The diagram below shows how electricity is generated in a hydroelectric power station

The illustration shows how a hydroelectric electricity plant generates power. Overall, power is generated during the day when water is released from a dam, and then at night, the water is pumped back to the dam.

Looking first at the daytime operation, this begins when water flows down a river to a high-level water storage facility with a dam. Next, the water passes through an intake of the dam and to a hydropower station. Subsequently, the water turns turbines which feeds power to the National Grid. At the end of the daytime, the water ends up in a low-level reservoir.

The nighttime operation starts with the turbines being reversed and water being pumped from the low-level reservoir. This water is then pumped up to the dam and into the high-level reservoir. All the water stays in the high-level reservoir because the intake to the dam is closed.