Power-generation company Calpine Corp. (CPN) is nearing a deal to sell itself to private equity firm Energy Capital Partners for $5.5 billion, or $15.25 per share in cash, sources told the Wall Street Journal Friday.

Calpine has a market value of $4.9 billion, and the purchase price would be a premium of 51% over the stock's closing price the day before the Wall Street Journal first reported a potential deal in May. 

Calpine shares were up 10.23% to $14.87 in premarket trading Friday. 

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