A handful of thermal solar energy plants, most of them experimental, have been developed over the last two decades. The Solar One power tower [13], developed in Southern California in 1981, was in operation from 1982 to 1986. It used 1,818 mirrors, each 40 m², for a total area of 72,650 m². The plant was transformed into Solar Two by adding a second ring of larger (95 m²) heliostats and molten salts as a storage medium. This gave Solar Two the ability to produce 10 MW and helped with energy storage, not only during brief interruptions in sunlight due to clouds, but also to store sufficient energy for use at night. Solar Two was decommissioned in 1999 but proved it could produce power continuously around the clock.