Independent power producers Mirant Corp and RRI Energy said on Sunday they have agreed to merge to create GenOn Energy, in an all-stock, tax-free transaction.

The new company will have a pro-forma market capitalization of $3.1 billion and expects to create annual cost savings of $150 million driven by reductions in corporate overhead which will fully kick in from Jan 2012.

Mirant stockholders will own approximately 54 percent of the equity of the combined company and RRI Energy stockholders will own around 46 percent.

Current Mirant Chief Executive Edward Muller will be chairman and CEO of the combined company until 2013, when the company said he plans to retire.

Mark Jacobs, currently CEO of RRI, will be the COO of GenOn. The company said Jacobs will succeed Muller as CEO in 2013.

The GenOn board will have five members of the present Mirant board and five from the RRI board.

(Reporting by Yinka Adegoke and Michael Erman; editing by Gunna Dickson)