LS Power is seeking to raise as much as $2.1 billion from the sale of a 12.3 million-share stake in NRG Energy Inc., according to people familiar with the matter. The shares are being offered for $163.50 to $168 each, representing a discount of up to 6.9% from NRG's closing price of $175.58 on Monday.

The stock sale follows the January closing of LS Power's acquisition by NRG, which agreed in May to purchase a fleet of natural gas-fired power plants from LS Power for about $12 billion including debt. The transaction reflects NRG's strategic bet that natural gas will be crucial to meeting growing electricity demand from data centers. Barclays Plc and Citigroup Inc. are managing the offering.

NRG shares fell 3.8% to $169 in after-hours trading following the announcement. The confidential marketing of shares before the public offering suggests LS Power is moving quickly to capitalize on market conditions following its business combination with NRG. The sale price range indicates LS Power is willing to accept a discount to accelerate the transaction.