How to Calculate the Electricity Running Cost of an Appliance
Working out what an appliance costs to run is one of the most practical things you can do when managing household energy bills. A tumble dryer, an electric shower, a games console left on standby — they all draw power, and that power costs money. This guide explains the calculation step by step, covers the most common sources of confusion, and helps you get accurate estimates rather than rough guesses.
Understanding watts, kilowatts and kilowatt-hours
Power is measured in watts (W) or kilowatts (kW), where one kilowatt equals one thousand watts. This is the rate at which an appliance uses energy at any given moment — like the speed of a car rather than the distance it has covered.
Energy is measured in kilowatt-hours (kWh). One kilowatt-hour is the amount of energy used when a one-kilowatt appliance runs for one hour. This is what appears on your electricity bill, and it is what your energy supplier charges you for. A 2 kW electric heater running for two hours uses 4 kWh. A 100 W light bulb running for ten hours also uses 1 kWh.
Your electricity unit rate — shown on your bill in pence per kWh — is the price you pay for each kilowatt-hour. In the UK this varies by supplier, tariff and region, but as a rough guide it has typically sat between 20p and 35p per kWh in recent years, though prices have fluctuated significantly.
The calculation
The formula is straightforward:
Energy used (kWh) = Power (W) ÷ 1000 × Hours used
Cost = Energy used (kWh) × Unit rate (p/kWh)
To get a daily, monthly or annual figure, multiply accordingly — or use the Watt Wise tool which does this automatically.
Worked examples
Example 1: Electric kettle
A typical electric kettle is rated at around 3,000 W (3 kW). If you boil it six times a day for an average of two minutes each time, that is twelve minutes of use — 0.2 hours. Energy used: 3 × 0.2 = 0.6 kWh per day. At 28p per kWh, that is about 17p a day, or roughly £5 a month.
Example 2: Fridge-freezer
A fridge-freezer may be rated at 150 W, but it does not run continuously — its compressor cycles on and off. A typical modern fridge-freezer uses around 1 to 1.5 kWh per day. At 28p per kWh, that is around 28p to 42p per day, or roughly £10 to £13 per month. This is why the rated wattage alone can be misleading for thermostatically controlled appliances.
Example 3: Electric shower
Electric showers are high-wattage but short-duration. A 9,500 W (9.5 kW) shower used for eight minutes (0.13 hours) uses 9.5 × 0.13 = 1.24 kWh. At 28p per kWh, one shower costs about 35p. Two showers a day would add up to around £21 per month.
Why rated wattage is not always actual consumption
The wattage printed on an appliance or in its manual is the maximum power draw — what it uses at full load. Many appliances do not run at full power all the time. A fridge cycles. A washing machine uses maximum power during the heating phase but much less during the spin. A laptop under light use draws far less than its charger's rated output. For cycling appliances, an energy monitor plugged in at the wall gives a much more accurate picture than the nameplate wattage alone.
Standing charges and other bill components
Your electricity bill contains more than just the unit rate. The standing charge is a fixed daily fee — typically between 40p and 60p per day — that covers the cost of being connected to the grid, regardless of how much electricity you use. This is separate from your consumption cost and should not be included when calculating the running cost of a specific appliance.
Some tariffs also include different rates at different times of day (time-of-use tariffs), where electricity is cheaper overnight. If you are on such a tariff, use the rate that applies when the appliance actually runs.
Comparing appliances
One of the most useful applications of this calculation is comparing appliances. A 500 W slow cooker running for eight hours uses 4 kWh. An 800 W microwave running for twenty minutes uses 0.27 kWh. For reheating food, the microwave is dramatically cheaper even though its wattage is higher — because duration matters as much as power.
Similarly, an A-rated appliance versus a D-rated equivalent can show a significant difference in annual running costs. When replacing a large appliance like a washing machine or fridge, calculating the annual energy cost at your current unit rate and comparing it against the purchase price of a more efficient model often reveals that the efficient model pays for the difference within a few years.
Tips for reducing appliance running costs
- Use appliances at off-peak times if you are on a time-of-use tariff. Overnight electricity can be significantly cheaper.
- Fill appliances before running them. A full dishwasher or washing machine uses roughly the same energy as a half-full one.
- Lower the temperature. Washing at 30°C instead of 60°C uses significantly less energy for the heating phase.
- Avoid standby where practical. Devices on standby still draw power, sometimes several watts continuously.
- Use a plug-in energy monitor for appliances you suspect are high consumers. The reading will be more accurate than an estimate based on rated wattage.
Common questions
How do I find my electricity unit rate?
Your unit rate appears on your electricity bill, usually expressed as pence per kWh (p/kWh). It may also be in the account section of your supplier's app or website. If you have a smart meter, your in-home display may also show it.
What is the average electricity unit rate in the UK?
Unit rates change regularly and vary by supplier, region and tariff type. Check your own bill for the most accurate figure — using a national average can give misleading results, especially if prices have recently changed.
Does the calculation work for three-phase appliances?
Most domestic appliances are single-phase. Three-phase equipment is more common in commercial or industrial settings. The same kWh calculation applies, but the power draw must account for all three phases.
Is it worth switching off appliances at the wall?
For appliances with a standby mode that draws several watts continuously, yes. For appliances with near-zero standby draw, the saving is negligible. A plug-in energy monitor can tell you which category a specific appliance falls into.
Related tools and guides
Use the Watt Wise electricity cost calculator to run these calculations instantly for any appliance. For budgeting your overall household costs, try Budget Wise. To track your savings goals, see Goal Saver. Browse all LibraireStudios guides for more practical help.
LibraireStudios tools are designed for everyday planning and general information. Energy prices change frequently — always use your current tariff rate for accurate results. Where energy decisions have financial or contractual consequences, consult your supplier or an energy adviser.