Electricity cost calculator: kWh and running cost of each appliance

An appliance's electricity use in kWh is its power in kilowatts multiplied by the hours it runs. A 2,000 W appliance on for 2 hours uses 4 kWh; the cost is that figure times the unit rate on your bill.

The formula: watts × hours ÷ 1,000

A kilowatt-hour (kWh), the "unit" on an electricity bill, is the energy used by one kilowatt of power for one hour. For an appliance, multiply its power in watts by the hours it runs and divide by 1,000. A 2,000 W heater on for 2 hours uses 2,000 × 2 ÷ 1,000 = 4 kWh.

The power is on the rating plate or in the manual. If the plate only gives voltage and current, the power of a resistive appliance is their product: 230 V × 10 A = 2,300 W.

Annual use by power rating and hours of use

Energy used in a year (365 days) by an appliance running at full power for the stated hours each day. Example: a 2,000 W appliance on for 2 hours a day uses 1,460 kWh a year. Multiply by the unit rate on your bill for the cost.

Per day, per month, per year

The calculator counts a year as 365 days and spreads the days of use across the week. The heater above, used 2 hours every day, comes to 1,460 kWh a year. A 60 W appliance left on 8 hours a day uses 0.48 kWh a day and 175.2 kWh a year. A 2,000 W appliance used for one hour on 3 days a week uses 312.9 kWh a year.

The monthly figure is the yearly figure divided by 12, an average month. For a heater used only in winter, work it out over the days it actually runs.

From kWh to pounds: unit rate and standing charge

Ofgem explains the two parts of a bill: the unit rate, the price per kWh of electricity, and the standing charge, a daily charge you pay even if you use no energy that day. Only the unit rate grows with use, so the running cost of an appliance is kWh × unit rate. Ofgem's price cap limits both for standard variable tariffs and is reviewed every three months; it does not limit the total bill.

The calculator has no default price, because it depends on your tariff. Bills usually show the unit rate in pence: enter 25p as 0.25. At £0.25 per kWh, a figure chosen only for this example, the heater costs £365.00 a year and the 60 W appliance £43.80.

Why your bill differs from the result

• Thermostats: heaters, immersion heaters and fridges switch off once they reach temperature. The calculation at full power is a maximum.
• Standby: an appliance left plugged in keeps drawing power. 5 W round the clock is 43.8 kWh a year, £10.95 at the example rate.
• Variable power: a computer or a variable-speed pump does not always run flat out. A plug-in energy monitor shows the real figure after a few days.

Special cases

• Caravans and 12 V: with direct current the power is still voltage × current. A 12 V appliance drawing 4 A uses 48 W, or 1.15 kWh if it runs for 24 hours.
• Several appliances: add a line for each one and the calculator shows each appliance's share of the total.
• Economy 7 and other time-of-use tariffs: run the calculation once for each rate, with the hours spent in each period.

Common mistakes

• Mixing up kW and kWh: kW is the power an appliance draws, kWh the energy the meter records.
• Forgetting to divide by 1,000: 1,500 W for 2 hours is 3,000 Wh, so 3 kWh.
• Entering the unit rate in pence as pounds: 25 instead of 0.25 multiplies the cost by 100.

For air conditioning, the BTU to kW converter gives cooling capacity, which is not the electrical power. A better-insulated home uses less energy for heating and cooling: the insulation thickness has its own calculator, and so do ventilation rates.