The Ivanpah Solar Electric Generating System is a concentrated solar thermal plant in the Mojave Desert. It is located at the base of Clark Mountain in California, across the state line from Primm, Nevada. The plant has a gross capacity of 392 megawatts (MW). It deploys 173,500 heliostats, each with two mirrors focusing solar energy on boilers located on three 459 ft (139.9 m) tall solar power towers. The first unit of the system was connected to the electrical grid in September 2013 for an initial synchronisation test. The facility formally opened on February 13, 2014. In 2014, it was the world's largest solar thermal power station.