kW versus kWh: power and energy on a battery spec sheet

A battery label usually shows two numbers that look almost the same: kW and kWh. They answer different questions. One says how fast the battery can deliver electricity. The other says how much it can hold.

Power is the speed

A watt measures power. Power is the rate at which electrical energy moves through a circuit, and the watt is defined as one joule per second¹. A kilowatt (kW) is a thousand watts.

Think of a garden hose. Power is how hard the water comes out of the nozzle at any moment. It tells you nothing about how big the tank behind it is.

On a battery label, a kW figure tells you how much load the battery can run at one time. A kettle, a fridge and a set of lights all draw power while they're on. Add up what's running and you get the total power the battery has to supply.

Energy is the amount

A kilowatt-hour (kWh) is energy. One kWh is the energy delivered by one kilowatt of power for one hour². It's the same unit your electricity bill uses, because kilowatt-hours are a common billing unit for electrical energy supplied by electric utilities².

In the hose picture, kWh is the size of the tank. It tells you how long the flow can last, not how hard it can flow.

Putting the two together

Energy is power multiplied by time. That one idea links the two numbers.

Here's a made-up example, only to show the arithmetic. Say a battery holds 10 kWh and your devices draw 2 kW. Ten divided by two gives five hours. Draw 1 kW and you'd get ten hours. The relationship comes from the definition of the kilowatt-hour². Real batteries won't hit these round numbers, for reasons covered on usable versus total capacity and round-trip efficiency.

Why both numbers matter

Two batteries can hold the same kWh and have very different kW. A large tank with a narrow outlet runs a few things for a long time. A small tank with a wide outlet can run many things, but only briefly.

That's why a label carries both. You need the kWh to judge how long it lasts. You need the kW to judge what it can run together. The kW limit is explained further on the continuous power rating page.

Common mix-ups

• "kW per hour" isn't a unit. A kilowatt already means a rate. Energy is kilowatts times hours, which is kWh.
• A bigger kWh number doesn't mean more power. Check the kW figure separately.
• A bigger kW number doesn't mean longer running time. Check the kWh figure.
• Watt-hours and kilowatt-hours differ only by a factor of a thousand. The kilowatt-hour is built from the watt-hour with a metric prefix².

When a label uses a unit you don't recognize, read the maker's own spec sheet or manual for its exact meaning.