Energy storage technologies can capture energy during periods when demand or costs are low, or when electricity (or heat) supply exceeds demand, and can surrender stored energy (electric or thermal) when demand or energy costs are high.
Global grid-connected and stationary energy storage capacity in 2016 totalled an estimated 156 GWii, with pumped storage hydropower accounting for the vast majority.
More than 6 GW of pumped storage capacity was commissioned in 2016, for a year-end total of approximately 150 GW. Storage also can participate in a range of market segments, particularly in power markets, acting as a direct energy provider to the broader system, as hardware to support energy delivery or as a supplementary system for individual households or businesses.
A number of different energy storage technologies exist and are under development, and their characteristics (response time, discharge time, output capacity and efficiency) and functions vary widely. As of 2016, most electric energy storage capacity relied on pumped storage.
Source: Renewables 2017 - REN21 (Renewable Energy Police Network for the 21st. Century)