Pumped-storage hydroelectricity

• 1. Introduction

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  Pumped-storage hydroelectricity is, by a wide margin, the world's dominant form of grid energy storage. As of 2025, it provides 200 gigawatts of power and 9,000 gigawatt-hours of storage capacity worldwide. To put that number in perspective, a global atlas of potential sites lists more than 800,000 locations with a combined storage potential equivalent to roughly 2 trillion electric vehicle batteries. The technology is not exotic or experimental. Its first installation opened in 1907 in Switzerland, more than a century before modern battery storage entered the conversation. How does an idea that old still account for the overwhelming share of the world's stored electricity? And why is it suddenly more important than ever? Those are the questions at the heart of this story.

• 2. Water Moving Uphill

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  At its core, a pumped-storage plant does something deceptively simple: it moves water uphill when electricity is cheap, then lets it fall back down when electricity is expensive. Two reservoirs sit at different elevations, connected by tunnels and reversible turbines. When demand on the grid is low, surplus power drives pumps that push water to the upper reservoir. When demand spikes, that water is released through generators. The machines used are typically reversible Francis turbine designs, capable of acting as both pump and generator depending on the direction of their rotation. Variable speed operation, a refinement introduced in recent decades, further improves the efficiency of that round trip. The round-trip energy efficiency of a pumped-storage system sits between 70% and 80%, meaning some energy is lost to friction and conversion in every cycle. Even so, the economics work because the plant buys cheap off-peak power and sells expensive peak power. In jurisdictions where electricity prices occasionally go negative, because supply exceeds demand, pumped hydro operators can earn in two ways: once when they are effectively paid to consume electricity for pumping, and again when they sell power during high-price periods.

• 3. The Geography Problem

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  Switzerland has pumped-storage capacity equal to 32.6% of its total installed generating capacity, a fraction that towers above every other major economy. That figure is not coincidental. The country is mountainous, and mountains are the primary requirement for pumped storage. The technology's dependence on elevation difference is a hard physical constraint: the only way to store large amounts of energy is to have a significant body of water located as high as possible above a second body of water. Where that natural arrangement does not exist, it must be created. Projects in which both reservoirs are artificial and fed by no natural inflows are called closed-loop systems. Most of the 800,000 potential sites identified in the global greenfield pumped hydro atlas are of this type, situated away from rivers. The atlas calculates their combined potential at 86 million gigawatt-hours of storage, roughly 100 times more than would be needed to support a fully renewable electricity system worldwide. That surplus of suitable sites has practical implications: planners can generally avoid protected landscapes and rivers, because alternatives are abundant. Some projects also pursue what planners call brownfield sites, meaning locations with existing infrastructure. Disused mines are one candidate. The Kidston project in Australia repurposes an old mine; a proposal in Bendigo, Victoria, would use more than 5,000 shafts sunk under the city during the second half of the 19th century, the deepest reaching 1,406 metres underground.

• 4. Services Beyond Storage

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  Bulk energy storage is what pumped hydro is most famous for, but grid operators value it for a range of ancillary services that conventional batteries struggle to match. Thermal power plants, whether coal or gas, respond slowly to sudden changes in electricity demand. A pumped-storage plant, like other hydroelectric facilities, can go from standby to full output within seconds. That response speed allows it to provide frequency regulation, spinning reserve, load following, and operating reserve, functions that keep the grid from tipping into instability during unexpected demand spikes or plant outages. Italy's experience illustrates a related dimension. The country reached peak usage of pumped storage in 2003, generating around 8 terawatt-hours that year. For decades before that, Italy had operated surplus pumping capacity largely because its own nuclear program was interrupted in the 1980s. With less base-load generation to balance, many pumping stations ran primarily at night, consuming cheap surplus nuclear electricity imported from France. The logic was the same as everywhere: buy low, sell high, while also keeping the grid steady. Pumped storage's operational flexibility has grown more valuable as renewable generation has expanded. In some regions, non-firm renewable sources already supply roughly 40% of annual electricity output. The emerging view among grid planners is that reaching 60% may require substantially more storage, and pumped hydro is positioned as the leading candidate at scale.

• 5. Pushing the Boundaries

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  The 30 megawatt Yanbaru project in Okinawa, which opened in 1999, was the world's first demonstration of seawater pumped storage. It has since been decommissioned, but it opened a line of inquiry that continues. A proposed pair of projects in the Atacama Desert in northern Chile would combine 600 megawatts of photovoltaic solar with 300 megawatts of pumped storage, lifting seawater 600 metres up a coastal cliff. Underground storage is another frontier. The Mount Hope project in New Jersey proposed using a former iron mine as the lower reservoir. A concept at the Callio site in Pyhäjärvi, Finland, would exploit the deepest base-metal mine in Europe, with an elevation difference of 1,450 metres. The United States alone has roughly half a million abandoned coal mines, and researchers have examined whether some could serve as lower reservoirs. A US-based company called Quidnet Energy is exploring abandoned oil and gas wells for the same purpose, with roughly 3 million such wells available across the country. In March 2017, a research project called StEnSea demonstrated a different approach entirely: a hollow sphere submerged and anchored on the sea floor, acting as the lower reservoir while the surrounding ocean serves as the upper one. The deeper the sphere is placed, the greater the water pressure above it, and the more energy it can store. That system inverts the usual logic of elevation, replacing gravitational height with vertical water pressure as the energy source. And in 2026, a company called RheEnergise commissioned a 500 kilowatt facility in Plymouth, England, using a fluid 2.5 times denser than water, with the stated aim of proving that pumped storage sites could be made 2.5 times smaller for the same power output.

• 6. A Century of Scale

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  The first pumped-storage installation in the United States came in 1930, when the Connecticut Electric and Power Company began pumping water from the Housatonic River to a reservoir 230 feet above, near New Milford, Connecticut. Decades of incremental growth followed. By 2009, world pumped storage generating capacity stood at around 104 gigawatts, already dwarfing every other form of utility-scale storage. Today, the largest single plant on earth is the Fengning Pumped Storage Power Station in China, rated at 3,600 megawatts with a storage capacity of 40 gigawatt-hours. The Bath County Pumped Storage Station in the United States holds second place at 3,003 megawatts. China now holds the largest national installed capacity in the world, with 58.69 gigawatts as of the end of 2024 after adding 7.75 gigawatts in that year alone. The country has more than 200 gigawatts under construction or approved, putting it on course to surpass a 2030 target of 120 gigawatts well ahead of schedule. In January 2019, the State Grid Corporation of China had announced plans to invest US$5.7 billion across five new plants totalling 6 gigawatts, spread across Hebei, Jilin, Zhejiang, Shandong, and Xinjiang. Pumped storage plants carry one additional advantage that does not appear in efficiency tables: longevity. Capital costs are high, but proven service lives run for decades, and in some cases over a century, which is three to five times longer than utility-scale battery installations.