The Electric Mountain
Wow. This is an amazing piece of engineering on a massive scale.
The station at Dinorwig has two lakes, one on the surface and one deep down inside the mountain. During the day when demand is high on the national grid, water flows down into the bottom lake to produce electricity. During the night when demand is low and electricity is cheaper, water is pumped back up to the top mountain.
The station is a net consumer of electricity and outputs around 75% of the electricity is consumes. That’s not a bad thing though, as the point of this is to help balance the grid and reduce those peeks and troughs that cause so many logistical problems for electricity generators.