The nameplate is a maximum, not a rate of use
A washing machine rated 2,100W does not draw 2,100W for an hour. It draws near that only while the heating element is warming the water, which is a few minutes at the start of the cycle, and very little for the rest of it.
That rating exists so an electrician can size a circuit. It answers "how much could this pull at once", which is a completely different question from "how much does it use in a wash". Multiply the nameplate by the run time and you can easily be out by a factor of three or four, and that is exactly what most running cost calculators do.
So the right input depends on what kind of thing it is
There are three, and they want three different numbers.
- Draws its full rating while on. Kettle, toaster, hob ring, fan heater, immersion, electric shower, hair dryer. The element really is on the whole time, so watts times hours is genuinely right. This is the only case where it is.
- Runs a cycle and stops. Washing machine, dishwasher, tumble dryer, oven, air fryer. Use the kWh per cycle off the energy label, which is a measured figure for a real cycle rather than a peak.
- Holds a temperature. Fridge, freezer, hot tub, storage heater. These cycle on and off to maintain a temperature, so they have a duty cycle rather than a run time. Nameplate times hours is meaningless here. Use kWh over a known period.
The tool asks which kind first and then asks for the right number, and where you have both it shows what the nameplate shortcut would have told you, so the size of the error is visible rather than theoretical.
The things that heat are the entire list
This is the most useful single fact about domestic electricity and it saves an enormous amount of pointless worrying. Anything that turns electricity into heat costs real money. Everything else is close to noise beside it.
A kettle, a tumble dryer, an oven, an immersion heater, an electric shower, a hot tub. One of those running for an hour costs more than a television does in a week. If you are looking for savings, that is where they are, and switching off a phone charger is not going to get you there.
Standby, in proportion
Most things with a clock, a remote or a display draw one to three watts around the clock. That is small per hour and it runs for 8,760 of them a year.
One appliance on standby is pence. A house full of them is a real if modest number. It is worth looking up once, on the rating plate or in the manual, rather than either ignoring it or treating it as the thing that is ruining your bill.
The rate matters as much as the appliance
The same appliance costs roughly a third as much on an off-peak overnight rate as it does at the standard day rate. That is a much bigger lever than replacing the appliance with a more efficient one, and it costs nothing but timing.
So if you have a tariff with a cheap window, running the dishwasher and the dryer inside it is worth more than almost anything else on this page. Put your own rate in rather than leaving the default.
Why no appliance figures are built in
There is no table here saying a tumble dryer uses a particular number of kWh, and that is deliberate rather than an omission.
Those figures vary enormously by model, by age and by which programme is selected, and almost none of the ones circulating online trace back to anything you could check. A made-up number in a running cost tool is the worst kind, because it looks like information and gets quoted onward.
Your appliance carries its own, and they are not hard to find: watts on the rating plateat the back or underneath, and kWh per cycle on the energy label. Those two numbers, plus your own rate off your own bill, are the whole calculation.
The same applies to the rate outside the UK. The arithmetic is identical in every market, so this works anywhere, but only the UK figure has been read at a primary source and so only that one is filled in. Everywhere else it comes off your bill.
Common questions
How much does a 2,000W appliance cost to run for an hour?
Two units, so at 26.11p that is 52p, but only if the appliance genuinely draws 2,000W for the whole hour. Most do not. A kettle or a fan heater does; a washing machine, a dishwasher or an oven does not, because the element only runs part of the time. For those the nameplate answer can be three or four times too high.
Why is the nameplate wattage the wrong number?
Because it answers a different question. The rating exists so an electrician can size the circuit: it is what the appliance could pull at once, at its peak. What it uses over a wash or a bake is a measured figure, and on anything with a thermostat the two are nowhere near each other.
Where do I find the right number?
It depends on the appliance. For anything that runs a cycle, the energy label gives kWh per cycle, which is the real measured figure. For a fridge, freezer or hot tub, use kWh over a known period, either from a plug-in meter or from the label's annual figure. Only for things whose element is genuinely on the whole time is watts times hours correct.
Does standby actually matter?
A little, and it is something to know rather than panic about. Most things with a clock, a remote or a display draw one to three watts around the clock. One appliance is pence a year. A house full of them adds up, and it is a number you look up once rather than guess at forever.
Which appliances actually cost money to run?
The ones that heat. A kettle, a tumble dryer, an oven, an immersion heater, an electric shower and a hot tub all turn electricity straight into heat, and one of them running for an hour costs more than a television does in a week. If you are hunting for savings, the heating appliances are the entire list and everything else is noise.
Why are there no appliance figures built in?
Because the published ones vary enormously by model, age and setting, and almost none of them trace back to anything. A made-up figure in a running cost tool looks like information without being any. Your own appliance carries the real numbers: watts on the rating plate at the back, kWh per cycle on the energy label.