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Pumped hydro storage: Engineering and performance

Why bigger reservoirs make each extra hour cheap

What this tab covers Engineering and operating performance: round-trip efficiency, whether power and energy can be scaled separately, footprint and siting, and safety.

Round-trip efficiency is around 80 percent and energy can be added cheaply by enlarging reservoirs, but each plant is a large, site-specific civil project.

Round-trip efficiency

Favourable

PNNL's 2023 review says modern plants can approach or exceed 80 percent round trip. NREL's Annual Technology Baseline uses 80 percent as its central value and reports a spread of roughly 70 to 87 percent across published estimates, the lower end reflecting older or smaller plants. That beats the 64 percent design figure for China's 300 MW Yingcheng compressed air plant and PNNL's 29 to 52 percent estimate for compressed air generally, but is below lithium batteries. Losses come from pump and turbine efficiency, friction in tunnels and some evaporation.456

Round-trip efficiency (PNNL) about 80%, can exceed 80%5 Range across published estimates (NREL ATB) about 70-87%; 80% central6 Comparison: CAES 64% (Yingcheng design); 29-52% (PNNL estimate)45

Power and energy decoupling

Favourable

Power is set by the turbines and powerhouse; energy is set by reservoir volume and head. In the US DOE and PNNL 2022 assessment for a 100 MW, 10-hour plant, the reservoir costs about USD 76 per kWh while the powerhouse costs about USD 742 per kW, so extra hours are relatively cheap. Cost per kWh falls from USD 263 at 10 hours to USD 143 at 24 hours.3

Reservoir cost (100 MW, 10 h) USD 76/kWh3 Powerhouse cost USD 742/kW3 Installed cost 10 h vs 24 h USD 263/kWh vs USD 143/kWh3

Footprint and siting flexibility

Unfavourable

Pumped hydro needs height, water and two reservoirs, often in mountains or valleys, and floods land. Siting is the main limit. Closed-loop schemes and mine conversions widen the options (Nova Scotia is studying a disused mine), but each project remains bespoke. Conventional schemes typically take six to eight years to build, against about two years for a large Chinese compressed air plant, a comparison made by that plant's developer.14

Typical build time 6-8 years4

Safety and thermal runaway risk

Mixed

There is no fire or toxic gas hazard from the storage medium. The serious risk is structural: failure of a dam, embankment or penstock can release a flood, so plants fall under national dam safety regimes rather than battery codes such as NFPA 855. Sustainability performance can be assessed under the Hydropower Sustainability Standard; 13 hydropower projects had been certified by April 2025.1

Projects certified under Hydropower Sustainability Standard 13 (April 2025), 25 in pipeline1

Sources

  1. 2025 World Hydropower Outlook International Hydropower Association, 2025
  2. 2022 Grid Energy Storage Technology Cost and Performance Assessment US Department of Energy / Pacific Northwest National Laboratory, 2022
  3. 'World's largest' compressed air energy storage project connects to the grid in China Energy-Storage.news, 2024
  4. Emerging Technologies Review: Pumped Storage Hydropower (PNNL-34458) Pacific Northwest National Laboratory, 2023
  5. Annual Technology Baseline 2022: Pumped Storage Hydropower US National Renewable Energy Laboratory, 2022