Pumped hydroelectric storage facilities store energy in the form of water in an upper reservoir, pumped from another reservoir to a lower elevation.During periods of high demand for electricity, the energy is generated by releasing the stored water through the turbines in the same way as a conventional hydroelectric station. During periods of low demand (usually nights or weekends when electricity is also lower cost), the top tank is recharged by using lower cost electricity from the grid to pump the water back to the upper tank . The pump-turbine / motor-generator reversible assemblies can act as pumps and turbines. Pumped storage stations are unlike traditional hydroelectric plants, since they are a net consumer of electricity, due to the hydraulic and electrical losses incurred in the pumping cycle of the lower and upper reservoirs