The giant water batteries that lose power yet still make money
A pumped storage plant uses more electricity than it ever produces, losing up to 30 percent on each cycle. It still pays. Cheap surplus power at night lifts water uphill, and at peak hours, when prices are highest, that water falls back through turbines, turning gravity into the grid's largest battery.
The setup is simply two reservoirs at different heights. When demand is low, spare generating capacity pumps water into the upper one. When demand rises, the water flows down through a turbine into the lower one. Most plants use reversible machines, usually Francis designs, that act as a pump in one direction and a generator in the other. Allowing for evaporation and conversion losses, 70 to 80 percent of the energy comes back.
Because storing water is not very energy-dense, the trick is height. A large lake as high as possible above a second one, relatively close by, stores the most. As of 2020 pumped storage made up around 95 percent of active grid storage worldwide, with over 181 gigawatts of throughput capacity, although by May 2025 China alone reported nearly 107 gigawatts of battery installations, a sign batteries are catching up fast.
Speed is another asset. Coal and nuclear plants struggle to react to sudden swings in demand, but hydro plants respond within seconds, steadying grid frequency and holding reserves. That flexibility matters more as solar and wind grow, since prices can drop to near zero or even negative when generation outstrips what users can absorb.
Suitable land is not scarce. A global atlas lists more than 800,000 potential hilly sites, holding roughly a hundred times the storage needed for fully renewable electricity, mostly closed loops away from rivers. Engineers have also tried seawater: the Yanbaru demonstration in Okinawa ran from 1999 before being shut, and a Chilean proposal would lift seawater 600 metres up a coastal cliff. A Finnish plan would use Europe's deepest base metal mine, with a 1,450 metre drop.
Source: Pumped-storage hydroelectricity
