How the running cost calculation works

Three numbers, multiplied

Energy = watts / 1000 x hours x duty. A kilowatt hour is a kilowatt held for an hour, so a 1,500 W device that runs for one hour uses 1.5 kWh.

Cost = energy x price. The price comes from your bill, in cents per kilowatt hour.

Month and year = the week x 30/7 and x 365/7. Weeks are the honest unit here, because few appliances run exactly seven days a week, and monthly billing periods are not four weeks long.

Why there is a duty cycle field

Almost nothing draws its label wattage continuously. A space heater on a thermostat, a fridge compressor, a heat pump in mild weather and an oven holding temperature all switch the element or compressor on and off. Entering 100 percent and guessing the hours is the same arithmetic, but it hides the guess. The field makes the guess visible and removable.

A worked example

A 1,500 W space heater at 17 cents per kWh costs 25.5 cents for every hour it draws power. Four hours a day at 50 percent draw is 3.00 kWh a day: $0.51 a day, $15.30 a month and $186.15 a year. Halving the duty cycle halves every one of those figures, and nothing else in the model changes.

What the calculator deliberately leaves out

Standing charges, taxes and time-of-use pricing. If your utility charges different rates in the evening peak, run the calculator twice with the two rates and add the results in the proportion of hours you use each.