Power company NRG Energy Inc has made an unsolicited bid to buy competitor Calpine Corp, the U.S. power producer that exited bankruptcy protection on January 31, for about $11 billion in stock, the companies said on Wednesday.

Calpine said it was reviewing the bid which values the company at $9.6 billion, to determine if it was in the best interests of its shareholders.

NRG offered to exchange 0.534 shares for each Calpine share, a premium of 6.7 percent based on the closing prices of the companies' stocks today.

The combined company would be the culmination of what we in this industry have aspired to become,'' NRG Chief Executive Officer David Crane said in that company's statement. He called it the right deal, at the right point in time, between the right partners.''

The deal would create a multi-fuel power company with four regional generation businesses in all the major competitive power markets in the United States.

It said each of those businesses would have at least 8 gigawatts of existing generating capacity.