Guide
How to Calculate the Electricity Cost of an Appliance (With Worked Examples)
How to calculate the electricity cost of an appliance: turn watts into kWh, multiply by your unit rate, and see worked examples in euros and pounds.
To calculate the electricity cost of an appliance, multiply its power in watts by the hours it runs, divide by 1,000 to get kilowatt-hours (kWh), then multiply by your unit rate. In one line: cost = watts × hours ÷ 1,000 × price per kWh. A 100 W device running for 10 hours uses 1 kWh, so it costs one unit on your bill. The catch is that many appliances don't draw their label wattage all the time, so for anything that cycles on and off, measuring beats calculating.
The formula, step by step
Step 1: find the power in watts
Look on the rating plate (usually on the back or underneath), the manual, or the power adaptor. It will show watts (W) or kilowatts (kW). 1 kW = 1,000 W.
If it only shows amps (A), multiply by the supply voltage: about 230 V in the UK and the EU. So 0.5 A × 230 V ≈ 115 W. Treat that as a maximum, not typical use.
Step 2: convert to kilowatt-hours
kWh = watts × hours ÷ 1,000
A kilowatt-hour is the unit on your bill: 1,000 watts for one hour, or 100 watts for ten hours, or 10 watts for a hundred hours.
Step 3: multiply by your unit rate
Cost = kWh × unit rate
Use the unit rate on your own bill. For orientation:
- UK: Ofgem's price cap for 1 October to 31 December 2026 sets an average direct debit electricity unit rate of about 26.32p per kWh (it varies by region and payment method). Ofgem lists the rates on its price cap unit rates page.
- EU: Eurostat figures for the first half of 2026, as reported by the Brussels Times, put the EU household average at around €0.28 per kWh, ranging from about €0.11 in Hungary to about €0.39 in Ireland.
Step 4: scale to a day, month or year
Multiply the daily cost by 30 for a rough month or 365 for a year. If use varies by season, calculate each season separately.
Quick reference: cost per hour by power
At full power, cost per hour = kW × unit rate. Here it is at the two reference prices above:
| Power | kWh per hour | Cost per hour at 26.32p | Cost per hour at €0.28 |
|---|---|---|---|
| 10 W | 0.01 | 0.26p | €0.003 |
| 60 W | 0.06 | 1.6p | €0.017 |
| 100 W | 0.1 | 2.6p | €0.028 |
| 500 W | 0.5 | 13.2p | €0.14 |
| 1,000 W (1 kW) | 1 | 26.3p | €0.28 |
| 2,000 W (2 kW) | 2 | 52.6p | €0.56 |
| 3,000 W (3 kW) | 3 | 79.0p | €0.84 |
Swap in your own rate: just multiply the kWh column by it.
Worked examples (hypothetical, arithmetic shown)
These are illustrations, not measurements of specific products. They use 26.32p and €0.28 per kWh.
A 60 W lamp on for 5 hours a day
- 60 × 5 ÷ 1,000 = 0.3 kWh per day
- UK: 0.3 × 26.32p = 7.9p per day; × 365 = 109.5 kWh = £28.82 a year
- EU: 0.3 × €0.28 = €0.084 per day; 109.5 kWh × €0.28 = €30.66 a year
Swap it for a 7 W LED and the same hours use 0.035 kWh a day, about 12.8 kWh a year: £3.36 or €3.58.
A 3 kW kettle boiling for 3 minutes
- 3 minutes = 0.05 hours. 3,000 × 0.05 ÷ 1,000 = 0.15 kWh per boil
- UK: 0.15 × 26.32p ≈ 4p per boil
- EU: 0.15 × €0.28 ≈ 4 cents per boil
- Four boils a day for a year: 0.6 kWh × 365 = 219 kWh = £57.64 or €61.32
High wattage, short time: the kettle costs little per use but adds up with frequency. Boiling only the water you need shortens the time.
A desktop PC at 150 W, 8 hours a day, 5 days a week
- 150 × 8 ÷ 1,000 = 1.2 kWh per working day
- 1.2 × 5 × 52 = 312 kWh a year
- UK: 312 × 26.32p = £82.12 a year; EU: 312 × €0.28 = €87.36 a year
A device drawing 5 W in standby, all year
- 5 × 24 ÷ 1,000 = 0.12 kWh per day; × 365 = 43.8 kWh a year
- UK: £11.53 a year; EU: €12.26 a year
Small loads matter when they never stop. Our standby power guide covers which devices are worth switching off at the wall.
