Item 6. Selected Financial Data

The following table presents NRG's historical selected financial data. This historical data should be read in conjunction with the Consolidated Financial Statements and the related notes thereto in Item 15 and Item 7, Management's Discussion and Analysis of Financial Condition and Results of Operations. The Company has completed several acquisitions and dispositions during the years shown below, as described in Item 15 — Note 4, Acquisitions, Discontinued Operations and Dispositions, to the Consolidated Financial Statements.

(a)Excludes impairment losses and impairment losses on investments

(b)Includes goodwill impairments recorded as described in Item 15 — Note 12, Goodwill and Other Intangibles, to the Consolidated Financial Statements

(c)Total stockholders' equity, divided by shares outstanding as of the end of the year

(d)Liquidity position is determined as disclosed in Item 7, Management's Discussion and Analysis of Financial Condition and Results of Operations, Liquidity and Capital Resources, Liquidity Position. It excludes collateral funds deposited by counterparties of $32 million, $33 million, $37 million, $2 million and $91 million as of December 31, 2019, 2018 , 2017, 2016 and 2015 respectively, which represents cash held as collateral from hedge counterparties in support of energy risk management activities. It is the Company's intention to limit the use of these funds for repayment of the related current liability for collateral received in support of energy risk management activities

(e)Net income attributable to NRG Energy Inc., divided by total stockholders' equity

(f)Total debt and capital leases minus cash and cash equivalents, divided by total capitalization (total debt and capital leases plus total stockholders' equity and non-controlling interest) minus cash and cash equivalents

The following table provides the details of NRG's operating revenues:

(a) Inter-segment sales and net derivative gains and losses included in operating revenues

Retail revenue consists of revenues from retail sales to residential, small business, commercial, industrial and governmental/institutional customers, revenues from the sale of excess supply into various markets, primarily in Texas, as well as product sales.

Energy revenue consists of revenues received from both physical and financial transactions billed to third parties at either market or negotiated contract terms, as well as from the Company's retail businesses, for sales of electricity in the day-ahead and real-time markets and bilateral sales. Energy revenue in 2018 and prior years also included energy sold through long-term PPAs for renewable facilities prior to the deconsolidation of Ivanpah and Agua Caliente, during 2018. In addition, energy revenue includes revenues from the settlement of financial instruments and net realized trading revenues.

Capacity revenue consists of revenues billed to a third party at either market or negotiated contract terms for making installed generation and demand response capacity available in order to satisfy system integrity and reliability requirements. Capacity revenue also includes revenues from the settlement of financial instruments where price is derived from capacity markets. In addition, capacity revenue includes revenues received under tolling arrangements, which entitle third parties to dispatch NRG's facilities and assume title to the electrical generation produced from that facility.

Mark-to-market for economic hedging activities includes asset-backed hedges that have not been designated as cash flow hedges and ineffectiveness on cash flow hedges.

Other revenues consists of operations and maintenance fees, or O&M fees, construction management services, or CMA fees, sale of emission allowances, and revenues from ancillary services. O&M fees consist of revenues received from providing certain third party and unconsolidated affiliates with services under long-term operating agreements. CMA fees are earned where NRG provides certain management and oversight of construction projects pursuant to negotiated agreements. Ancillary services are comprised of the sale of energy-related products associated with the generation of electrical energy such as spinning reserves, reactive power and other similar products. Other revenues also include unrealized trading activities.