Pumped-storage hydroelectricity (
PSH), or
Pumped Hydroelectric Energy Storage (
PHES), is a type of
hydroelectric energy storage used by
electric power systems for
load balancing. The method stores energy in the form of
gravitational potential energy of water, pumped from a lower elevation
reservoir to a higher elevation. Low-cost off-peak electric power is used to run the pumps. During periods of high electrical demand, the stored water is released through
turbines to produce electric power. Although the losses of the pumping process makes the plant a net consumer of energy overall, the system increases
revenue by selling more electricity during periods of
peak demand, when electricity prices are highest.