Electricity bills use kilowatt-hours, usually written kWh. It sounds technical, but a kilowatt-hour is simply a way of measuring how much electrical energy was used over time. Once you know an appliance's power rating and roughly how long it runs, you can make a sensible estimate of its energy use.
The important distinction is between power and energy. Watts tell you how quickly electricity is being used at a particular moment. Kilowatt-hours tell you how much has been used across a period. A high-power appliance run briefly can use less energy than a low-power appliance left on for days.
Know the difference between watts and kWh
Power is measured in watts (W). One kilowatt (kW) equals 1000 watts. A 2000W heater has a rated power of 2kW.
Energy used over time is measured in watt-hours (Wh) or kilowatt-hours (kWh). Running a steady 1000W load for one hour uses 1kWh. Running a steady 100W load for ten hours also uses 1kWh.
The US Energy Information Administration explains this distinction between power and energy in its electricity measurement guide. You should not write 'kW per hour' when you mean kWh.
Use the basic energy formula
The formula for a constant-power device is kWh = watts × hours ÷ 1000. You can also use kWh = kilowatts × hours when the power is already in kW.
Suppose a 500W appliance runs continuously for three hours. Calculate 500 × 3 ÷ 1000 = 1.5kWh.
This is an estimate for a device actually drawing 500W throughout those three hours. Many appliances cycle on and off or change their power consumption, so the result may not match a meter exactly.
Convert minutes into hours
If you run a device for thirty minutes, the time is 0.5 hours. Ten minutes is 10/60, approximately 0.1667 hours.
For a 1200W appliance used for fifteen minutes, convert fifteen minutes to 0.25 hours. Then 1200 × 0.25 ÷ 1000 = 0.3kWh.
Do not multiply watts by the number of minutes directly and divide only by 1000. You must convert the time into hours first or use a formula that includes the minutes-to-hours conversion.
Example: an electric heater
Imagine a 2kW electric heater working continuously at full power for four hours. Its calculated energy use is 2 × 4 = 8kWh.
Real thermostatic heaters often switch their heating element on and off. If the element is active for only half that time, the actual energy use attributable to the heating element will be lower.
A rating describes maximum or nominal power, not necessarily the average power measured across a whole afternoon. For a more accurate estimate, use measured consumption or manufacturer energy figures.
Example: an LED light
Suppose a 10W LED bulb runs for six hours every evening. Daily use is 10 × 6 ÷ 1000 = 0.06kWh.
Over thirty days at the same usage, multiply 0.06 by 30 to get 1.8kWh. This shows why even a small device uses measurable energy over time.
You should not confuse the bulb's brightness with its wattage. Modern efficient bulbs can produce substantial light at much lower power than older designs.
Work out estimated running cost
Once you have kWh, multiply by the electricity unit price in pounds per kWh. For example, 1.5kWh at an illustrative 25p per kWh costs 1.5 × £0.25 = £0.375, or 37.5p.
That calculation covers only the energy unit charge. Actual bills may also include standing charges and other tariff-specific components.
Use the actual unit price shown on your bill or tariff rather than treating the illustrative figure as a current market rate. Prices change, and different households pay different rates.
Monthly and annual calculations
Suppose a 100W device operates for five hours a day. Daily energy is 100 × 5 ÷ 1000 = 0.5kWh.
Across thirty days, that is 15kWh. Across 365 days of identical use, it is 182.5kWh. These figures are straightforward projections, not measurements.
If actual usage varies by season or weekday, calculate those periods separately. A heater used in January and not in July should not be projected from its busiest week across the whole year.
Use ratings carefully for variable appliances
A kettle rated 3000W may operate near that power while boiling, but only for a short time. A washing machine changes power through heating, tumbling and spinning.
Fridges and freezers cycle to maintain temperature. Their annual energy labels or actual monitored consumption can be more useful than multiplying a single rated wattage by twenty-four hours every day.
Citizens Advice specifically notes that it is harder to estimate energy use for appliances with changing demand from their power rating alone. Check an energy label or measured cycle figure when available.
Read a home energy monitor
A plug-in energy meter designed for the appropriate appliance can measure cumulative kWh as well as instantaneous power. Follow its electrical rating and safety instructions.
Do not connect equipment that exceeds its rated current or use a domestic plug-in monitor on hardwired circuits. For those, use appropriate professional measurement methods.
Leave the meter connected long enough to capture representative use, particularly for devices that cycle. A fifteen-minute reading may not tell you what a fridge uses over a normal day.
Calculate from a smart meter or bill
Your electricity meter records cumulative energy usage. Subtract an earlier reading from a later one to obtain kWh used over that period.
For example, a reading of 12,580kWh followed by 12,623kWh indicates 43kWh of recorded electricity consumption between readings, subject to correct meter register and unit interpretation.
That total covers the whole property, not one appliance. If you want to estimate an individual device's contribution, account for other equipment operating at the same time.
Handle multiple appliances
If a 500W heater and 100W fan run at steady rated power for two hours, together they draw 600W. Energy is 600 × 2 ÷ 1000 = 1.2kWh.
You can instead calculate 1kWh for the heater and 0.2kWh for the fan, then add them. Both methods give the same answer.
Do not add wattage ratings and call the result kWh. Total power needs a time period before it becomes energy.
Frequent calculation mistakes
Mixing minutes and hours is a common source of results sixty times too large. Another is forgetting to divide watts by 1000 before multiplying by hours.
Some people calculate a device's running cost using its power rating multiplied directly by pence per kWh. That is incorrect because the tariff price applies to energy, not instantaneous power.
Rounding every intermediate step also creates errors for low-power devices. Keep enough decimal places until the final cost calculation.
Why actual consumption can differ
Thermostats, standby modes, load size, efficiency and ambient temperature all affect how much electricity devices use. A 2kW oven does not necessarily draw 2kW every moment it is switched on.
The formula still provides a useful baseline, but measured usage is best when you need high accuracy. For business accounting or technical work, use suitable metering and consistent periods.
Citizens Advice's consumer guide offers a practical method for estimating usage and running cost from rated watts and daily hours, with warnings about variable-load appliances.
A simple check before trusting the number
For any estimate, write down the rated watts, the actual running time in hours and the unit price. Calculate kWh first, then money.
If a ten-watt lamp produces a wildly larger daily kWh result than a two-kilowatt heater used for the same time, revisit the conversion. Units help catch arithmetic errors.
The short formula to remember
Multiply watts by hours, divide by 1000, and multiply the resulting kWh by your tariff price if you want an estimated unit cost. Distinguish the rated power from actual average power whenever an appliance cycles.
That is enough to make everyday energy figures less mysterious—and to identify which calculations are estimates rather than promises about your bill.