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Dinorwig Power Station

Snowdonia, United Kingdom




10 years


£4.7bn (in today's money)


United Kingdom
Project achievements

Solved the problem

Build a power station to even peaks in demand for electricity in the UK.

Used engineering skill

Biggest ever government-backed civil engineering project at the time.


Power station was put in a Welsh mountain and cables buried out of sight

Add electricity capacity and storage to the National Grid to even out supply

Dinorwig was built in caverns inside Elidir Fawr, a mountain in north Wales.

There are 11 caverns altogether; the largest is 180m long, 23m wide and 51m high. They’re connected by shafts and tunnels up to 11.5m in diameter. Some of the tunnels carry water, others are road tunnels for trucks and vans to service the plant.

Engineers built a 35m high dam on the surface to increase the capacity of the lake Marchlyn Mawr, which became the upper reservoir for the plant. The lake is 503m above Llyn Peris, a used as the lower reservoir for the plant.

Opened by Prince Charles in 1984, the station can generate 1728MW of power within 12 seconds to stabilize demand on the National Grid.

Dinorwig Power Station

Dinorwig Power Station was built in caverns inside Elidir Fawr (a mountain in north Wales) to provide rapid response to sudden demands for electricity. The power station also stores cheap energy produced at night for use during times of peak demand.

Did you know …

  1. Dinorwig was the biggest civil engineering project ever commissioned by the UK government at the time.

  2. The power station is made up of 16km (about 10 miles) of tunnels, 1m tons of concrete, 200,000 tons of cement and 4,500 tons of steel.

  3. Like other electricity grid control centres, Dinorwig monitors popular television channels. When a high rating show such as ‘Strictly Come Dancing’ finishes millions of people all over the country get up to make a cup of tea. This sudden surge of demand is known as a ‘TV pick-up’.

Difference the power station has made

Dinorwig was built to provide rapid response to sudden demands for electricity. The power station also stores cheap energy produced at night for use during times of peak demand.

The station significantly reduced the need for other power plants to hold ‘spinning reserve’ – the extra generating capacity of stations connected to the grid.

Dinorwig helped improve the stability of power supply in the UK and made it less likely that a surge in demand could lead to power cuts.

How the power station was built

The scheme was constructed in the abandoned Dinorwig slate quarry. To preserve the landscape in Snowdonia National Park engineers built the power station itself inside the mountain Elidir Fawr.

Early stages of the project saw engineers making extensive improvements to local roads and nearby Bangor harbour to help move materials to the construction site.

Protecting the environment was a key part of the plan. Work included moving fish from Llyn Peris (the lower reservoir for the plant) to other lakes nearby. The project also collected spores from local heather so the mountain side could be replanted after building was finished.

Engineers had to excavate 12m tonnes of rock from inside the mountain to build the station. The tunnels they dug had to be wide enough for 2 lorries to pass comfortably.

The construction team had to connect Dinorwig to the National Grid substation at Pentir. Rather than using transmissions towers or pylons – and spoiling the local environment – they buried the cables underground in tunnels approximately 10km long.

People who made it happen

  • Commissioned by: the Central Electricity Generating Board
  • Designers: James Williamson and partners (now part of Mott McDonald)
  • Built as a joint venture between MacAlpine, Brand and Zschokke