The Exelon Corporation, the largest US operator of nuclear power plants, made an unsolicited offer to buy rival NRG Energy Inc. for $6.2 billion. In a letter to NRG president and chief executive office David Crane and chairman Howard Cosgrove, Exelon proposed to acquire all of NRGÔÇÖs outstanding common stock in an all-stock transaction. The deal would create the largest power company in the US, worth about $60 billion and with a generating capacity of about 47,000 megawatts, enough electricity to serve nearly 45 million homes, according to Exelon.