Pumped-Storage Hydro — The World's Water Batteries

It's the world's oldest and largest form of grid storage: two reservoirs at different heights connected by a reversible pump-turbine. When electricity is cheap and plentiful, the plant runs the machine as a pump and pushes water from the lower reservoir up to the higher one — storing energy as the gravitational potential of all that lifted water. When the grid runs short, it lets the water fall back down through the same machine, now spinning as a turbine, to generate electricity on demand. Nothing is consumed; the same water cycles up and down for decades. Like a battery, a pumped-storage plant makes no net power of its own — it moves power through time. The difference is scale and duration: a single plant can store the energy of a small country's evening peak and pour it out for many hours. This map plots 327 of these stations worldwide, drawn as two-reservoir marks and sized by their power in megawatts.

For the same reason batteries do — wind and solar generate on the weather's schedule, not the grid's — but pumped storage solves the bulk, long-duration end of that problem. Solar floods the grid at midday and vanishes at dusk; wind surges and stalls with the gusts. A pumped-storage plant soaks up the cheap midday solar glut or a windy night by pumping water uphill, then releases it across the evening peak or a calm, still morning by letting that water fall. Because the reservoirs can be enormous, the plant can shift gigawatt-hours of energy over many hours — far more than a battery field — which is exactly what a grid running on weather-driven power needs to stay firm through the long gaps. That's why pumped storage is being revived alongside the renewables on the wind and solar layers: it's the heavy lifting behind a clean grid.

They're two halves of the same job, split by speed and duration. Lithium batteries — the kind on the battery-storage layer — respond in milliseconds and are perfect for fast, short bursts: smoothing a passing cloud, steadying frequency, covering minutes to a few hours. Pumped storage is slower to spin up but vastly larger and longer-lasting, banking energy for the multi-hour and overnight shifts that would drain a battery field dry. It's also the incumbent by a huge margin: pumped hydro is roughly 90% of the world's installed grid-storage capacity, even as batteries grow fast. The trade-off is geography — a battery can go almost anywhere, but a pumped-storage plant needs two large reservoirs with a big height difference between them, which is why they cluster in mountainous country. Think of batteries as the grid's reflexes and pumped storage as its deep reserves.

Much bigger than any battery plant. The largest on this map — and on Earth — is Fengning in northern China, which can deliver about 3.6 gigawatts, more than the biggest nuclear station. Behind it sit the long-standing American giants, Bath County in Virginia (2.9 GW) and Ludington in Michigan (about 2 GW), more recent Chinese stations, and Japan's deep mountain schemes. The map sizes every mark by its capacity in megawatts on a square-root scale, so these giants anchor the world view while smaller stations reveal as you zoom in. Capacity is recorded on about 88% of plants — cleaner than the battery layer — and any tag above 4 GW is clamped so a single mis-tag can't crown the map; the rest draw at the smallest size rather than invent a figure. Tap a mark for its name, capacity and country.

It shows every plant mapped in OpenStreetMap, which is close to a census for the giants but not for the long tail. Unusually for an OSM layer, the size ranking here is broadly true to the real world: China genuinely runs the largest and fastest-growing fleet and tops the list outright, with the US and Japan close behind — so you can trust the headline ordering. Where to be careful is the overall count: about 55% of the mapped plants sit in Europe, which is partly real (the Alps, Pyrenees and UK highlands pioneered pumped storage and are dense with it) and partly a mapping artifact (China operates more capacity than any nation yet shows up at only around a sixth of the set by count). So read Europe's density as genuine but over-counted, and China's share as understated even though its plants top the size ranking. A few of the biggest plants appear under their native Chinese or Japanese name, because that's what OSM records.