📋 Project Overview
Dinorwig — known as Electric Mountain — is carved inside Elidir Fawr in Snowdonia. Six 288 MW Francis pump-turbines sit in a cavern the size of St Paul's Cathedral, 600 metres underground. It can respond to a system emergency and go from cold standby to full 1,320 MW output in just 12 seconds — making it the fastest responding large power station in the world. Built between 1974 and 1984 at a cost of £425 million, over 12 million tonnes of rock were excavated.
Site Schematic
Schematic diagram — not to scale. Illustrative layout based on project specifications.
⚡ Key Facts
🔧 Technical Specifications
| Capacity (MW) | 1,728 MW |
| Storage (MWh) | 9,100 MWh |
| No. of turbines/units | 6 |
| Turbine / unit model | Reversible Francis pump-turbines (Boving/GEC) |
| Unit capacity | 288 MW each |
| Head (hydro) | 542 m |
| Upper reservoir | Marchlyn Mawr (7 million m³) |
| Lower reservoir | Llyn Peris (7 million m³) |
| Tunnel / penstock | 16 km total tunnel network |
| Response time | 0 to 1,320 MW in 12 seconds (cold start) |
| Annual output | 1,700 GWh/year |
| Homes powered | 490,000 homes |
🔌 Grid Connection & Infrastructure
🏢 Development & Ownership
| Developer | Central Electricity Generating Board (CEGB) |
| Owner / operator | First Hydro Company (Engie) |
| Year consented | 1973 |
| Construction started | 1974 |
| Commissioned / target | 1984 |
| Capex estimate | Historic: ~£425 million (1984 prices) |
| Location | Llanberis, Snowdonia National Park, North Wales |
| Region | Wales |
| Coordinates | 53.12°N, 4.1°W |
📅 Project Timeline
🌿 Environmental & Planning
Located within Snowdonia National Park. Construction required extensive landscaping of surface features to minimise visual impact. The water pipelines and surface structures are painted in muted colours. The upper reservoir is a natural mountain lake modified to increase capacity.