Dinorwig in Wales is pumped hydro and stores 9.1GWh https://en.m.wikipedia.org/wiki/Dinorwig_Power_Station https://en.m.wikipedia.org/wiki/Dinorwig_Power_Stati

Dinorwig in Wales is pumped hydro and stores 9.1GWh https://en.m.wikipedia.org/wiki/Dinorwig_Power_Station https://en.m.wikipedia.org/wiki/Dinorwig_Power_Station Cruachan in Scotland stores 7GWh https://en.m.wikipedia.org/wiki/Cruachan_Power_Station https://en.m.wikipedia.org/wiki/Cruachan_Power_Station Ffestiniog in Wales stores 1.4GWh https://en.m.wikipedia.org/wiki/Ffestiniog_Power_Station https://en.m.wikipedia.org/wiki/Ffestiniog_Power_Station Foyer at Loch Ness exists, can’t quickly see how many Wh it stores. Coire-Glas is proposed and could double the existing electricity storage apparently, which would make it huge, but not yet approved https://www.bbc.co.uk/news/uk-scotland-highlands-islands-65015217 https://www.bbc.co.uk/news/uk-scotland-highlands-islands-650... Still, powering a country though a cold dim calm winter is a lot of power; the UK uses around 800GWh/day of electricity, not counting gas/energy for heating. To store a week or a month or a winter of that is not a solved problem.