12. Revenue

Disaggregation of revenue


Major products/services lines
	Three months ended March 31,
	2019		2018
Sales of fuel cell systems	$	2,220	$	5,983
Sale of hydrogen installations and other infrastructure		—		4,630
Services performed on fuel cell systems and related infrastructure		6,213		5,483
Power Purchase Agreements		4,707		5,372
Fuel delivered to customers		5,453		4,950
Net revenue	$	18,593	$	26,418

Contract balances

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


	March 31,		December 31,
	2019		2018
Accounts receivable	$	32,062	$	37,347
Contract assets		3,521		3,328
Contract liabilities		38,104		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 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 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	Three months ended
	March 31, 2019
Transferred to receivables from contract assets recognized at the beginning of the period	$	(94)
Revenue recognized and not billed as of the end of the period		287
Net change in contract assets	$	193


Contract liabilities	Three months ended
	March 31, 2019
Revenue recognized that was included in the contract liability balance as of the beginning of the period	$	2,819
Increases due to cash received, net of amounts recognized as revenue during the period		(447)
Net change in contract liabilities	$	2,372

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):



	March 31,
	2019		2018
Sales of fuel cell systems	$	26,354	$	17,318
Sale of hydrogen installations and other infrastructure		9,072		9,141
Services performed on fuel cell systems and related infrastructure		68,878		73,381
Power Purchase Agreements		105,441		111,533
Other rental income		6,427		6,633
Total estimated future revenue	$	216,171	$	218,005

Contract costs

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

Capitalized contract costs at March 31, 2019 and December 31, 2018 were $0.3 million and $0.2, respectively. Expense related to the amortization of capitalized contract costs was not significant for the three ended March 31, 2019.