To fill it up, the Danube river would need 30 minutes, and it could feed Niagara Falls over half an hour – in the summer of 2017, after five years, we discharged all water from the upper reservoir of the largest Slovak pumped-storage hydroelectric power plant for a two-month general overhaul. Pumped hydroelectric power plants are the most widely available, cheapest and most reliable energy storage. Čierny Váh can save up to almost 4000 megawatt-hours of energy, roughly as much as batteries in 160-thousand conventional electric cars, or enough to supply 1300 households per year.