kWh and electricity cost calculator
One kilowatt-hour runs a 1,000 W heater for an hour, a 100 W bulb for ten hours, or a 10 W LED strip for four days. It boils about ten kettles, or drives an electric car roughly six kilometres. It is the unit every energy bill is denominated in, and putting an appliance into it is the fastest way to see whether it is worth worrying about.
Kilowatt-hours are watts times hours divided by a thousand. A 1,000 W heater running three hours a day uses 3 kWh daily and 1,096 kWh a year, about £307 at 28p a unit. The same panel costs a running appliance, an LED installation, and a load that only runs part of the time.
How to calculate kWh
The gap between rated wattage and real consumption is where estimates go wrong. A fridge is rated at its compressor power but runs maybe a third of the time. A washing machine draws most of its energy in the heating phase and very little for the rest of the cycle. A television rated at 120 W might average 60. For anything that cycles or modulates, the rating plate is an upper bound rather than an average, and a plug-in energy monitor left on for a week gives a truer figure than any calculation.
Duty cycle is what the annual mode exists for. Fridges, freezers, pumps, heating and air conditioning all cycle, and the fraction of time they run swamps every other variable: a freezer in a cold garage runs a fraction as often as the same freezer in a warm kitchen. Appliance energy labels quote an annual kWh figure measured under a standard test cycle, which is why the label number and your bill can diverge sharply. The label is a comparison between products, not a prediction about your house.
Two things make the yearly cost the useful output. Small hourly costs are easy to dismiss and annual ones are not: a 3p-an-hour device left on constantly is £263 a year. And the annual figure is the right basis for comparing a replacement. An appliance that saves £80 a year justifies a very different purchase price from one that saves £8. Your bill also carries a standing charge that no appliance calculation will explain, and a time-of-use tariff makes when you run something matter as much as how long.
For an LED run the energy answer is only half of it. The headline saving is real — a 10 W LED replacing a 60 W incandescent cuts that fitting by 83% — but lighting is rarely the largest load in a house, so the household saving is modest. What is understated is heat: a kitchen with twenty halogen downlights was running a kilowatt of lighting and a kilowatt of heating at the same time. Size the driver at 1.2 times the strip wattage, and feed runs longer than five metres from both ends, or the far end dims visibly.
What people use it for
- Working out what an appliance costs to run
- Comparing an old appliance against a replacement
- Checking a standby load is worth eliminating
- Sizing a solar or battery system from real consumption
- Costing electric heating against gas
- Estimating what a fridge or freezer uses over a year
- Costing an LED strip run and sizing its driver
- Splitting a shared electricity bill by what each device actually uses
- Costing a pond or pool pump for a season rather than a full year
- Checking an appliance energy label against your own running hours
- Comparing LED against halogen on running cost, not just on watts
Questions
Multiply watts by hours of use and divide by 1,000. A 500 W device for 2 hours uses 1 kWh.
A 1 kW heater for an hour, a 10 W LED for four days, or about ten kettle boils.
About 56p an hour at 28p per kWh. Four hours a day is £2.24, or over £800 a year.
The one on your bill, including VAT, not the headline advertised rate. They frequently differ.
Rarely, and duty cycle is the name for the gap: the fraction of time a device actually runs. Anything with a thermostat or a motor cycles, so the rating plate is a peak rather than an average. A fridge compressor typically runs 30–40% of the time, which puts its annual use far below its rating times 24 hours.
A plug-in monitor with a cumulative kWh reading, left in place for a week and multiplied up, beats any calculation for a cycling appliance.
Standing charges, VAT, and consumption you have not counted. This tool costs one device; heating, hot water and cooking usually dominate a household bill.
Individually no, collectively sometimes. A house full of 2 W standby loads is perhaps £30 a year. It is not counted here: standby watts × 8,760 ÷ 1,000 gives its own annual kWh.
Per kilowatt-hour, usually three to four times gas. Heat pumps change that arithmetic because they deliver three or four units of heat per unit of electricity.
Labels use a standard test cycle for comparison between products. Your ambient temperature, door openings and settings all move the real figure.
It is printed per metre; commonly 4.8, 9.6 or 14.4 W/m. Multiply by the length for the total.
At least 20% above the total strip wattage. Running a driver at exactly its rating shortens its life.
Voltage drop in the strip copper. Feed from both ends, or split into runs of five metres or less.
Less, but not always in proportion. PWM dimming, which is what most LED drivers use, scales close to linearly with the setting. Some analogue and trailing-edge dimmers waste part of the difference as heat instead, so half brightness is not reliably half the watts.