Pumped Hydro runs at about 80% inefficient compared to Battery technology which is about 90%. It can deliver power over a much longer time period. Power switch for batteries is almost instantaneous were Pumped Hydro takes about 15 min. The two together vastly stabilizes grid power.
The technology requires 2 reservoirs of different elevation. Surplus power from the grid is used to pump water from the lower reservoir is pumped to the higher reservoir. When the grid needs power the reversible generator produces power by letting the water pumped up drain though the turbine to the lower reservoir.