US power companies Duke and Cinergy have announced plans for a merger that will see Duke’s utility operations in North and South Carolina combined with Cinergy’s utility operations in Indiana, Ohio and Kentucky to create a new company with more than 5 million customers.

Cinergy, with almost exclusively cheap coal-fired generation, will add Duke’s coal, gas and nuclear power plants to an impressive combined portfolio.

However, the $9.1 billion deal still requires regulatory approvals and Federal authorities are likely to require Duke Energy to join a regional wholesale power market as a condition.

Cinergy already belongs to the Midwest independent system operator, but Southeast-based Duke is in a region that has been resistant to formation of regional transmission operators (RTOs).