When the reservoir is full, Cruachan Power Station in Scotland can operate for 22 hours before it is emptied. Also, it is required to leave 12 hours water in th

When the reservoir is full, Cruachan Power Station in Scotland can operate for 22 hours before it is emptied. Also, it is required to leave 12 hours water in the lake at all times to provide emergency cold start backup on the grid. If it wasn't pumped hydro and was only working from runoff, then it wouldn't work.