NorthWestern Energy Group and Black Hills have agreed to combine to form a large $7.8 billion regulated electric and natural gas utility company at a time of climbing energy demand across the U.S.

The utility will have a market capitalization of approximately $7.8 billion, based on each company’s closing stock price as of Monday.

The combined company will be worth $15.4 billion, based on each company's closing stock price on Monday, and have about 2.1 million customers.