The following table represents a disaggregation of revenue from contracts with customers for the three and six months ended June 30, 2020 and 2019 along with the reportable segment for each category (in thousands):


		Three Months Ended June 30,						Six Months Ended June 30,
Category	Segment	2020		2019		2020		2019
Solar modules	Modules	$	371,518	$	228,745	$	765,199	$	427,560
Solar power systems	Systems	215,474		217,755		305,550		375,049
O&M services	Systems	31,701		26,452		61,176		54,152
Energy generation (1)	Systems	22,576		9,547		40,549		20,122
EPC services	Systems	1,142		102,457		2,061		240,051
Net sales		$	642,411	$	584,956	$	1,174,535	$	1,116,934

(1)During the three and six months ended June 30, 2020, the majority of energy generated and sold by our PV solar power systems was accounted for under ASC 840 consistent with the classification of the associated PPAs.

We recognize revenue for module sales at a point in time following the transfer of control of the modules to the customer, which typically occurs upon shipment or delivery depending on the terms of the underlying contracts. Such contracts may contain provisions that require us to make liquidated damage payments to the customer if we fail to ship or deliver modules by scheduled dates. We recognize these liquidated damages as a reduction of revenue in the period we transfer control of the modules to the customer.

For certain sales of solar power systems and/or EPC services, we recognize revenue over time using cost based input methods, in which significant judgment is required to evaluate assumptions including the amount of net contract revenues and the total estimated costs to determine our progress toward contract completion. If the estimated total costs on any contract are greater than the net contract revenues, we recognize the entire estimated loss in the period the loss becomes known. The cumulative effect of revisions to estimates related to net contract revenues or costs to complete contracts are recorded in the period in which the revisions to estimates are identified and the amounts can be reasonably estimated.

Changes in estimates for sales of systems and EPC services occur for a variety of reasons, including but not limited to (i) construction plan accelerations or delays, (ii) module cost forecast changes, (iii) cost related change orders, or (iv) changes in other information used to estimate costs. Changes in estimates may have a material effect on our condensed consolidated statements of operations.

The following table outlines the impact on revenue of net changes in estimated transaction prices and input costs for systems related sales contracts (both increases and decreases) for the three and six months ended June 30, 2020 and 2019 as well as the number of projects that comprise such changes. For purposes of the table, we only include projects with changes in estimates that have a net impact on revenue of at least $1.0 million during the periods presented. Also included in the table is the net change in estimate as a percentage of the aggregate revenue for such projects.


	Three Months Ended June 30,									Six Months Ended June 30,
	2020			2019			2020			2019
Number of projects	5			3			8			5

(Decrease) increase in revenue from net changes in transaction prices (in thousands)	$	(8,524)		$	(4,064)		$	(9,435)		$	4,697
Decrease in revenue from net changes in input cost estimates (in thousands)	(9,576)			(1,749)			(8,750)			(16,736)
Net decrease in revenue from net changes in estimates (in thousands)	$	(18,100)		$	(5,813)		$	(18,185)		$	(12,039)

Net change in estimate as a percentage of aggregate revenue	(1.3)		%	(0.4)		%	(1.0)		%	(0.7)		%

The following table reflects the changes in our contract assets, which we classify as “Accounts receivable, unbilled” or “Retainage,” and our contract liabilities, which we classify as “Deferred revenue,” for the six months ended June 30, 2020 (in thousands):


	June 30, 2020		December 31, 2019		Six Month Change
Accounts receivable, unbilled (1)	$	58,323	$	162,057
Retainage	18,388		21,416
Allowance for credit losses	(734)		—
Accounts receivable, unbilled and retainage, net	$	75,977	$	183,473	$	(107,496)	(59)	%

Deferred revenue (2)	$	187,640	$	394,655	$	(207,015)	(52)	%

(2)Includes $57.1 million and $71.4 million of long-term deferred revenue classified as “Other liabilities” on our condensed consolidated balance sheets as of June 30, 2020 and December 31, 2019, respectively.

During the six months ended June 30, 2020, our contract assets decreased by $107.5 million primarily due to billings associated with ongoing construction activities at the GA Solar 4, Sun Streams, and Sunshine Valley projects. During the six months ended June 30, 2020, our contract liabilities decreased by $207.0 million primarily due to the recognition of revenue for sales of solar modules for which payment was received in 2019 prior to the step down in the U.S. investment tax credit, partially offset by advance payments received for sales of solar modules in the current period. During the six months ended June 30, 2020 and 2019, we recognized revenue of $285.9 million and $69.8 million, respectively, that was included in the corresponding contract liability balance at the beginning of the periods.

The following table represents our remaining performance obligations as of June 30, 2020 for sales of solar power systems, including uncompleted sold projects and projects under contracts subject to conditions precedent. Such table excludes remaining performance obligations for sales arrangements that had not fully satisfied the criteria to be considered a contract with a customer pursuant to the requirements of ASC 606. We expect to recognize $19.7 million of revenue for such contracts through the substantial completion dates of the projects.

Project/Location

Project Size in MW_AC

Revenue Category

Customer

Expected Year Revenue Recognition Will Be Completed

Percentage of Revenue Recognized

GA Solar 4, Georgia

200

Solar power systems

Origis Energy USA

2020

92%

Seabrook, South Carolina

Solar power systems

Dominion Energy

2020

97%

Total

272

As of June 30, 2020, we had entered into contracts with customers for the future sale of 10.8 GW_DC of solar modules for an aggregate transaction price of $3.5 billion. We expect to recognize such amounts as revenue through 2023 as we transfer control of the modules to the customers. While our contracts with customers typically represent firm purchase commitments, these contracts may be subject to amendments made by us or requested by our customers. These amendments may increase or decrease the volume of modules to be sold under the contract, change delivery schedules, or otherwise adjust the expected revenue under these contracts. As of June 30, 2020, we had entered into O&M contracts covering approximately 13 GW_DC of utility-scale PV solar power systems. We expect to recognize $0.4 billion of revenue during the noncancelable term of these O&M contracts over a weighted-average period of 9.1 years.

Following an evaluation of the long-term cost structure, competitiveness, and risk-adjusted returns of our O&M business, we received an offer to purchase certain portions of the business and determined it is in the best interest of our stockholders to pursue this transaction. Accordingly, in August 2020 we entered into an agreement with a subsidiary of Clairvest Group, Inc. (“Clairvest”), pursuant to which Clairvest will acquire our North American O&M operations. The completion of the transaction is contingent on a number of closing conditions, including the receipt of certain third-party consents, the expiration of the mandatory waiting period under U.S. antitrust laws, a review of the transaction by the Committee on Foreign Investment in the United States, and other customary closing conditions. Assuming satisfaction of such closing conditions, we expect the sale to be completed in late 2020.