Battery and pumped storage: MW, MWh and duration
Read storage power and energy ratings, calculate duration, and understand pumped hydro, losses and charging sources.
By Power Stations Worldwide · Reviewed 28 September 2026 · Editorial approach
How is energy storage different from an energy source?
Storage keeps energy available for later use instead of supplying an independent primary energy resource. A grid battery charges from electricity and subsequently discharges. To understand its role, ask both where the charging energy comes from and when the stored energy can be delivered to the system.
Why does a battery have both MW and MWh ratings?
MW describes its power capability, while MWh describes its energy capacity. Two batteries with the same MW rating can store different amounts of energy. A capacity-only map record cannot tell you how long a battery can sustain discharge, so look for a separately reported energy rating.
Sources: EIA: Battery storage data concepts
How do I calculate storage duration?
Divide usable energy in MWh by discharge power in MW. An illustrative 40 MWh system delivering 10 MW has four hours of idealised duration. Actual duration also depends on the usable operating range and losses. State whether the energy rating is usable or nominal before comparing projects.
Sources: EIA: Battery storage data concepts
How does pumped-storage hydropower work?
It uses electricity to pump water to a higher reservoir and later releases water through turbines to recover energy. The generating MW rating describes discharge capability, not the total stored energy. Reservoir arrangement and usable water volume also matter when interpreting the system’s storage role.
What is the difference between open-loop and closed-loop pumped storage?
Open-loop pumped storage has an ongoing connection with a natural water body; closed-loop arrangements do not have that continuing hydrological connection. These descriptions concern the water system. They do not establish zero environmental impact or identify the electricity source used for pumping.
What does round-trip efficiency measure?
It compares electricity returned from storage with electricity consumed in charging over a consistent cycle or period. The difference represents losses within the chosen measurement boundary. Do not copy a fleet-average efficiency from another year into a particular plant record as though it were a measured project specification.
Sources: EIA: Storage round-trip efficiency
Why can storage help even though it loses energy?
Timing and controllability have value: stored energy can be delivered when the system needs it, and some technologies respond quickly to changes. Storage can therefore support grid operation despite consuming more energy than it returns. Its usefulness depends on the service and system, not efficiency alone.
How should I read storage entries on this map?
Inspect the technology, capacity unit and source notes before comparing entries. A pumping unit, battery block and entire hydro complex may represent different reporting boundaries. Keep storage energy separate from generation capacity, and avoid treating every hydro-labelled record as conventional renewable generation when the underlying source describes pumping.
Atlas context: Data coverage and methodology
Textbook references
The science on this page follows these standard engineering textbooks. See the full bibliography.
- Linden's Handbook of BatteriesBattery technologies and ratings.
- Renewable and Efficient Electric Power SystemsPumped hydro and battery storage in grids.