PRINCETON, N. J. — NRG Energy said Wednesday that it has turned down Exelon Corp.’s revised hostile takeover bid of $8 billion in stock because it undervalues the business.

Last week Exelon, the nation’s largest nuclear power company, sweetened its offer by about $1 billion because of newly identified cost savings and NRG’s recent $287.5 million deal for Reliant Energy’s Texas retail business.

In a letter to Exelon CEO John Rowe, Princeton-based NRG said it found the new bid was not in its shareholders’ best interest but said it represented a “step in the right direction.”

NRG said it is still open to any proposal that properly accounts for its “fundamental value and extraordinary growth prospects.” Exelon, Chicago, previously said that its most recent bid was its “best and final offer.”

If NRG does manage to work out a deal with Chicago-based Exelon, the new company would be the largest U. S. power generator, providing energy to about 45 million homes. A potential acquisition would also give Exelon gas generation and coal plants — assets that would be valuable even under pending emissions caps — and expand its presence in Texas, California and the Northeast.

NRG owns power plants in Dunkirk and the Town of Tonawanda.