Pumped hydro storage is the only large energy storage technique widely used in power systems. For decades, utilities have used pumped hydro storage as an economical way to utilise off-peak energy, by pumping water to a reservoir at a higher level. During peak load periods the stored water is discharged through the pumps, then acting as turbines, to generate electricity to meet the peak demand. Thus, the main idea is conceptually simple. Energy is stored as hydraulic potential energy by pumping water from a low-level into a higher level reservoir. When discharge of the energy is required, the water is returned to the lower reservoir through turbines which drive electricity generators.