Last week the largest PV power plant in the U.S. quietly went online in Boulder City, Nevada, about 40 miles southeast of Las Vegas. Sempra Generation’s 48-MW Copper Mountain Solar Facility began construction in January 2010 and on Dec 1st, the company announced that it had finished the project and the facility was now generating electricity.

At its peak, Sempra said, more than 350 construction workers were installing the 775,000 First Solar panels that power the plant on the 380-acre site.

The power from Copper Mountain Solar and Sempra Generation’s adjacent 10-MW El Dorado Solar plant has been sold to Pacific Gas & Electric (PG&E) under separate 20-year contracts. California utilities are required to procure 20 percent of their energy supply from alternative sources by the end of 2010, increasing to 33 percent by 2020. RenewableEnergyWorld.com profiled the El Dorado plant in 2008. Play the video below to see a tour of it.

The completion of the project eclipsed the 20-MW DeSoto PV plant in Arcadia Florida, which was the previous record holder for the largest U.S. solar power plant.

This moves the U.S. into the “top five” when it comes to large PV power plants; only Canada, Italy, Germany and Spain have bigger plants, according PV Power Plants 2010, which ranks the top 50 facilities.