six months ended
	June 30, 2018
Transferred to receivables from contract assets recognized at the beginning of the period	$	(5,502)
Revenue recognized and not billed as of the end of the period		2,432
Net change in contract assets	$	(3,070)


Contract liabilities	Six months ended
	June 30, 2018
Revenue recognized that was included in the contract liability balance as the beginning of the period	$	4,608
Increases due to cash received, net of amounts recognized as revenue during the period		(6,919)
Net change in contract liabilities	$	(2,311)

Estimated future revenue

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


	June 30, 2018		December 31, 2017
Sales of fuel cell systems	$	13,478	$	26,298
Sale of hydrogen installations and other infrastructure		8,281		15,512
Services performed on fuel cell systems and related infrastructure		77,496		89,079
Power Purchase Agreements		122,659		130,042
Total estimated future revenue	$	221,914	$	260,931

All consideration from contracts with customers is included in the amounts presented above.

Contract costs

Contract costs consists of capitalized commission fees and other expenses related to obtaining or fulfilling a contract.

Capitalized contract costs at June 30, 2018 and December 31, 2017 were $0.1 million and zero, respectively. Expense related to the amortization of capitalized contract costs was not significant for the three or six months ended June 30, 2018.