Short answers, pulled from the story.
The Engeweiher facility near Schaffhausen in Switzerland began pumping water uphill to store energy on the 1st of January 1907. This simple mechanical process defines pumped-storage hydroelectricity globally.
As of 2025, pumped-storage hydroelectricity accounts for over 94% of all long-duration energy storage worldwide according to the International Hydropower Association. Global installed capacity reached nearly 200 gigawatts during this period.
China leads with 58.69 GW of installed generation capacity after adding 7.75 GW in 2024 alone. Japan holds 28.3 GW while the United States maintains 21.6 GW.
The Ffestiniog scheme in North Wales generates 360 MW of electricity within 60 seconds of need arising. Frequency stabilization occurs within seconds allowing rapid response to sudden changes in electrical demand.
Reversible hydroelectric turbines became available during the 1930s enabling machines to function as both generators and pumps. Connecticut Electric and Power Company installed the first United States system in 1930 using a reservoir above New Milford.