NRG Solar and SunPower will build a 250MW California Valley Solar Ranch in San Luis Obispo County, the US.
Under the agreement, NRG will assume ownership and financing responsibilities, while SunPower will continue to develop the project and also design, build, operate and maintain the plant.
The firms plan to begin work on the project in the second half of 2011 with an estimated budget of $450m.
Once fully operational, the plant will power about 0.1 million homes and help California achieve its 33% renewable portfolio target.
The California Valley Solar Ranch is currently seeking a loan guarantee from the US Department of Energy's (DOE) Loan Guarantee Program Office.