Pumped Hydro How It Works

Roughly 60% of all grid-scale energy storage on Earth is one technology: water pumped uphill, still comfortably the largest single source even as batteries have grown fast. Not flywheels, not compressed air. Water and gravity, the simplest physics in this curriculum, working at enormous scale.

Pumped hydro storage uses two reservoirs at different elevations connected by a pipe and a turbine. When the grid has excess electricity (low demand or surplus renewables), pumps push water from the lower reservoir to the upper one, converting electrical energy into gravitational potential energy. When the grid needs power, water flows downhill through a turbine, converting that stored potential energy back into electricity. Round-trip efficiency is 70-85%: for every 100 MWh pumped uphill, 70-85 MWh come back.

The energy stored depends on three variables: the mass of water, gravitational acceleration, and the height difference. E = mgh. A facility with a 500-meter head (height difference) and a 5-million-cubic-meter upper reservoir stores roughly 6,800 MWh, enough to supply 680 MW for 10 hours.

Worked Example

A pumped hydro plant has a 300-meter head and an upper reservoir holding 2 million cubic meters of water.

• Calculate mass. Water density is 1,000 kg/m³. Mass = 2,000,000 x 1,000 = 2 x 10⁹ kg.
• Calculate stored energy. E = mgh = 2 x 10⁹ x 9.81 x 300 = 5.886 x 10¹² J = 5,886 GJ.
• Convert. 5,886 GJ / 3.6 = 1,635 MWh (gross). At 80% round-trip efficiency: 1,308 MWh usable.

Why does pumped hydro dominate grid storage despite batteries getting cheaper?

Duration. This single facility stores 1,308 MWh. A utility-scale battery installation typically stores 100-400 MWh. Pumped hydro provides days of storage from geography that lasts 50+ years with minimal degradation.

Pumped hydro turns gravitational potential energy into the grid's largest storage resource, limited mainly by geography and permitting, not physics.

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