Nuclear power stations take much longer. We need something that can go from nothing to full power immediately, and keep us supplied for around half an hour until the other power stations catch up. Pumped storage reservoirs are the answer we've chosen. How it works Between 1976 and 1982 at Dinorwig, in North Wales, a huge project was built. Yet there's little to see as you drive past, as most of it is deep inside a mountain.
Water is pumped up to the top reservoir at night, when demand for power across the country is low.
The water then collects in the bottom reservoir, ready to be pumped back up later. The greater the height, the more energy is stored. This is one of the reasons that the Dinorwig site was chosen - there was a big height difference between two existing lakes, so less work was needed to build the station. The water falls 600 metres on its way to the turbines , so it's under a great deal of pressure when it arrives. For this reason, the tunnels are lined with steel at the bottom end.