Independent power producer NRG Energy Inc. (NRG) said Monday it has signed an agreement with solar-thermal power plant developer eSolar to build three solar plants in California and a nearby state that will produce a total of 500 megawatts of electricity.

NRG said it will invest $10 million as part of the deal for equity and development rights for the three projects and for power purchase agreements to develop, build, own and operate up to 11 eSolar modular solar generating units.

"This is NRG's first venture into solar power and it brings an exceptionally important component to the low- and no-carbon focus of our RepoweringNRG program," said, Michael Liebelson, NRG's chief development officer for low carbon technology, in a statement.

The announcement, by NRG of Princeton, N.J., and privately held eSolar of Pasadena, Calif., comes on the heels of a deal announced this month between Edison International (EIX) unit Southern California Edison and solar-thermal power plant developer BrightSource Energy of Oakland, Calif., to build 1,300 megawatts of solar power plants in California's Mojave Desert.

Edison has also signed a contract with eSolar to take the output from a series of concentrating solar power plants eSolar plans to build in Kern County, Calif. that would produce up to 245 megawatts of electricity. California regulators have approved Edison's request to recover the costs of the contract from customers.

NRG shares were trading flat recently at $19.80 apiece.

-By Cassandra Sweet, Dow Jones Newswires; 415-439-6468; cassandra.sweet@dowjones.com