Almost every figure on your energy bill, and almost every running cost on this site, comes back to one small unit: the kilowatt hour, written kWh and often just called a unit. Understand what it is and what one costs, and the whole bill stops being a mystery. You can price any appliance in your head, spot which habits actually matter, and tell real savings from the ones that barely register. It takes about two minutes to get straight, and it is the most useful two minutes you can spend on your bills.
The short answer. A kilowatt hour is the energy used by a 1,000-watt appliance running for one hour. It is the unit your meter counts. In mid 2026 one kWh of electricity costs about 26p under the Ofgem price cap, and one kWh of gas about 7p. To cost anything, take its power in kilowatts, multiply by the hours it runs, then multiply by the unit rate. A 2kW heater on for three hours uses 6 kWh, or about £1.57.
What a kilowatt hour actually is
Start with the watt, which measures how fast something uses energy. A dim LED bulb might draw 8 watts; a kettle around 3,000. A kilowatt is simply 1,000 watts, a tidier size for household appliances. The kilowatt hour then adds time to power: it is one kilowatt used for one full hour. So a 1kW electric heater left on for an hour uses exactly 1 kWh. The same heater on for half an hour uses 0.5 kWh, and a 2kW heater on for an hour uses 2 kWh. Power multiplied by time, nothing more.
The reason the kWh matters is that it is the unit your electricity and gas meters count, and the unit your supplier bills you for. Every reading you send in, every figure on the statement, every running cost worth knowing is measured in these units. Learn to think in them and the bill turns from a wall of numbers into something you can actually check.
How much does one kWh cost?
For most homes on a standard variable tariff, the price is set by the Ofgem energy price cap, which caps the rate per unit and the daily standing charge, and changes every three months. For July to September 2026 the cap works out at roughly 26p per kWh of electricity and around 7p per kWh of gas. Those are the figures used across this site, so if your own tariff differs, swap in your rate from your bill and every sum still holds.
| Fuel | Cost per kWh (mid 2026) | What that means |
|---|---|---|
| Electricity (standard rate) | ~26p | The dearest everyday fuel |
| Gas | ~7p | Roughly a quarter the price of electricity |
| Off-peak electricity (Economy 7 night) | ~13p to 15p | Cheaper overnight on the right tariff |
Illustrative rates around the Ofgem cap for July to September 2026. Your own unit rate and standing charge are printed on your bill and tariff sheet.
Two points are worth holding onto. First, a unit of electricity costs several times more than a unit of gas, which is why the same warmth from an electric heater costs far more than from a gas boiler, and it is the whole reason gas heating is cheaper to run than direct electric. Second, that headline rate is not the only thing you pay: every day carries a fixed standing charge whether you use any energy or not, so a very low bill is never zero.
Turning wattage into money
Here is the one sum that prices everything in the house. Take the appliance's power in kilowatts, multiply by the hours you run it, then multiply by your unit rate:
kilowatts × hours × rate = cost
Most appliances show their wattage on a rating plate or near the plug. Divide that figure by 1,000 to get kilowatts. A 2,000-watt appliance is 2kW; a 900-watt microwave is 0.9kW. Then multiply by how long it runs and by the price of a unit. A worked example: a 2kW heater on for three hours uses 2 × 3 = 6 kWh, and at 26p that is about £1.57. A 900-watt microwave for five minutes uses 0.9 × 0.083 = 0.075 kWh, barely 2p.
| Appliance | Power | Time | Energy used | Cost at 26p |
|---|---|---|---|---|
| LED bulb | 8 W | 5 hours | 0.04 kWh | ~1p |
| Kettle boil | 3 kW | 3 minutes | 0.15 kWh | ~4p |
| Washing machine cycle | ~0.7 kWh a load | one cycle | 0.7 kWh | ~18p |
| Fan heater | 2 kW | 3 hours | 6 kWh | ~£1.57 |
| Tumble dryer, conventional | ~3 kWh a load | one load | 3 kWh | ~78p |
Rounded worked examples at 26p per kWh. Appliances that heat cost the most; a plug-in monitor shows your own figures exactly.
Once you have done this a few times, a pattern jumps out that makes the whole bill sensible: anything that makes heat is expensive, and anything that does not is usually cheap. A kettle, a shower, a heater and a tumble dryer all turn electricity into warmth, which takes a great deal of energy, so they dominate the bill. A phone charger, a television or an LED bulb move only a trickle of power, so they cost pennies however long you leave them on. This is why the advice to unplug a charger barely shows up, while cutting a long shower genuinely does. The what uses the most electricity guide ranks the whole house on exactly this logic.
Reading a kWh off the energy label
For appliances that run for long, unpredictable stretches, like a fridge or a washing machine, you do not need to time anything. The energy label gives a figure in kWh per year, already worked out under standard test conditions. Multiply that by your unit rate in pounds and you have the yearly running cost straight away. A fridge-freezer labelled at 200 kWh a year costs 200 × 0.26 = about £52 a year to run. It is the quickest running-cost sum there is, because the hard part is done for you.
kWh, kW and other units that trip people up
The commonest confusion is between the kilowatt and the kilowatt hour. A kilowatt (kW) is a rate, how fast energy is used at a given moment, like the speed on a car's dial. A kilowatt hour (kWh) is a quantity, how much energy was used over time, like the miles the car has covered. A 3kW kettle uses power at a high rate, but because it runs for only a couple of minutes it uses very little total energy. A 0.1kW fridge uses power slowly but never switches off, so over a year it gets through a fair amount. High power does not mean high cost; power multiplied by time does.
You will also meet the watt hour (Wh), which is simply a thousandth of a kWh and turns up on phone and laptop batteries, and megawatt hours (MWh) when people talk about the grid, which are a thousand kWh each. Gas meters sometimes read in cubic metres or hundreds of cubic feet, which your supplier converts to kWh on the bill using a standard calorific value, so you never have to do that conversion yourself.
Why thinking in kWh pays off
The value of all this is that it lets you check your own bill and aim your effort where it counts. When you can price any appliance in a few seconds, you stop worrying about the wrong things and start on the ones that move the needle: the heating, the hot water, the shower and the dryer. It also lets you sanity-check a statement, because an estimated reading that looks wrong will show up once you know roughly how many units your home should be using. If you want to see that number, how much electricity a home uses gives typical annual kWh figures, and a plug-in energy monitor shows the exact draw of anything you plug into it. To turn any figure into money without doing the arithmetic, the kWh to cost calculator does it for you.
Common questions
How much does 1 kWh of electricity cost in the UK?
Around 26p on a standard variable tariff under the Ofgem price cap for July to September 2026. Overnight off-peak rates on tariffs like Economy 7 can be roughly half that, and your own rate is printed on your bill.
What is a kWh in simple terms?
A kilowatt hour is the energy used by a 1,000-watt appliance running for one hour. It is the unit your meter counts and your supplier bills you for. A 2kW heater on for one hour uses 2 kWh.
How do I work out what an appliance costs to run?
Take its power in kilowatts, multiply by the hours you run it, then multiply by your unit rate. A 2kW heater on for three hours uses 6 kWh, which at 26p is about £1.57. Divide any wattage by 1,000 to get kilowatts.
Why is electricity so much more expensive than gas per kWh?
A unit of electricity costs roughly four times a unit of gas, around 26p against 7p in mid 2026. That gap is why gas central heating is cheaper to run than direct electric heating, and why heat pumps, which multiply each electric unit into several units of heat, matter.