The most important examples of mechanical storage systems are the familiar pumped storage power plants (PSPP). These essentially consist of an upper and a lower reservoir. Both are filled with water and connected together using pipes. When electricity is fed in, a pump conveys water from the lower reservoir via the pipes into the upper reservoir.
The stored energy is present in the form of the potential energy of the water. The greater the volume of the pumped water and the greater the height difference between the upper and lower reservoirs, the greater the stored energy will be. To release the energy from storage, the water is allowed to flow from the upper reservoir via the pipes, through a turbine and generator, back to the lower reservoir, thus recovering the previously stored electricity.