Cost = watts ÷ 1,000 × hours used × price per kWh. A 3,000 W kettle used for 15 minutes a day uses 0.75 kWh per day, which costs about 20p at an example price of 26p per kWh. Your own price is on your bill or app, so swap it in.
What a kilowatt-hour is
Electricity is billed in kilowatt-hours (kWh). One kWh is one kilowatt (1,000 watts) used for one hour. So a 1,000 W heater running for an hour uses 1 kWh, and so does a 100 W bulb left on for ten hours.
The power of an appliance, in watts, is usually on a label on the back or underneath it, or in the manual. Heating appliances such as kettles, heaters and ovens use the most power while they are heating, but they may cycle on and off, so real use can be lower than the label suggests.
The formula
- Convert watts to kilowatts: watts ÷ 1,000.
- Multiply by hours used to get kWh.
- Multiply by your price per kWh to get the cost.
The price per kWh is your own tariff, which differs between suppliers and changes over time, so we do not quote a current rate. Examples here use 26p per kWh purely to show the method.
Worked example 1: a kettle
Take a 3,000 W kettle that is on for 15 minutes (0.25 hours) a day, every day, with the Electricity cost of appliances tool.
- Energy per day: 3 kW × 0.25 h = 0.75 kWh.
- Cost per use at 26p: about 20p.
- Energy per year: about 273.8 kWh.
- Cost per year: about £71.18, or £5.93 a month.
Worked example 2: a 100 W device left on
Suppose a 100 W device is used for 5 hours a day, every day.
- Energy per day: 0.1 kW × 5 h = 0.5 kWh.
- Energy per year: 182.5 kWh.
- Cost per year at 26p: about £47.45, or £3.95 a month.
A low-power device used for long hours can cost as much as a high-power one used briefly. That is why the hours matter as much as the watts.
Typical appliances to check
The calculator has presets for common items, including an LED bulb, television, laptop, gaming PC, kettle, microwave, electric oven, air fryer, washing machine, tumble dryer, dishwasher, heater, hair dryer, electric shower and EV home charger. These are typical figures only, so enter the watts from your own label if you can.
For a washing machine or dishwasher, the cost depends on the programme and temperature, so use the figure on the energy label or a plug-in meter if you want precision. For an electric car, the same method applies: kWh added to the battery × your price per kWh. The guide to Fuel cost and mpg: UK gallons, litres and trip costs covers the equivalent for petrol and diesel.
Ways to bring the cost down
- Switch off devices at the wall if they use standby power.
- Choose a lower-temperature wash and fuller loads.
- Only boil the water you need.
- Use a plug-in energy meter to find out what your real appliances use.
- Check whether your tariff has cheaper off-peak periods, and run big appliances then if it suits you.
If you want to know how much a change saves in percentage terms, see How to calculate percentages.
Note: These are estimates, and standing charges are not included. Check your tariff and bill for your real costs. This is general information, not advice.
Making sense of the result
A yearly figure can look larger than expected, so put it in context. Multiply by the share of the year you really use the appliance, and be honest about the hours. A tumble dryer used twice a week is very different from one used daily. Setting the days per week in the calculator handles this, and you can compare two appliances by running both through the same tool with the same price.
If you are thinking of replacing an old appliance, work out its running cost now, then the running cost of the new model from its energy label, and compare the saving over several years with the purchase price. That tells you whether an upgrade pays for itself.
Try the calculator
Pick an appliance in the Electricity cost of appliances tool, enter your own price per kWh and the hours you use it, and it shows the cost per use, per day, per month and per year.
Open the free Electricity cost of appliances
Worked example 3: a heater, an always-on device and an EV charger
Heating appliances and always-on appliances cost money in different ways. Using the House of Calculator electricity cost calculator at the example price of 26p per kWh:
| Appliance | Power | Use | Energy per year | Cost per year | Cost per month |
|---|---|---|---|---|---|
| Electric heater | 2,000 W | 3 hours a day, 5 days a week | 1,564.3 kWh | £406.71 | £33.89 |
| Device left on all day | 60 W | 24 hours a day, 7 days a week | 525.6 kWh | £136.66 | £11.39 |
| Home EV charger | 7,000 W | 7 hours, 3 days a week | 7,665 kWh | £1,992.90 | £166.08 |
The heater costs 52p an hour to run, and one three-hour session is £1.56. The 60 W device costs only about 2p an hour but runs for 8,760 hours a year, so it adds up to over £136. The EV figure is for 7 kW delivered for 7 hours three times a week; if your charger or car draws less, or you charge on a cheaper night tariff, enter your own numbers.
