The Ffestiniog Power Station is a 360 megawatts (480,000 hp) pumped-storage hydroelectricity scheme near Ffestiniog, in Gwynedd, north-west Wales, United Kingdom. The power station at the lower reservoir has four water turbines, which can generate 360 megawatts of electricity within 60 seconds of the need arising. The station, commissioned in 1963, was the first major pumped storage system in the UK. The upper reservoir is Llyn Stwlan which discharges 27 cubic metres per second (950 cu ft/s) of water to the turbine generators at the power station on the bank of Tan-y-Grisiau reservoir. The building of the lower reservoir flooded the route of the Ffestiniog Railway which had to build a deviation around the reservoir and power station.
The plant is operated by First Hydro, a UK company owned by a joint venture of International Power and Mitsui & Co., and has an average efficiency of 72-73% - i.e. it uses 39% more electricity (when pumping the water back up to the Llyn Stwlan) than it actually produces.