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.

This chapter discusses a multitude of energy storage mechanisms that include pumped storage hydro (PSH) systems and various forms of battery storage, as well as other forms of energy storage with varying levels of technical and commercial maturity. The role and importance of energy storage is changing with the introduction of renewable energy generation such as wind and solar photovoltaics whose output is inherently variable. This increasing generation variability has created a need for energy storage to provide energy balancing. This chapter discusses the different requirements for energy balancing within renewable-based power systems over various timescales. The requirements for balancing services will be met by different forms of energy storage, highlighting the need for a portfolio of energy storage technologies. Energy storage also provides other benefits for modern power systems including to provide network and systems services and to enhance system flexibility and resilience. This chapter concludes by exploring issues related to the integration of energy storage into electricity grids and reviews social research related to energy storage uptake.