This kind of energy storage is very simple. At night, when energy demand is low, the pumps pump water uphill from the river. The water is pumped through the pipe to the reservoir. During the day, when energy demand is high, the reservoir releases water to flow downhill. The flowing water turns the turbine to make electricity.
The pump which pumps the water uphill from the river can although be generated with solar energy during the day. At night, when there is no solar energy the stored water turns the turbine to make electricity.
The efficienty of a pumped water storage plant is about 50%. When you pump water uphill, you lose 30% of energy. When the water flows down, you lose 20% of energy.
A pumped water storage plant operates for more than 20 years.
When the energy is needed the plant only needs 30 seconds to reach 100% of its power.