Bid for NRG may increase after rejection

Still, shares of NRG fell in early trading.

Exelon's initial bid to acquire NRG was aggressive, but it may be forced to raise its offer by as much as 15 percent, Deutsche Bank's John Kiani wrote in an analyst note.

"We continue to agree that NRG is worth substantially more than (Exelon's) offer (our target is $41), but also believe (Exelon's) large cap currency offers similar financial and commodity upside."

Deutsche Bank said the aggressive offer likely means the buyout will go through, but that keeping some of NRG's management will give the deal credibility.

NRG said Sunday they rejected the deal because it significantly undervalued the company and because Exelon Corp. did not line up the financing for the deal.

Exelon, a Chicago-based nuclear power giant and utility operator, made the all-stock bid offer last month. Shares of NRG have risen 57 percent since then and closed at $23.86 on Friday.

NRG shares fell 4.7 percent, or $1.16, to $22.70. Exelon shares fell 31 cents to $53.51.