Electricity Cost Calculator

Three numbers give you the running cost of anything: the power it draws, how many hours a day it runs, and what you pay per kilowatt-hour. The calculator converts power to energy — kilowatt-hours are kilowatts multiplied by hours, nothing more — applies your rate, and projects the daily figure out to a week, a thirty-day month and a year.

The input that quietly ruins most of these estimates is the hours. A nameplate wattage is what a device draws while it is actually working, not an average over the day, and anything with a thermostat spends most of its time not working. A fridge rated at 150 watts is not 3.6 kWh a day; it cycles, and at a realistic thirty-five percent duty it uses about 1.26 — a factor of nearly three between the obvious answer and the right one. The same applies to a kettle, an oven, an air conditioner and a water heater. For a cycling appliance, put in the hours it actually runs, or read the annual kWh figure off its energy label and work backwards.

The other quiet error is on the rate. Many residential tariffs are tiered, so the last kilowatt-hour of a heavy month costs considerably more than the first, and the number worth multiplying by is the marginal rate in the band you are already in rather than the average on your bill. Time-of-use tariffs move the same problem to the clock, and a fixed standing charge does not change with consumption at all, so it should not be in this calculation.

Two places the arithmetic is more interesting than it looks. Standby power is small and permanent: five watts left on all year is 43.8 kWh, which is a real line on a bill for a device nobody is using. And electric heating is the one case where efficiency comparisons get strange — a resistive heater turns essentially all of its electricity into heat, so it cannot be beaten on that measure, but a heat pump moves heat rather than making it and delivers three to four times as much per kilowatt-hour. Against a heat pump, the resistive heater's hundred percent is the worse number.