Clear answer, explained.
Power and energy answer two different questions about a battery.
The battery’s kW rating tells you the maximum rate at which it can deliver electricity. The kWh rating tells you how much energy is available to deliver over time. The U.S. Energy Information Administration uses the same distinction: power capacity is measured in kW or MW, while energy capacity is measured in kWh or MWh. U.S. Energy Information Administration
Battery duration is therefore related to both numbers. At a simplified level, duration at full rated output is calculated as energy capacity divided by power capacity. A 1,000 kWh battery discharging continuously at 200 kW has a theoretical five-hour duration. Actual usable duration depends on system efficiency, operating limits, state of charge and control strategy.
This distinction matters commercially because different objectives require different combinations of power and energy. A short, sharp facility peak requires a different system than a sustained multi-hour load. That is why Green Integrations sizes commercial battery storage against the shape and duration of the facility’s actual demand profile rather than selecting a battery from annual electricity consumption alone. See also: What size battery storage system does a commercial facility need?
What this means in practice.
- kW = power.
- kWh = stored energy.
- Duration is approximately kWh ÷ kW.
- Two batteries with the same kW can have very different operating durations.
- System sizing should follow the intended operating objective.
Best-fit environments.
- You are sizing a commercial battery storage system.
- You are planning battery storage for peak shaving.
- You are planning backup power for critical loads.
- You are designing a solar-plus-storage system.
- You are building a financial model for a BESS project.
