How can water store electricity for a later time?

Electricity can be stored by changing the height of water. A pumped-storage hydropower plant connects reservoirs at different elevations. During charging, an electric motor drives a pump that transfers water uphill. During generation, water travels downhill through a turbine linked to a generator. Some plants use reversible equipment for these two roles.

The stored quantity is gravitational potential energy. In an ideal fixed-head model it increases by E = mgh, where m is the moved water mass, g is gravitational acceleration and h is the effective height difference. The illustration moves 1,000 kilograms through 100 meters. With g = 9.81 meters per second squared, that stores 981,000 joules, or 0.981 megajoule. A real reservoir’s effective head can change as its water level changes.

Charging needs more electrical energy than the increase in potential energy because pumps, motors and water flow have losses. With a stipulated charging efficiency of 0.80, electrical input is 0.981 ÷ 0.80 = 1.22625 megajoules. On descent, a generating efficiency of 0.90 yields 0.8829 megajoule of electricity. The round-trip efficiency is 0.80 × 0.90 = 0.72: 72 percent of the charging input returns as electricity.

The plant therefore shifts energy in time; it does not create extra energy. Its storage capacity and its power rating answer different questions: how much energy can be stored and how quickly it can be delivered. Reservoir volume, head, flow and equipment limits all matter. The same water can make repeated trips while energy must be supplied anew for each charging cycle.