Step-by-step: finding the real wattage
The label gives the maximum rating, but real use is often lower. To get a better figure:
- Check the label for watts. If it only shows amps (A), multiply amps by volts (230 V in the UK) to get watts.
- Plug the appliance into a plug-in energy monitor and run a normal cycle.
- Read the kWh figure after the cycle. That is your real energy use for one run.
- Multiply by your price per kWh, and by the number of runs per week or year.
A fridge or freezer is a good example. Its motor switches on and off, so a label of, say, 150 W does not mean 150 W for 24 hours. A monitor reading over a full day gives the true daily kWh.
Common mistakes when working out running costs
- Mixing up watts and kilowatts. 3,000 W is 3 kW. Forgetting to divide by 1,000 makes the answer 1,000 times too large.
- Using minutes as hours. 15 minutes is 0.25 hours, not 15.
- Assuming full power all the time. Ovens, kettles, heaters and fridges cycle, so the label overstates average use. A thermostat-controlled heater may run far less than its hours “on”.
- Ignoring standby. Small standby loads, such as a television or console left on, are easy to overlook but run all day.
- Forgetting the unit is per kWh, not per hour. Your tariff price is for energy used, not for time.
- Leaving out the standing charge. It is a separate daily fee, so it never changes with the appliance you choose.
Comparing an old appliance with a new one
Say an old appliance uses 300 kWh a year and a new model’s energy label shows 180 kWh a year. The difference is 120 kWh. At 26p per kWh, that is 120 × £0.26 = £31.20 a year (a hand calculation). If the new model costs £400 more, the running cost saving alone would take well over a decade to pay it back, so the decision would rest on other things too, such as reliability or capacity. Run the same sum with your own tariff and the labelled kWh figures, because both numbers vary.
For more on turning a saving into a percentage, see How to calculate percentages. For a similar cost-per-use calculation on the road, Fuel cost and mpg: UK gallons, litres and trip costs explains fuel costs. If you are planning a decorating or home project that involves heating or drying time, How much paint, wallpaper or tile do you need? helps with the materials side.
Glossary of terms
- Watt (W): the rate at which an appliance uses energy at a given moment.
- Kilowatt (kW): 1,000 watts.
- Kilowatt-hour (kWh): the energy used by 1 kW running for one hour. This is the billing unit.
- Unit rate: the price per kWh on your tariff.
- Standing charge: a fixed daily charge for being connected, regardless of use.
- Standby power: the small amount of electricity an appliance draws while switched on but not in use.
- Off-peak tariff: a tariff with cheaper rates at certain times, which may suit appliances you can schedule.
When to check with your supplier or a professional
Your price per kWh, any time-of-use rates and your standing charge are set out on your bill, in your supplier’s app and in your contract, so use those rather than a general figure. If an appliance appears to use much more than its label suggests, it may be faulty, and a qualified electrician or the manufacturer can advise. Never open or modify mains appliances yourself.
Frequently asked questions
How do I work out the cost of running an appliance?
Multiply its power in kilowatts by the hours used, then by your price per kWh.
What is a kWh?
A kilowatt-hour is the energy used by a 1,000 W appliance running for one hour. It is the unit your supplier bills you in.
Where can I find my price per kWh?
On your bill, your supplier’s app or your tariff documents. It varies by supplier and tariff.
Does the calculation include the standing charge?
No. The standing charge is a daily fee that you pay regardless of use, so it is separate from the cost of running an appliance.
How do I convert amps to watts?
Multiply amps by volts. UK mains is nominally 230 V, so a 13 A rating corresponds to a maximum of about 2,990 W. That is the limit, not necessarily what the appliance uses.
Is it cheaper to leave something on standby or switch it off?
Switching it off at the wall saves the standby draw, which can be small per device but runs all day. A plug-in monitor shows the figure for each one.
Why is my electricity bill higher than my appliance estimates?
Bills include every appliance, lighting, heating and the standing charge. Estimates for individual items are only part of the total.
Does running an appliance at night cost less?
Only if your tariff has a cheaper off-peak rate. On a single flat rate, the cost per kWh is the same at any hour.