kWh cost calculator UK: convert usage into bill cost
Use this calculator when you know your kWh and unit rate but need to estimate the usage part of a UK energy bill before checking standing charge and billed days.
kWh to cost calculator
Use this as a sense check. Some bills include more than one unit rate, an Economy 7 split, account adjustments or balances from earlier periods.
When this calculator is useful
- Checking whether the variable part of the bill feels broadly right.
- Comparing one period against another when the tariff stayed the same.
- Working out whether a bill is mostly a usage problem or mostly a standing-charge problem.
- Preparing a clearer supplier message with your own rough maths attached.
When it is not enough
- The bill covers a tariff change mid-period.
- You are on a multi-rate tariff such as Economy 7.
- The supplier is correcting earlier estimated bills.
- The monthly payment is the problem rather than the billed usage itself.
What to check after the maths
- Look at the bill dates and billed days.
- Check whether the reading used was actual or estimated.
- Confirm the meter serial number matches your meter.
- Check whether the bill shows one unit rate or multiple rates.
- Only then decide whether you need the complaint route.
Official and reference sources
The formula, and the four things people get wrong
cost = ( units × unit rate ÷ 100 ) + ( days × standing charge ÷ 100 )
| Mistake | What happens | Fix |
|---|---|---|
| Forgetting the standing charge | Your figure is £26 a month under on dual fuel. | Always add days × p/day. |
| Using the headline cap figure as a rate | £1,862 is an older-usage annual illustration at July–September 2026 rates, not a unit rate or the revised Ofgem headline. Nothing works if you multiply by it. | Use p/kWh from your bill. |
| Counting days wrong | Off by a few pounds, and it hides genuine double-billed days. | Use the supplier’s billed days and check that consecutive periods do not overlap. |
| VAT confusion | Out by exactly 5%. | Ofgem's published cap rates include VAT. Some bills quote rates excluding it and add 5% at the end. Check which yours does. |
Gas has one extra step
Gas meters measure volume, not energy. If your meter reads in cubic metres, the bill converts it:
kWh = m³ × 1.02264 × calorific value ÷ 3.6
The calorific value is printed on your bill and is typically around 39–40. The volume correction factor 1.02264 is standard. An imperial meter reading in hundreds of cubic feet is multiplied by 2.83 first to get cubic metres. In practice: use the kWh figure the bill has already calculated, and only do this arithmetic if you are checking whether the conversion itself is right.
What a kWh buys
Useful for working out whether a specific appliance is worth worrying about. At the current GB average electricity rate of 26.11p per kWh:
| Appliance | Rating | Per hour | Per month if used |
|---|---|---|---|
| LED bulb | 0.01 kW | 0.3p | 6h/day: 4.7p |
| Laptop | 0.05 kW | 1.3p | 8h/day: £3.13 |
| Fridge-freezer | — | — | continuous: £7.83 |
| Washing machine | — | 21p/load | 5 loads/week: £4.55 |
| Electric oven | 2.0 kW | 52p | 1h/day: £15.67 |
| Tumble dryer | 2.5 kW | 65p | 4 loads/week: £10.44 |
| Electric shower | 9.0 kW | £2.35 | 10 min/day: £11.75 |
| Fan heater | 2.0 kW | 52p | 4h/day: £62.66 |
| Immersion heater | 3.0 kW | 78p | 2h/day: £47.00 |
| EV charging | 7.0 kW | £1.83 | 200 miles/week: £60.05 |
Ratings vary between models — check the label on yours and substitute the real figure. Used here for orders of magnitude.
Anything under about £5 a month is noise. Turning off standby saves real money in aggregate but it will not explain a £100 jump. The three bold rows will. When a bill changes sharply, look for a kilowatt-scale load running for hours, not a watt-scale one running all day.
Frequently asked questions
How do I convert kWh to cost?
Multiply the kWh by the unit rate in pence and divide by 100. Then add the standing charge separately: days billed times the daily rate, divided by 100. Missing that second part is the most common mistake.
What is a kWh in practical terms?
One kilowatt hour runs a 1kW appliance for an hour. At 26.11p that is about nine kettle boils, half a tumble dryer cycle, or a fridge-freezer for a day.
Why is gas so much cheaper per kWh?
Gas is 7.33p per kWh against 26.11p for electricity under the current cap — roughly three and a half times cheaper. It is why moving heating or hot water from gas to electric changes a bill so dramatically.