Hydropower, geothermal, and tidal electricity generators can serve as baseload (constant-output) generators because their output can be held steady for long periods. However, wind, solar PV, and wave outputs vary during a day and seasonally. As such, these electricity sources provide variable output. Given that solar and wind may end up supplying 90 percent or more of all WWS energy generation worldwide, on average, it is important to have electricity storage technologies available to provide backup power when both solar and wind are unavailable. Major electricity storage options include existing hydroelectric dams, pumped hydroelectric storage, batteries, concentrated solar power coupled with thermal energy storage, flywheels, compressed air energy storage, and gravitational storage with solid masses. These technologies are discussed herein.