12. Revenue

Disaggregation of revenue


Major products/services lines
	Three months ended June 30,				Six months ended June 30,
	2019		2018		2019		2018
Sales of fuel cell systems	$	38,547	$	13,371	$	40,767	$	19,354
Sale of hydrogen installations and other infrastructure		—		5,449		—		10,079
Services performed on fuel cell systems and related infrastructure		5,282		5,691		11,495		11,174
Power Purchase Agreements		6,307		5,438		11,014		10,810
Fuel delivered to customers		6,932		5,280		12,385		10,230
Net revenue	$	57,068	$	35,229	$	75,661	$	61,647

Contract balances

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


	June 30,		December 31,
	2019		2018
Accounts receivable	$	26,592	$	37,347
Contract assets		4,372		3,328
Contract liabilities		36,652		40,476

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 unaudited interim condensed consolidated balance sheet.

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 on the accompanying unaudited interim condensed consolidated interim balance sheet.

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, 2019
Transferred to receivables from contract assets recognized at the beginning of the period	$	(137)
Revenue recognized and not billed as of the end of the period		1,181
Net change in contract assets	$	1,044


Contract liabilities	Six months ended
	June 30, 2019
Revenue recognized that was included in the contract liability balance as of the beginning of the period	$	4,456
Increases due to cash received, net of amounts recognized as revenue during the period		(1,792)
Net change in contract liabilities	$	2,664

Estimated future revenue

The following table includes estimated revenue expected to be recognized in the future (sales of fuel cell systems and hydrogen installations are expected to be recognized as revenue within one year; sales of services and PPAs are expected to be recognized as revenue over five to seven years) related to performance obligations that are unsatisfied (or partially unsatisfied) at the end of the reporting period, excluding provision for common stock warrants as it is not readily estimable as it depends on the valuation of the common stock warrants when revenue is recognized (in thousands):



	June 30,
	2019		2018
Sales of fuel cell systems	$	16,467	$	13,478
Sale of hydrogen installations and other infrastructure		10,072		8,281
Services performed on fuel cell systems and related infrastructure		61,596		77,496
Power Purchase Agreements		121,744		122,659
Other rental income		5,793		5,463
Total estimated future revenue	$	215,672	$	227,377

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, 2019 and December 31, 2018 were $0.5 million and $0.2 million, respectively. Expense related to the amortization of capitalized contract costs was not significant for the three or six months ended June 30, 2019.