Electricity Cost Calculator

Every electrical appliance has a power rating, measured in watts (W), which indicates how much electricity it consumes each second it is running. A standard electric kettle might use 3,000 W (3 kW), while an LED light bulb might use just 9 W. Energy suppliers charge you not per watt, but per kilowatt-hour (kWh) — also known as a 'unit' of electricity. One kWh is the energy consumed by a 1,000 W appliance running for exactly one hour. In the UK, the October 2024 Energy Price Cap set the unit rate at approximately 24.5p per kWh, though actual tariffs vary by supplier and region.

The formula used is straightforward: multiply the appliance wattage (W) by the hours used per day and the number of days, then divide by 1,000 to convert to kWh. Finally, multiply by your tariff rate to get the cost. Written formally: Cost = (P × t × d) ÷ 1000 × R, where P is power in watts, t is hours per day, d is the number of days, and R is the rate in pence per kWh. This is the exact formula used by energy providers and consumer comparison tools such as those endorsed by Ofgem and the Energy Saving Trust.

This calculator is useful across a wide range of practical scenarios: working out the monthly cost of running a fridge-freezer, comparing the running cost of a gas versus electric oven, estimating the annual spend on home office equipment, or deciding whether to replace an old appliance with a more energy-efficient model. The projected annual cost output is particularly useful for budgeting, showing what the device would cost if used at the same rate all year round.

This calculator assumes constant, steady-state power consumption equal to the appliance's rated wattage. In reality, many devices do not draw their full rated power continuously. For example, a refrigerator cycles its compressor on and off, typically consuming only 30–50% of its rated wattage on average over time. Similarly, washing machines draw peak power only during the heating phase of a cycle. For such appliances, the actual energy consumed will be lower than this calculator suggests unless you use the true average wattage rather than the rated peak wattage. Check the appliance's energy label or a smart plug energy monitor for a more accurate average figure.

The calculator uses a single flat tariff rate. If you are on a time-of-use tariff (such as Economy 7 or Octopus Go), where the rate varies by time of day, you should use the rate that applies to when the appliance is actually running. This calculator also does not account for standing charges, VAT, or any discounts applied by your supplier, which are separate components of your overall electricity bill. Finally, wattage information should be taken from the appliance's rating plate or official product documentation — manufacturer marketing figures are sometimes approximate.