Constellation Energy agreed to buy Calpine for $16.4 billion, combining two of the country’s largest electricity generators at a time when their product is in high demand from tech companies. Including the assumption of debt,

The company is betting that data-center owners will be hungry for all sorts of power—from nuclear to conventional.

Constellation Energy is nearing a roughly $30 billion deal to acquire power producer Calpine, people familiar with the matter said on Wednesday, a move that would significantly expand Constellation's generation assets at a time of rising U.