The water from this lower reservoir at Electric Mountain is pumped up to a higher reservoir at night ready for use the following day. During the daytime and evening, when demand for electricity is at its highest, the water flows from the higher reservoir turning the giant turbines which generate electricity for the national grid.
Other than the waters of the two reservoirs, the whole of the operations of the hydroelectric power station is hidden in the 6 miles of tunneling inside the mountain (entrance to Electric Mountain is to the left of the photo). A bus tour of the hydro station is available from the town of Llanberis.