What Is a Kilowatt-hour (kWh)?

At a glance

  • A kilowatt-hour (kWh) measures energy. A kilowatt (kW) measures power: the rate at which energy is used or produced.
  • Energy used equals average power in kW multiplied by time in hours.
  • Multiply kWh by your unit rate to estimate the consumption charge. Standing charges and taxes may also apply.
  • An appliance’s maximum power rating is not necessarily its average consumption, and a kWh does not have one fixed carbon footprint.

Energy bills use kilowatt-hours to describe how much electricity or gas you have used. Understanding this unit makes it easier to check a bill, compare appliances and investigate changes in consumption. The calculations below apply to homes and workplaces.

On this page

Power and energy: the difference

Power describes how quickly energy is being used at a particular moment. Energy describes the total used over a period.

A 2 kW heater running continuously for half an hour uses 1 kWh. A 1 kW heater running continuously for one hour also uses 1 kWh. Their power ratings differ, but in these examples their energy use is the same.

There are 1,000 watts in a kilowatt. A 100 W device therefore uses power at a rate of 0.1 kW while operating at that level. The “hour” in kWh is part of the energy unit; kWh does not mean “kilowatts per hour”.

How to calculate electricity use

Use this calculation when you know the average power over the period:

Energy used in kWh = power in kW × time in hours

For a device using a steady 60 W for five hours:

  • Convert watts to kilowatts: 60 ÷ 1,000 = 0.06 kW.
  • Multiply by time: 0.06 × 5 = 0.3 kWh.
  • At an illustrative unit rate of 30p/kWh, the consumption costs 0.3 × 30p = 9p.

The 30p rate is an example, not a current tariff quotation. Use the rate on your own bill and check whether it includes VAT.

For an appliance used for 15 minutes, use 0.25 hours. A device drawing 2 kW continuously for that period uses 0.5 kWh. Changing minutes into hours first avoids a common calculation error.

The Centre for Sustainable Energy explains how appliance power ratings and operating time affect electricity consumption.

Why the label is not the whole answer

An appliance’s rated power may describe its maximum input, rather than what it draws throughout a normal cycle. An oven heats up and then cycles its heating element. A fridge’s compressor starts and stops. A washing machine uses different amounts during heating, washing and spinning.

Multiplying the maximum rating by every hour an appliance is plugged in can therefore exaggerate consumption. Where available, use measured kWh over a representative period or the product’s stated kWh per cycle or year, while allowing for differences in how you use it.

A lower power rating does not automatically mean lower energy use for the same job. A lower-powered device may need to run longer. Compare the energy needed to complete the task, along with capacity, performance and suitability.

Measuring your own consumption

The difference between two electricity meter readings shows the energy recorded between them. For example, readings of 15,400 kWh and 15,425 kWh show 25 kWh used. Record the dates and times too: 25 kWh over one day means something different from 25 kWh over a week.

With a multi-rate meter, record each relevant register separately. Do not confuse an electricity export register with the import reading used for purchased electricity.

A smart meter’s in-home display or supplier account can help reveal patterns. A suitable plug-in energy monitor can measure an individual plug-connected appliance within the monitor’s rated limits. It will not measure every hard-wired appliance, and equipment should not be disconnected if doing so would affect safety or essential services. See the Centre for Sustainable Energy’s guide to energy monitors.

For a cycling appliance such as a fridge, a longer representative measurement is more useful than a momentary watt reading. Note any unusual conditions, such as an unusually warm room or an empty appliance being tested.

Why gas bills also use kWh

Gas meters normally record a volume, in cubic metres or cubic feet. Suppliers convert that volume into energy for billing, using factors that include the gas’s calorific value.

The volume shown on the gas meter is therefore not the same quantity as the kWh on the bill. Imperial and metric meters require different conversion steps. GOV.UK explains how gas readings are converted into kWh. Check the meter units and bill calculation rather than applying a guessed multiplier.

Turning consumption into a bill

For a simple tariff, the consumption charge is kWh multiplied by the unit rate. The whole bill may also contain daily standing charges, taxes, previous balances, payments and adjustments.

If a bill covers a price change, calculate each part using its applicable rate. If prices vary by time of day, use the consumption and rate for each period. An average rate can be useful for a rough estimate, but it may not reproduce the bill exactly.

For businesses, additional contract charges and Climate Change Levy may apply. Household price protections should not be assumed to cover a business contract. Read our guide to reading an energy bill for the main checks.

Energy, efficiency and carbon emissions

Using fewer kWh to deliver the same useful result is one way to improve energy efficiency. Comparing kWh per item produced, per occupied hour or per square metre can also help, provided the comparison accounts for differences in use and conditions.

Cost and emissions are separate calculations. A cheaper tariff does not prove that less energy was used. A kWh of gas and a kWh of electricity do not necessarily produce the same greenhouse gas emissions.

For organisational emissions reporting, use the appropriate fuel, electricity and reporting-year factors and follow the method’s instructions. The government publishes greenhouse gas conversion factors and supporting methodology. Avoid treating a saving in pounds as evidence of an equivalent saving in carbon.

Sources and further reading

Browse the Hub: Household resources · SME resources · Environmental resources

Contributors

Scroll to Top