A type of hydroelectric power station with a lower as well as an upper storage pool or reservoir: the water that generated electricity during the day is stored in the lower storage pool or reservoir and can then be pumped back up to the upper storage pool at a time of day when energy demand is lower (for instance, at night). This means the water that is pumped back up using power can be reused to generate energy at peak demand times. This enables users to take advantage of price differences and provide services grid stabilization services.