Electric power systems that use hydroelectric energy storage for load balancing are known as "Pumped Hydro Storage" (PHS)
Pumped Hydro Storage facilities are used to address the grid's load stability needs as well as peak load energy requirements. The two water reservoirs of a Pumped Hydro Storage system are at various heights. The water kept in the top reservoir is used to power turbines to produce electricity when demand is at its highest. The water is pumped back, replenishing the top reservoir, using electricity from the grid at times of low demand (often late at night when electricity prices are low). The system controls excess generation capacity, facilitating the grid's efficient and reliable operation as well as the incorporation of renewable energy sources.
