Salt River Project has agreed to buy more geothermal energy from the Imperial Valley of southern California.

SRP already has an agreement with CalEnergy to buy 50 megawatts of geothermal energy. Now, it’s adding another 37 megawatts to the previous contract. The total of 87 megawatts is enough energy to supply 21,750 Phoenix area homes for a year.

The power will come from the Salton Sea Known Geothermal Resource Area, one of the world’s most prolific producers of renewable energy. SRP says it will begin by buying 18 megawatts of geothermal energy in 2016 and grow to the full 87 megawatts by 2020.

Under the agreement SRP will be able to provide sustainable energy for its customers through 2039. The energy is produced naturally, when steam in superheated water reservoirs thousands of feet below the Earth’s surface is tapped by production wells. It’s considered renewable energy because no fuel is consumed to produce it.