kW vs kWh: the difference, and why your bill uses kWh

A kilowatt (kW) is a rate: how fast something uses or delivers energy. A kilowatt-hour (kWh) is an amount of energy: 1 kW used for one hour. Electricity bills charge per kWh because you pay for the energy used, not the speed. A 2 kW heater running for three hours uses 6 kWh.

The speed and distance analogy

Think of kW as speed and kWh as distance. Speed alone does not tell you how far you went; you need the time too. Power alone does not tell you how much energy was used; multiply by the hours:

• Energy (kWh) = power (kW) × time (hours)
• Power (kW) = energy (kWh) ÷ time (hours)
• Time (hours) = energy (kWh) ÷ power (kW)

Saying “my house used 10 kW yesterday” or “this charger is 7 kWh” mixes the two up.

From appliance to bill

1. Find the power on the label, in watts or kilowatts. 1,000 W = 1 kW.
2. Multiply by the hours it runs. Minutes need converting: 20 minutes is 1/3 hour.
3. Multiply the kWh by your price per kWh.

Examples, at an illustrative price of 0.25 per kWh:

• A 2 kW kettle for 3 minutes: 2 × 0.05 = 0.1 kWh → 0.025
• A 100 W television for 4 hours: 0.1 × 4 = 0.4 kWh → 0.10
• A 2 kW heater for 3 hours: 6 kWh → 1.50

Label figures are maximums. Fridges, washing machines and heat pumps cycle on and off, so their real use is lower than power × hours switched on. Many labels give an annual kWh estimate, which is more useful. For minute and hour conversions, the Time Converter helps; for the arithmetic, the Scientific Calculator.

Electric cars: battery kWh and charger kW

• Battery size is energy, in kWh: how much the car can store.
• Charger speed is power, in kW: how fast energy flows in.
• Charging time is roughly the kWh you need ÷ the charging kW. Adding 40 kWh on a 7 kW home charger takes about 6 hours.

Real charging takes longer than the simple sum. The car may accept less power than the charger offers, rapid charging slows as the battery fills, and some energy is lost as heat, so the meter records more kWh than the battery gains. The EV Charging Cost Calculator works out the cost of a charge from battery size, charge level and your price per kWh; see also how much it costs to charge an electric car.

Reading an energy bill

Your meter counts kWh, and the bill multiplies the kWh used in the period by the unit rate. A smart meter display may also show current power in kW or watts — that is how fast energy is being used right now, like a car’s speedometer, while the kWh total is the odometer. Switching a 2 kW heater on shows up immediately as a jump in power, and over time as kWh on the bill.

Other places the units appear

• Solar panels are rated in kW (often kWp, for peak) — the most they produce in ideal conditions. What they generate over a day or a year is in kWh.
• Standing charges and capacity charges on some tariffs are per day or per kW of connection, separate from the per-kWh price.
• kW per hour is not a meaningful unit for energy use; if you see it, the writer meant kWh.

Mistakes to avoid

• Multiplying watts by hours and treating the answer as kWh without dividing by 1,000.
• Estimating a fridge’s cost from its label power running 24 hours a day.
• Expecting a 50 kW charger to fill a 50 kWh battery from empty in exactly an hour.

Checklist

• Power in kW (watts ÷ 1,000)?
• Time in hours?
• kW × hours = kWh; kWh × price = cost.