The £300m investment taking place 500 metres under North Wales mountain
A £300m investment more than 500 metres underneath an Eryri mountain has been signed off by an energy giant.
ENGIE plans to invest £1bn over a number of years at the Dinorwig and Ffestiniog pumped storage hydro stations through First Hydro Company, a 75:25 joint venture between ENGIE and La Caisse.
Dinorwig pumped storage hydro station Image ENGIE
Ffestiniog opened in 1964 and Dinorwig in 1984 - providing bursts of power when the needed for the grid.
This week investment for the next stage of this programme was approved by the ENGIE board and La Caisse. This will enable the first two units at Dinorwig to be replanted in a c.£300 million project, with the units expected to reach commercial operation in 2028 and 2029. During this period, the remaining four units in Dinorwig will remain fully operational.
The works will see major components replaced in Dinorwig, including turbine and generator shafts of around 160 tonnes and generator motor rotors of up to 460 tonnes. The cavern itself is large enough to house St Paul’s Cathedral, and the station can generate up to 1,800MW of power, enough to supply around 1.5 million homes at peak output.
Image: University of Southampton
Dinorwig is Europe’s largest and fastest-acting pumped storage station, capable of reaching full output in just 30 seconds. Water from Marchlyn Mawr runs through high-pressure tunnels to generate electricity within the underground power station, before being released at Llyn Peris.
This investment will support a highly skilled workforce in North Wales, with peak construction activity expected to involve more than 250 people. The project will draw on local expertise alongside national and international supply chains, creating opportunities for Welsh businesses and workers.
Dinorwig Turbine Hall Image: Fully Charged/You Tube
Miya Paolucci, UK CEO of ENGIE, said: “Wales plays a central role in ENGIE’s UK strategy. Reaching financial close on this latest tranche of our pumped hydro investment demonstrates our long-term confidence in these vital assets and in the role flexible storage will play in delivering a secure, low carbon power system.”
Miya Paolucci
ENGIE is also strengthening the talent pipeline in North Wales working with Bangor and Wrexham universities, collaborating with Ambition Wales and the North Wales skills portal. This year, four new apprentices and two new graduates will join the business.
In a separate related development, Morgan Sindall Infrastructure have shared that the second of three new high‑voltage circuits connecting the Dinorwig pumped storage station to the North Wales transmission network has now been energised. Morgan Sindall Infrastructure, alongside National Grid and Siemens Energy, has been upgrading the original 1970s infrastructure since 2021.
When complete, the three circuits will ensure clean, reliable power from Dinorwig continues to support communities for decades to come.
Peter Kirk, MSI’s managing director, Energy said
“Energising the second circuit is a great example of strong collaboration and innovative engineering in a challenging environment. With the second circuit now live, our focus shifts to completing the final circuit and advancing works at Dinorwig and Pentir substations. We're proud to help power a cleaner, more resilient energy future”.