Electricity cost calculator

Running cost is watts times hours divided by 1,000, multiplied by your price per kWh. A 2,000 W heater for four hours a day at 28p per unit costs £2.24 daily, £68 a month and £818 a year.

Two things make the yearly figure the useful one. First, small hourly costs are easy to dismiss and annual ones are not: a 3p-an-hour device left on constantly is £263 a year. Second, it is the right basis for comparing a replacement — an appliance that saves £80 a year justifies a very different purchase price than one that saves £8. Remember that your bill also carries a standing charge that no appliance calculation will ever explain, and that a variable or time-of-use tariff makes the timing matter as much as the total.