12. Revenue

Disaggregation of revenue

The following table provides information about disaggregation of revenue (in thousands):


Major products/services lines
	Three months ended September 30,				Nine months ended September 30,
	2019		2018		2019		2018
Sales of fuel cell systems	$	31,264	$	21,594	$	72,031	$	40,948
Sale of hydrogen installations and other infrastructure		6,901		15,074		6,901		25,153
Services performed on fuel cell systems and related infrastructure		5,920		5,156		17,415		16,330
Power Purchase Agreements		5,599		5,555		16,613		16,365
Fuel delivered to customers		6,557		5,786		18,942		16,016
Other		135		—		135		—
Net revenue	$	56,376	$	53,165	$	132,037	$	114,812

Contract balances

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


	September 30,		December 31,
	2019		2018
Accounts receivable	$	24,392	$	37,347
Contract assets		4,319		3,328
Contract liabilities		43,248		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	Nine months ended
	September 30, 2019
Transferred to receivables from contract assets recognized at the beginning of the period	$	(180)
Revenue recognized and not billed as of the end of the period		1,171
Net change in contract assets	$	991

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):



	September 30,
	2019		2018
Sales of fuel cell systems	$	25,954	$	21,029
Sale of hydrogen installations and other infrastructure		11,448		6,705
Services performed on fuel cell systems and related infrastructure		60,196		73,484
Power Purchase Agreements		134,871		114,757
Other rental income		5,373		7,056
Total estimated future revenue	$	237,842	$	223,031

Contract costs

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

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