🧮How to Work Out a Running Cost

By the What It Costs editorial desk

The arithmetic takes ten seconds. Getting the inputs right is where the work is.

The formula

watts × hours ÷ 1,000 × price per kWh = cost

A 1,500 W heater, four hours, at $0.17 per kWh: 1,500 × 4 = 6,000 watt-hours = 6 kWh × 0.17 = $1.02. That is the whole method, and everything else on this site is that line with better inputs.

Where the two numbers live

Watts are on the rating plate: the back of a television, the base of a kettle, inside the washing machine door, on the power supply brick. If it lists volts and amps instead, multiply them. If the plate has gone, the wattage table has typical figures with the usual range.

Price per kWh is on your electricity bill, usually near the meter readings. Look for a unit rate rather than the total — the total includes standing charges, which are not part of what an appliance costs. If you have more than one rate, use the one that applies when the appliance actually runs.

The four mistakes

1. Hours plugged in, not hours running. A fridge is powered 24 hours a day and its compressor runs about eight. A thermostatic heater in a warm room may draw power half the time it is on. This one mistake alone can treble an answer.

2. Watts from the wrong place. A power supply rated 850 W is a ceiling, not a consumption; a gaming PC with one typically draws 350–500 W. An air conditioner’s BTU rating is cooling produced, not electricity used — divide BTU by roughly 10 for a rough wattage.

3. Per hour on a machine measured per cycle. Washing machines, dishwashers and tumble dryers swing between 100 W and 2,500 W during a programme. Multiplying a plate rating by the cycle length overstates the cost two or three times. Use the kWh-per-cycle figure from the energy label instead.

4. Forgetting that a month is not 30 days. Twelve months of 30 days is 360, so monthly figures multiplied by twelve fall short of the year. Everything here uses 30.44 days — a twelfth of a year — so the monthly and annual numbers on a page never contradict each other.

When in doubt, measure

A plug-in energy monitor costs less than a month of the appliance you are arguing about. Put it between the plug and the socket, leave it a week, and read the kilowatt-hours. It settles the question for a specific machine in a specific house better than any table can — including this one.

Then take the number to the electricity cost calculator and put your own price against it.