Mumbai: Electricity generator Exelon Corporation has made an unsolicited offer to acquire rival NRG Energy Inc for $6.2 billion in an all-stock deal, to form the largest power utility in the US.

Exelon has offered to exchange 0.485 of its own share for each NRG share - worth $26.43 a share at Friday's closing price - a premium of roughly 37 per cent.

NRG, based in New Jersey, has a diversified generation portfolio in terms of geography, fuel type and dispatch level and has a global project portfolio of over 24,000 megawatts.

A combination of the two energy majors could create the largest power company in the US, with a generation capacity of around 47,000 megawatts - enough to power nearly 45 million homes.

NRG Energy Inc has confirmed the receipt of a proposal from Exelon Corporation to acquire all of its outstanding shares and said its board will discuss the proposal with their advisors. NRG also advised its shareholders to take no action pending the review by NRG's board of directors.

Citigroup Global Markets Inc and Credit Suisse Securities LLC are serving as financial advisors and Kirkland & Ellis LLP is serving as legal counsel to NRG.

A Fortune 500 company, NRG Energy Inc, based in Princeton, NJ,  owns and operates 48 plants with a generating capacity of approximately 24,000 megawatts of generation capacity - enough to power nearly 20 million homes.

Warren Buffett's Berkshire Hathaway Inc owned 3.24 million shares of NRG as of 30 June, according to SEC filing released in August.

NRG, meanwhile, had offered to acquire bankrupt power utility Calpine Corp, in May, for about $11 billion, in an all-stock deal. However, Calpine, which emerged from bankruptcy protection earlier this year, rejected NRG's takeover bid saying that the offer was too low.

''This combination would not only diversify Exelon's generation portfolio geographically, it would also create immediate earnings and cash flow accretion,'' said Exelon CEO John Rowe.

A merger would give Exelon an extended geographical reach and immediate cash gains while NRG, which has over $8 billion of debt, could improve its credit rating.