A space heater
A 2 kW heater running flat out for 4 hours uses 8 kWh: about £2.11 a day at 26.32p or €2.24 at €0.28. In practice the thermostat cycles it, so real use is usually lower. The full breakdown, plus safety rules for plugs and sockets, is in our space heater running cost guide.
Appliances where the simple formula misleads
The formula assumes constant power. Many appliances don't work that way:
- Fridges and freezers run their compressor in cycles. Multiplying the rated watts by 24 hours overstates use. The EU energy label shows annual consumption in kWh per year; multiply that by your unit rate instead. For example, a label showing 250 kWh a year would cost £65.80 at 26.32p, or €70.00 at €0.28.
- Washing machines and dishwashers heat water at the start and then use much less power. The current EU energy label shows energy per 100 cycles on the eco programme, so divide by 100 for a per-cycle figure. A hypothetical 50 kWh per 100 cycles is 0.5 kWh a wash: 13.2p or €0.14. Hotter programmes use more.
- Heaters, irons and ovens with thermostats switch their element on and off to hold temperature.
- TVs, consoles and computers vary with what they're doing.
For these, measure energy (kWh) over a whole cycle, day or week, then multiply by your rate. That is more accurate than any label. Our guide to measuring appliance energy use compares plug-in meters, smart plugs and smart meters.
A checklist for accurate results
- Use your own unit rate, not an average. If you're on a time-of-use tariff, cost each band separately: kWh in that band × that band's rate.
- Ignore the standing charge. It's fixed per day whatever you use (54.83p a day on average under the UK cap for October to December 2026), so it doesn't belong in an appliance calculation.
- Use real hours. Estimate honestly, or time it for a week.
- Measure cycling appliances over at least a full day, ideally a week.
- Check the units. Watts are power (a rate); kWh are energy (an amount). A 2 kW heater doesn't use "2 kWh"; it uses 2 kWh for every hour at full power.
- Round at the end, not in the middle, to avoid small errors compounding.
Common mistakes
- Mixing up kW and kWh. Your bill charges for kWh.
- Forgetting to divide by 1,000. 100 W for 10 hours is 1 kWh, not 1,000 kWh.
- Using pence and pounds (or cents and euros) inconsistently. 26.32p is £0.2632.
- Treating label watts as typical. It's usually a maximum.
- Costing hard-wired appliances by guesswork. For an electric shower or oven you can't use a plug-in meter; use the rating plate for a ceiling estimate and your home meter for the real effect. See what uses the most electricity at home for typical big users.
Where Wattly fits
The Wattly smart plug does this arithmetic for you for any plug-in appliance: it shows live watts, cost per hour, day, week or month, and daily to yearly usage history in the Tuya Smart app. That's most useful for exactly the appliances where the formula misleads, such as fridges, heaters and computers. It can't measure hard-wired appliances, and it only saves money if you act on what it shows. For the honest view, read do smart plugs save money, or see the Wattly smart plug and current price.
Related reading
Frequently asked questions
What is the formula for the running cost of an appliance?
Multiply the power in watts by the hours used and divide by 1,000 to get kWh, then multiply the kWh by your unit rate. For example, a 100 W device used for 10 hours uses 1 kWh, which costs whatever one unit costs on your bill.
How much does it cost to run a 1 kW appliance for an hour?
A 1 kW appliance running at full power for one hour uses 1 kWh, so it costs exactly your unit rate. At the UK price cap rate of 26.32p per kWh for October to December 2026 that is about 26p; at the EU average of around 0.28 euros it is about 28 cents.
Why is my calculated cost higher than what the appliance really uses?
The wattage on a label is usually the maximum. Fridges, heaters with thermostats, washing machines and dishwashers cycle on and off or vary their power, so their real average is lower. Measuring energy over a full day or cycle gives a truer figure.
Should I include the standing charge in appliance costs?
No. The standing charge is a fixed daily amount you pay regardless of how much you use, so it does not change when you switch an appliance on or off. Use only the unit rate when costing an appliance.
How do I convert amps to watts?
Multiply the amps by the supply voltage, which is about 230 V in the UK and EU. A label showing 0.5 A means a maximum of roughly 115 W, but typical use is often lower.