Exelon ( EXC) proposed to acquire NRG Energy ( NRG) for $6.2 billion.

In an unsolicited offer, Exelon proposed to buy NRG in an all-stock transaction with a fixed exchange ratio of 0.485 an Exelon share for each NRG share, representing a value of about $26.43 for each NRG share. The offer is a premium of 37% to NRG's closing price Friday of $19.33. Exelon closed Friday at $54.50.

"An Exelon-NRG combination would result in a total enterprise value of approximately $60 billion with a generating capacity of around 47,000 megawatts, or enough electricity to serve nearly 45 million homes," said John W. Rowe, chairman and CEO of Exelon, in a statement late Sunday.

The combined company would be the largest power company in the U.S., according to Exelon. The company's market capitalization would be $40 billion.

NRG's board said it plans to review Exelon's proposal and "determine the appropriate response in due course." NRG advised its stockholders to take no action at this time.

In a letter to NRG President and CEO David Crane, Rowe disclosed the companies met on Sept. 30. He said Exelon is prepared to begin discussions and due diligence immediately.