kW vs kWh: how to read the size of a commercial battery

kW vs kWh comes down to this: kW is power, how fast a battery can deliver energy right now. kWh is energy, how much it can deliver in total. A commercial battery is specified by both, and the ratio between them is its duration: 250 kWh at 125 kW is a two-hour system. This page explains the difference, the related terms on a datasheet, and how a business turns its own load into a size. Our buying guide to commercial battery storage shows where the size decision fits in the whole purchase.

Power and energy in one picture

Think of a water tank. The tank's volume is the energy, kWh. The width of the tap is the power, kW. A large tank with a narrow tap holds a lot but empties slowly. A small tank with a wide tap empties fast. In the same way, 1 kWh is the energy delivered by 1 kW sustained for one hour, so a 100 kW discharge for 30 minutes delivers 50 kWh. Both numbers belong to the battery, and the inverter limits the kW however much the cells could hold.

Duration: dividing one by the other

Duration in hours is energy divided by power. Keeping the power at 100 kW, three different batteries look like this:

The C-rate is the same information expressed relative to capacity: 1C empties the battery in an hour, 0.5C in two, 0.25C in four. It is the reciprocal of the duration in hours.

Which number does your site need?

Work from what the battery is for.

• Peak shaving. Power is how much you want to take off the peak, and energy is that power multiplied by how long the peak lasts. To cut a peak by 100 kW for a three-hour window you need at least 100 kW and 300 kWh of usable energy at the output, before allowing for losses and reserve. See peak shaving and DUoS charges for why the peak is expensive.
• Backup power. Power is the load you want to keep running and energy is that load multiplied by the hours you need. A 50 kW load for two hours needs 100 kWh usable, plus whatever reserve you want to hold at all times. Backup power covers the trade-offs, including holding a reserve that peak shaving would otherwise use.
• Load shifting and solar storage. Here energy usually matters more, because the aim is to move a block of kWh from one period to another. Energy arbitrage and storing solar work through it.

None of these can be sized from an annual total. They need the shape of your demand, which is why a year of half-hourly data comes first. How to read half-hourly data before buying a battery shows what to look for.

Nominal capacity, usable capacity and depth of discharge

Datasheets usually list the battery's nominal energy, the total the cells could hold. The usable energy is lower, because the battery management system keeps the cells from being fully emptied or fully charged. The share you can use is related to the depth of discharge, how far down the battery is allowed to go. Two batteries with the same nominal kWh can therefore deliver different usable kWh, and quotes that state only the nominal figure flatter the price per kWh. Ask for usable energy, and ask whether it is measured at the battery or at the AC output after the inverter.

Round-trip efficiency: what comes back

Charging and discharging both lose some energy as heat and in the inverter. Round-trip efficiency is the share you get back. The US National Renewable Energy Laboratory's 2024 Annual Technology Baseline uses 85% for commercial batteries, so 100 kWh going in returns about 85 kWh. That is a planning assumption rather than a guarantee, and a supplier's claim should state what it is measured on. The loss matters for any strategy that buys cheap energy and uses it later, because the price gap has to cover the missing 15%.

MW and MWh

The same logic applies at larger scales: 1 MW is 1,000 kW and 1 MWh is 1,000 kWh. A one-megawatt battery holding two megawatt-hours is a two-hour system, just as 100 kW with 200 kWh is. Containerised commercial systems are usually quoted in MWh.

How the two numbers move the price

NREL's cost model for commercial storage splits the price into a part that scales with the energy in the pack, a part that scales with the power of the inverter and balance-of-system equipment, and a fixed part. That is why more kW for the same kWh costs more, and why bigger systems cost less per kWh. Our guide to commercial battery storage cost sets out the published UK bands, and how to compare commercial battery quotes shows how to put quotes on the same basis. If you want the equipment itself explained, start with what a BESS is.

A datasheet checklist

1. Nominal energy (kWh) and usable energy (kWh), and where each is measured.
2. Continuous power (kW) at the AC output, and how long it can be held.
3. Peak power for short bursts, and for how many seconds.
4. Round-trip efficiency, with the conditions.
5. Duration at continuous power, which should equal usable kWh divided by kW.
6. Temperature limits and any derating of power in hot or cold conditions.
7. Warranty: years, energy throughput or cycle count, and guaranteed remaining capacity.