Watts and kilowatt-hours are not the same thing
Watts are a rate, not an amount, and mixing the two up is the single most common mistake in reading an appliance label. Think of watts as a car's speed and kilowatt-hours as the distance it actually covers: a 3,000W kettle is not "3,000 units of electricity" any more than a car doing 70mph is "70 miles". It is drawing energy at that rate for as long as it runs, and only the combination of rate and time tells you how much energy was actually used.
Divide the wattage by 1,000 to get kilowatts, multiply by the hours it ran, and the result is the kilowatt-hours your meter and your bill both use as the unit of sale. A 2,000W appliance run for three hours uses 6 kWh, whatever it is, a heater, a washing machine's heating element or three separate one-hour uses of something smaller. Multiply that by your price per kWh and you have the cost. Nothing about that formula cares what the appliance actually is; it only cares about power and time.
Small appliances make the same point from the other direction. A phone charger left plugged in and switched on might draw 5W, which sounds negligible next to a kettle's 3,000W. Run it for eight hours overnight, though, and it only uses 0.04 kWh, a fraction of a penny at typical rates, while the kettle's ten minutes a day already costs several times as much across a year. The gap in wattage between the two is enormous; the gap in yearly cost is much smaller than that wattage difference suggests, because the kettle's high draw only lasts minutes and the charger's low draw runs for hours.
Why heating appliances cost so much more
Look at a wattage label on its own and the ranking looks obvious: kettles, tumble dryers, electric showers and grills sit at the top, often 2,000–3,000W or more, while a TV or a games console sits down at 60–200W. The reason is resistive heating. Turning electricity directly into heat has no efficient shortcut, so anything that heats water or air draws a very high rate of power while it runs. Electronics, lighting and control boards, by contrast, only need enough power to drive a chip or a light source, which is inherently a much smaller job.
What that means in practice is that wattage alone does not predict cost, because the appliances at the top of the wattage table tend to run for minutes rather than hours. A 3,000W kettle boiling for ten minutes a day and a 60W TV left on for four hours can end up costing roughly the same over a year, despite one appliance being fifty times more powerful than the other on paper. Time of use matters just as much as the label.
It also explains why a handful of appliances dominate most household electricity bills. The Energy Saving Trust names washing machines, dishwashers and tumble dryers as the single biggest group, and fridges and freezers as another large one, between them accounting for a large share of a typical bill, not because any one cycle is expensive but because heating water or air is inherently power-hungry and a fridge-freezer runs continuously all year. Comparing your own appliance against the typical figures in the table below is a quick way to see whether it is one of the ones actually worth paying attention to.
Standby power: the vampire load
Plenty of devices keep drawing a small trickle of electricity even when they look switched off, powering a clock, a remote-control sensor or a standby light. The Energy Saving Trust estimates that this standby, or "vampire", power adds up to somewhere between 9% and 16% of a typical UK household's electricity use, worth roughly £65 to £100 a year. Games consoles, set-top boxes, routers and phone chargers left plugged in around the clock are the usual culprits, and a smart plug with a schedule, or simply switching off at the wall, is the easiest fix.
Reading your bill's unit rate and standing charge
Every electricity bill splits into two separate charges: a unit rate, in pence per kWh, for the energy actually used, and a standing charge, a fixed amount per day that applies whether or not any electricity was used at all. This calculator only works out the cost of running the appliance itself, using the unit rate; it does not add the standing charge, which is a separate flat cost on top of every bill regardless of what you run.
Unit rates and standing charges are reviewed and capped every three months, and they vary by region and supplier, so always check your current unit rate on a recent bill or in your supplier's online account rather than assuming an old figure still applies.
It is also worth checking whether a bill is based on an actual meter reading or an estimate. An estimated bill spreads a supplier's guess of your usage evenly across the billing period, which can hide the fact that one appliance, run in short, heavy bursts, accounts for far more of the total than an averaged figure suggests. Submitting a real reading, or reading a smart meter directly, keeps the unit rate honest and makes a figure from this calculator easier to check against an actual bill.
Smart meter sanity checks
A smart meter's in-home display is the fastest way to check a wattage figure rather than trust a label alone. Switch everything off, note the baseline reading on the display, then switch on just the appliance in question and watch how much the live reading jumps: that jump, in kW, is roughly the wattage to enter here (multiply by 1,000 for watts). Checking the overnight baseline, when almost everything in the house should be off, is a good way to catch a bigger-than-expected standby load, or a fridge-freezer whose compressor is working harder than its rating plate suggests.
Questions people ask
What's the difference between watts and kilowatt-hours?
Watts measure the rate at which an appliance draws electricity, at any given instant. Kilowatt-hours measure the total energy used over time, which is what a bill actually charges for. A high-wattage appliance used briefly and a low-wattage one left running for hours can use the same number of kilowatt-hours.
Why does the calculator ask for hours per day and days per week separately?
Because most appliances are not used every single day. Separating the two lets you describe something used for an hour every evening, or a washing machine run four times a week, without having to average it into a single misleading daily figure. Leave the days field at seven for anything that runs continuously, like a fridge-freezer.
Why is my fridge-freezer's power draw hard to pin down?
A fridge-freezer's compressor cycles on and off to hold its temperature rather than running continuously, so its rating-plate wattage overstates what it actually draws averaged across a day. Typical measured use for a combination unit works out at around 1 kWh a day, which is the figure used in the comparison table, rather than a plain watts-times-hours sum.
Does this include VAT?
Yes, as long as the unit price you enter is the one printed on your bill, which is already VAT-inclusive for domestic customers in the UK. This calculator does not add or remove VAT itself; it simply multiplies the price you give it by the energy used.
How much does standby power actually cost?
The Energy Saving Trust puts it at roughly £65 to £100 a year for a typical UK household, around 9–16% of total electricity use, spread across devices like set-top boxes, routers, games consoles and chargers left permanently plugged in.
Where do I find my exact unit rate?
On a recent electricity bill or in your supplier's online account, listed separately from the standing charge, usually in pence per kWh. Rates are reviewed every three months and vary by region and supplier, so check the current one rather than reusing an old bill.
Why does a low-wattage appliance sometimes cost more than a high-wattage one?
Because cost depends on energy, not power: a 60W TV left on for six hours uses more energy, 0.36 kWh, than a 3,000W kettle boiled for five minutes, 0.25 kWh. A high wattage only costs a lot if it is also sustained for a meaningful stretch of time.
This calculator estimates the running cost of a single appliance from its wattage, hours of use and your own unit price. It excludes the standing charge, does not account for time-of-use or off-peak tariffs, and the comparison table uses typical published figures rather than a measurement of your own appliances.