The concept of Coire Glas is simple. It involves two reservoirs at different heights in the Great Glen, the geological fault which slices through Scotland between Inverness and Fort William.

When power is plentiful and cheap, water would be pumped 500 metres uphill for storage in an upper reservoir with the capacity of 11,000 Olympic-sized swimming pools.

When supply is tight and prices high, it would be released, using gravity to generate electricity by spinning four turbines way below on the banks of Loch Lochy, before flowing into the lower reservoir.

By storing electricity generated in windy or sunny weather for use on cold, still or dark days, Coire Glas could help smooth the transition from oil, gas and coal to more sustainable but intermittent sources of energy such as wind and solar.

“We believe strongly it could play a huge role in enabling a decarbonised energy system,” said Finlay McCutcheon, SSE’s director of onshore renewables.