GenOn Energy Holdings, formerly Mirant Corporation, was a subsidiary of GenOn Energy, and is now a part of NRG Energy. The company was spun off from its former parent, Southern Company, on April 2, 2001. The company was merged into GenOn Energy on 3 December 2010. The company then became part of NRG Energy in December 2012. Mirant operated 13 plants in the states of California, Georgia, Maine, Maryland, Massachusetts, Michigan, New York, and Virginia and has the capacity to generate approximately 10,300 MW of electricity.