NRG Energy said on Monday it would acquire power generation assets from energy infrastructure investment firm LS Power in a deal valued at $12 billion, as the U.S. utility bets on surging electricity demand,

The deal, to acquire a portfolio of natural gas generation facilities from LS Power Equity Advisors, will double NRG’s power-generation capacity, the company says.

Shares of NRG Energy Inc. NRG rallied 4.05% to $156.71 Tuesday, on what proved to be an all-around mixed trading session for the stock market, with the S&P 500 Index SPX rising 0.72% to 5,886.55 and the Dow Jones Industrial Average DJIA falling 0.64% to 42,140.43.