Another conventional power producer succumbs to low prices. : Power generator Calpine Corp (cpn), which owns some 80 power plants across the U.S. and Canada, said it has agreed to be bought by a group of investors led by Energy Capital Partners for $5.6 billion, having struggled in recent months with persistently low power prices. The all-cash $15.25 per share offer represents a 13 percent premium to Calpine's closing price on Thursday, and the company's shares were up 9.7 percent at $14.81 in premarket trading on Friday.