Separately, Calpine and Constellation are already among the largest power generation companies in the country.

Constellation is buying natural gas and geothermal power provider Calpine for $16.4 billion, joining together two of the country's biggest power companies. The acquisition would create the nation’s leading retail electric supplier,

Houston's largest private company will be acquired by a publicly traded utilities company based in Baltimore in a deal worth $26.6 billion.