Short answers, pulled from the story.
Pumped-storage hydroelectricity stores energy by pumping water from a lower reservoir to a higher one using surplus off-peak electricity, then releasing that water through turbines to generate power during periods of high demand. Reversible Francis turbine designs handle both pumping and generation. The round-trip energy efficiency is 70-80%.
As of 2020, pumped-storage hydroelectricity accounted for around 95% of all active grid energy storage installations worldwide, with a total installed storage capacity of over 1.6 terawatt-hours. As of 2025, global PSH capacity stands at 200 gigawatts and 9,000 gigawatt-hours.
The first use of pumped-storage hydroelectricity was in 1907 at the Engeweiher facility near Schaffhausen, Switzerland. The first US installation followed in 1930, operated by the Connecticut Electric and Power Company near New Milford, Connecticut, pumping water from the Housatonic River to a reservoir 230 feet above.
China has the largest pumped-storage capacity in the world. As of the end of 2024, China's total installed PSH generation capacity reached 58.69 gigawatts after adding 7.75 gigawatts in 2024 alone, with more than 200 gigawatts under construction or approved.
The Fengning Pumped Storage Power Station in China is the largest operational pumped-storage plant, with an installed generation capacity of 3,600 megawatts and a storage capacity of 40 gigawatt-hours. The Bath County Pumped Storage Station in the United States ranks second at 3,003 megawatts.
Yes, pumped-storage plants can operate with seawater, though challenges such as saltwater corrosion and barnacle growth must be managed. The 30 MW Yanbaru project in Okinawa, opened in 1999, was the world's first seawater pumped-storage demonstration. Proposed projects in Chile's Atacama Desert would lift seawater 600 metres up a coastal cliff.