The transaction was announced along with Centrica's half-year earnings, where the group reported a profit slump due to the COVID-19 impact and low commodity prices.
Both sets of shareholders welcomed the deal. Centrica's shares ended 16.7% higher, having surged almost 40% early on Friday, and NRG closed up 2.9%.
Centrica plans to use the cash from the sale to reduce net debt and contribute to its pension schemes, with its operations subsequently centred on the U.K. and Ireland.
"We had a number of expressions of interest in Direct Energy but it came down to the right price and the right buyer," Centrica Group Chief Executive Chris O'Shea told reporters on a conference call. O'Shea took over permanently as CEO in April.
The transaction comes at a time of flux for power providers on both sides of the Atlantic, as companies shape their businesses to best cope with changed energy demands amid the coronavirus pandemic, such as greater working from home.
For NRG, the transaction doubles the number of retail customers it serves to 6 million people, and strengthens its operations in the eastern United States, Chief Executive Mauricio Gutierrez told Reuters in an interview.
"It's really exciting for NRG as we have national reach," said Gutierrez. Until now, NRG's business was primarily focused on Texas.
The sale of Direct Energy is part of a wider restructuring effort at Centrica which has gained additional significance amid the challenges of the pandemic.
Last month, Centrica said it planned to cut around 5,000 jobs, almost 20% of its global workforce.
The group reported a 14% drop in half-year operating profit to 343 million pounds ($436.3 million) from 399 million a year earlier. This was due to the pandemic which reduced energy use and weakened commodity prices.
The net impact on pre-tax profit from the pandemic was around 60 million pounds as the company took mitigating measures such as not paying senior management bonuses, cost savings and using government job retention schemes.
Centrica gave no full-year financial guidance due to the possibility of customers delaying or deferring payments because of an uncertain economic outlook and its implications for unemployment.
It did not declare an interim dividend.
Centrica said it remains committed to exiting exit oil and gas production and nuclear power generation but has paused planned divestment of its 20% stake in Britain's nuclear energy fleet and the planned sale of its 69% shareholding in North Sea oil and gas producer Spirit Energy.
It plans to restart the Spirit Energy process when commodity and financial markets have settled and is considering options for the sale of its nuclear stake.
($1 = 0.7862 pounds)
(Reporting by Nina Chestney in London and David French in New York, additional reporting by Shanima A in Bengaluru; editing by Emelia Sithole-Matarise, Jane Merriman and Tom Brown)
By Nina Chestney and David French