The projected footprint of the California Valley Solar Ranch project, which can supply energy for 100,000 homes.

NRG Solar and SunPower said today they expect to begin construction on a large 250-megawatt solar installation next year in California's central coastal region.

The project would be one of the largest solar plants in the U.S., the companies said. NRG Solar, a subsidiary of utility NRG Energy, is purchasing the project from SunPower for about $450 million over four years.

The California Valley Solar Ranch plant will use SunPower's solar photovoltaic panels, which are mounted on trackers that position the panels to follow the sun to optimize power output. SunPower will build and operate the plant. NRG Solar will finance and own it.

In 2008, California utility Pacific Gas & Electric signed a power purchase agreement to buy the electricity generated at California Valley Solar Ranch. At the time, SunPower estimated it would produce on average 550,000 megawatt-hours a year which could supply about 50,000 average U.S. homes.

California is one of the primary drivers for large-scale solar plants in the U.S. because the state has a mandate that utilities generate a percentage of their electricity from renewable sources. This plant in San Luis Obispo County is expected to come online in 2012 and 2013.

The companies have applied for a Department of Energy loan guarantee to finance the construction and have a draft term sheet. From a technology perspective, the project is win for solar photovoltaic panels over solar concentrating power systems, which use heat to make steam that drives a turbine to generate electricity.

Updated at 11:20 a.m. PT with additional details. Updated at 11:45 with different estimate for number of homes that could be supplied.