<P> While a hydroelectric dam does not directly store energy from other generating units, it behaves equivalently by lowering output in periods of excess electricity from other sources . In this mode, dams are one of the most efficient forms of energy storage, because only the timing of its generation changes . Hydroelectric turbines have a start - up time on the order of a few minutes . </P> <P> Worldwide, pumped - storage hydroelectricity (PSH) is the largest - capacity form of active grid energy storage available, and, as of March 2012, the Electric Power Research Institute (EPRI) reports that PSH accounts for more than 99% of bulk storage capacity worldwide, representing around 127,000 MW . PSH reported energy efficiency varies in practice between 70% and 80%, with claims of up to 87% . </P> <P> At times of low electrical demand, excess generation capacity is used to pump water from a lower source into a higher reservoir . When demand grows, water is released back into a lower reservoir (or waterway or body of water) through a turbine, generating electricity . Reversible turbine - generator assemblies act as both a pump and turbine (usually a Francis turbine design). Nearly all facilities use the height difference between two water bodies . Pure pumped - storage plants shift the water between reservoirs, while the "pump - back" approach is a combination of pumped storage and conventional hydroelectric plants that use natural stream - flow . </P> <P> Compressed air energy storage (CAES) uses surplus energy to compress air for subsequent electricity generation . Small scale systems have long been used in such applications as propulsion of mine locomotives . The compressed air is stored in an underground reservoir . </P>

This is used for long term storage of energy