COLUMBUS, Ohio—Exelon Corp. on Thursday sweetened its hostile takeover bid for NRG Energy by 12 percent to $8 billion in stock, citing newly identified cost savings along with NRG’s recent deal for Reliant Energy’s Texas retail business.

Exelon Chairman and CEO John Rowe called it a “best and final offer.” The new bid ups the price for NRG by about $1 billion.

“Together the two company’s shareholders get something very rare, a truly effective combination in an industry in which significant profitable growth is very hard to come by,” Rowe said in a conference call.

NRG, which rejected Exelon’s previous bid, said it would review the latest offer. NRG owns power plants in the Town of Tonawanda and Dunkirk.

If NRG combines with Chicago- based Exelon, the new company would be the largest U. S. power generator, providing energy to about 45 million homes. Exelon is already the nation’s largest nuclear power company.

Rowe told analysts that the deal would further expand Exelon’s nuclear fleet.

Exelon would also add gas generation and coal plants, assets that would be valuable even under pending emmissions caps, and broaden its footprint in Texas, California and the Northeast.

Exelon is now offering 0.545 shares of its stock for each NRG share. The old offer had been 0.485 shares.

The new offer represents value of more than $3 billion to NRG shareholders, Exelon said. Exelon said its shareholders would see additional value of $1 billion to $3 billion.

The bid was increased after Exelon said it found an additional $1.5 billion in potential savings from the tie-up, and because of NRG’s acquisition of Reliant Energy’s retail business in May for $287.5 million.

The new bid is still low, said Jefferies & Co. analyst Paul Fremont, because it is just a 7 percent premium from NRG’s closing stock price Wednesday. Jefferies has a $30 price target for Princeton, N. J.-based NRG.

NRG shares fell $1.25, or 4.8 percent, to close at $24.80 Thursday. It shares have traded between $14.39 and $43.95 over the past year.

Exelon shares fell $2.19, or 4.3 percent, to $49.37. It shares have traded between $38.41 and $92.13 in the past year.