Appliance running cost calculator
The wattage is on the label or in the manual. Everything else is arithmetic — and the yearly figure is usually the one that surprises people.
Watts, kilowatt-hours, and the number on your bill
Electricity is billed in kilowatt-hours: one kilowatt drawn for one hour. A 150 watt device left on for eight hours uses 1.2 kilowatt-hours, and your rate turns that into money. That is the whole calculation, and it works for anything from a lightbulb to a gaming PC.
The gap between the daily and yearly figures is where the useful insight lives. A device costing pennies a day is easy to ignore, but the annual number is what appears on twelve bills, and it is often the deciding argument for replacing something old or switching it off at the wall.
Two caveats worth knowing. Rated wattage is usually maximum draw, not average — a fridge compressor cycles on and off, so its real consumption is well below its plate rating, while a space heater genuinely pulls its rating the whole time. And devices in standby still draw a small amount continuously; individually trivial, collectively a measurable share of a household bill. A plug-in energy meter costs very little and turns all of this from estimate into measurement.
Questions
Where do I find the wattage?
On the appliance label, the power supply, or the manual. If it lists amps and volts instead, multiply them: 2 amps at 120 volts is 240 watts.
Why is my actual bill different?
Rated wattage is peak draw, and many appliances cycle rather than run continuously — fridges and air conditioners especially. Tiered rates and time-of-use pricing also mean your marginal rate may differ from your average.
What rate should I use?
Take a recent bill, divide the total by the kilowatt-hours used, and use that. It captures the fees and taxes that a headline rate leaves out.