The Dinorwig Pumped Storage Hydroelectric Power Plant is a 1,800 MW pumped-storage hydroelectric scheme, near Dinorwig, Llanberis in Snowdonia national park in Gwynedd, north Wales. It comprises 16 km of tunnels, 1 million tons of concrete, 200,000 tons of cement and 4,500 tons of steel.

The scheme of Dinorwig Pumped Storage Hydroelectric Power Plant was constructed in the abandoned Dinorwic slate quarry. In order to preserve the natural beauty of Snowdonia National Park, the power station itself is located deep inside the mountain Elidir Fawr, inside tunnels and caverns. The project - begun in 1974 and costing £425 million and taking ten years to complete - was the largest civil engineering contract ever awarded by the UK government at the time. The work was undertaken by an Alfred McAlpine / Brand / Zschokke consortium. 12,000,000 tonnes of rock had to be moved from inside the mountain, creating tunnels wide enough for two lorries to pass comfortably, and an enormous cavern 51m tall, 180m long, and 23m wide known as "the concert hall". This has also given rise to the station's alternative name of Electric Mountain. The power station is connected to the National Grid substation at Pentir by 400kV cables that are buried for approximately six miles, rather than use unsightly, transmission towers or pylons to transmit the electricity across what is an area of outstanding natural beauty.


Water is stored at a high altitude in Marchlyn Mawr reservoir and is discharged into Llyn Peris through the turbines during times of peak electricity demand. It is pumped back from Llyn Peris to Marchlyn Mawr during off-peak times. Although it uses more electricity to pump the water back than it generates on the way down, pumping is generally done at periods of low demand, when the energy is cheaper to consume.

The Dinorwig Pumped Storage Hydroelectric Power Plant comprises six 300MW GEC generator/motors coupled to Francis-type reversible turbines. The generators are vertical shaft, salient pole, air cooled units each having 12 electromagnetic poles weighing 10 tonnes a piece, producing a terminal voltage of 18kV, synchronous speed is 500rpm. From standstill, a single 450-tonne generator can synchronise and achieve full load in approximately 75 seconds. With all six units synchronised and spinning-in-air (compressed air), 0MW to 1800MW load can be achieved in approximately 16 seconds. Once running, the station can provide power for up to 6 hours before running out of water.

Another important feature of Dinorwig is that it has been designed to assist restarting the National Grid on the occasion of a complete power failure (a black start). It includes diesel generators and large batteries which would allow the plant to restart even in the event of a complete shutdown of the grid (Dinorwig is not unique in this respect as some fossil-fuel plants are also able to self-start).

The Dinorwig Pumped Storage Hydroelectric Power Plant runs on average at 74-75% efficiency - i.e. it uses 33% more electricity (when pumping the water back up to the Machlyn Mawr) than it actually produces. It fills an important need in the system by responding to sudden surges in electricity demand because of its rapid ability to deliver power on load spikes. One of the alternatives would be to be to have spare capacity from conventional power stations running part loaded - spinning reserve, hence at lower efficiency than otherwise, and thus capable of being rapidly run up to full load. This would mean generating an additional 1330-1590GWh of power each year and releasing more than 140,000 tonnes of CO2 into the atmosphere each year. Other forms of power plant compete in this market for reserve power such as gas turbines and diesel generators for the National Grid Reserve Service.

Excess water overflows to Llyn Padarn and is lost from the Reservoir system. Both Llyn Peris and Llyn Padarn were ancestral homes to the Arctic char, a rare fish in the UK. When the scheme was commissioned, a fish rescue was undertaken to transfer the char from Llyn Peris to other local suitable lakes and it is believed that due to the very variable water levels in Llyn Peris, Arctic char are now absent from the lake.

The Dinorwig Pumped Storage Hydroelectric Power Plant is also promoted as a Welsh tourist attraction, with visitors able to take a minibus trip to see the workings inside the so called "Electric Mountain".

Dinorwig Pumped Storage Plant
Locale Dinorwig, Wales
Construction began 1974
Construction cost £425 million
Creates Upper: Marchlyn Mawr
Lower: Llyn Peris
Power station
Type Yes
Turbines 6 × 300 MW
Installed capacity 1,800 MW