Ffestiniog Pumped Storage Scheme was opened in 1963. The upper lake, Llyn Stwlan (nprn 306315), has a capacity of 2,000,000 cubic metres of water which is used to drive the four turbines in the power station. The generators are capable of producing ninety megwatts of electricity in under sixty seconds. Electricity is produced at 16kV and stepped up to 400kV to feed into the National Grid. In off-peak periods, the turbines act as pumps to force water back to Llyn Stwlan; 75MW of electricity per pump is required to return water to the reservoir.
Sources: Ffestiniog Power Station; David Gwyn & Merfyn Williams (1996) `A Guide to the Industrial Archaeology of North West Wales?. Association for Industrial Archaeology.
RCAHMW, 12 December 2011.