The FERC said units of six utilities failed at least part of an initial market power test the agency proposed this year to promote competition.
The companies include Alliant Energy Corp., American Electric Power Co. and Duke Energy Corp.
Entergy Corp., Southern Co. and Kansas City Power and Light Co., a unit of Great Plains Energy Inc., were also named by regulators.
The FERC gave the companies 60 days to propose ways to dilute their market dominance, or forfeit their right to sell electricity at market-based rates.
Future power sales by the utilities will be subject to refunds until the market power issues are resolved, the FERC said.
