Pumped Hydro Storage Sizing Calculator
Pumped hydro energy storage (PHES) is the oldest, largest, and most commercially mature form of grid-scale energy storage in the world -- it has been deployed for over a century and still represents the vast majority of installed storage capacity worldwide by both power and energy. This calculator takes the usable reservoir volume, the head (elevation difference between the upper and lower reservoirs), and a round-trip efficiency figure, then reports the usable energy the system can store and deliver. It pairs naturally with our planned Pumped Hydro LCOS Calculator for translating these physical quantities into levelized cost of storage, and our planned LDES Technology Comparison Calculator for sizing pumped hydro against flow batteries, compressed air, and thermal storage.
The usable water volume that can be cycled between the upper and lower reservoirs.
The vertical elevation difference between the upper and lower reservoirs -- higher head means more energy stored per unit of water volume.
Pumped hydro typically achieves 70-85% round-trip efficiency, among the highest of any grid-scale storage technology, and the most commercially mature by far -- pumped hydro represents the vast majority of installed grid storage capacity worldwide.
(reservoir volume (million m³) × 1,000,000 × head (m) × 0.002725 × (round-trip efficiency (%) ÷ 100)) ÷ 1000
Results update live as you type. For planning and field-check estimates — always verify against applicable standards and equipment ratings.
How we calculate this →Pumped hydro's energy storage capacity comes down to a simple physics relationship: mass times gravity times height. A modest 10 million cubic meter reservoir with 300 meters of head -- a real, achievable combination at many mountainous or hilly sites -- stores over 6,500 MWh (6.5 GWh) at 80% round-trip efficiency. That's enough to discharge at 500 MW for 13 hours straight, illustrating why pumped hydro remains the dominant grid-scale storage technology worldwide by a wide margin, even as battery storage grabs most of the current headlines.
How pumped hydro storage sizing is calculated
This calculator estimates the usable energy a pumped hydro storage system can store and deliver, given the usable reservoir volume, the head (elevation difference), and the round-trip efficiency. One quantity ties the calculation together.
Usable Energy Storage (MWh) = (Reservoir Volume (million cubic meters) × 1,000,000 × Head (meters) × 0.002725 × (Round-Trip Efficiency (%) ÷ 100)) ÷ 1000. The calculation is built directly from gravitational potential energy. A cubic meter of water has a mass of 1,000 kilograms, and gravity accelerates it at 9.81 m/s², so each cubic meter of water raised one meter stores 1,000 × 9.81 = 9,810 joules of potential energy. Converting joules to kilowatt-hours (dividing by 3,600,000 J/kWh) gives 0.002725 kWh per cubic meter per meter of head -- the constant embedded in the formula. Multiplying that by the reservoir volume (converted from millions of cubic meters to cubic meters via × 1,000,000) and by the head gives the gross stored energy in kWh; applying the round-trip efficiency accounts for pumping, generating, hydraulic, and electrical losses; and dividing by 1,000 converts the final result from kWh to MWh. At the defaults (10 million m³, 300 m head, 80% round-trip efficiency), that is (10 × 1,000,000 × 300 × 0.002725 × 0.80) ÷ 1000 = 6,540.0 MWh.
Two notes on the model. First, the 0.002725 constant assumes a water density of 1,000 kg/m³ and standard gravity (9.81 m/s²); it does not account for the small density variation with temperature or salinity, nor for penstock friction losses, turbine and generator efficiency separately, or the usable operating range (reservoirs are rarely drawn down completely), all of which a detailed feasibility study would refine. Second, this calculator isolates the energy-capacity question and excludes power rating (set by turbine and pump sizing), discharge duration, annual cycles, construction cost, and project economics, all of which matter for real pumped hydro project evaluation. Data sources: Pumped hydro energy storage physics and gravitational potential energy from mechanical engineering and hydropower fundamentals; pumped hydro round-trip efficiency ranges from DOE (Department of Energy), NREL (National Renewable Energy Laboratory), and IRENA (International Renewable Energy Agency) energy storage technology assessments; global installed pumped hydro storage capacity share from EIA (Energy Information Administration) and IRENA storage databases; pumped hydro site selection and head/volume relationships from hydropower engineering references and industry feasibility studies. Verification: with defaults (10 million m³, 300 m head, 80% RTE), Usable Energy Storage = 6,540.0 MWh.