Constellation Energy (CEG) has agreed to acquire Calpine in a cash and stock transaction valued at about $16.4 billion, the energy companies said Friday.

The transaction cost includes 50 million shares of Constellation stock, $4.5 billion in cash, and the assumption of about $12.7 billion of Calpine debt, for a net purchase price of about $26.6 billion, the companies said.

The transaction is expected to close within 12 months of signing, according to the companies.

Constellation President and Chief Executive Joe Dominguez said the deal combines his firm's expertise in nuclear energy with Calpine's natural gas and geothermal generation to offer an "array of energy products and services" to about 2.5 million customers.

Constellation shares were up about 5% in Friday's premarket activity.