kW (kilowatt)

What it is

A kilowatt (kW) is 1,000 watts. It describes how fast something uses energy at a given moment, like the speed on a car's speedometer. The amount of energy used over time is measured in kWh, like the miles on the trip counter.

Where you will see it

• On appliance labels, such as a 3 kW heater.
• On bills and contracts for half-hourly sites, where demand in kW can affect charges.
• In discussions about solar panels, batteries and EV chargers, which are rated in kW.

Why it matters

You are billed on kWh, not kW. But kW tells you how much capacity you need and whether you might reach a limit. If lots of equipment runs at once, your peak kW rises, which matters for larger sites with a kVA capacity. Our energy monitoring guide shows how to see your own peaks.

Example

A 3 kW heater, left on for 4 hours, uses 3 x 4 = 12 kWh. At the DESNZ Q2 2026 non-domestic average electricity price of 24.51p/kWh, that costs 12 x £0.2451 = about £2.94. Three of those heaters running together draw 9 kW at once, even though the energy used per hour is just three times larger.

To estimate a bill, multiply the appliance kW by hours used and then by your unit rate. Our business energy cost calculator does the sum for whole sites.

Related terms

Unit rate and half-hourly meter.