What this calculator does
An appliance's running cost comes down to three things: how much power it draws, how long it runs, and what you pay per kilowatt hour. The wattage is on the label, the hours are up to you, and the price is on your bill.
What surprises people is which appliances matter. A device that draws a lot of power briefly can cost far less than something modest that runs constantly, because the hours do as much work in the calculation as the watts do.
The formula
Multiply the power draw in watts by the hours used, then divide by 1,000 to convert watt-hours into kilowatt hours. Multiplying that by the price per kilowatt hour gives the daily cost, and by the days used gives the annual figure.
| Term | Meaning |
|---|---|
| Watt (W) | The rate at which the appliance uses energy while it is running. |
| Kilowatt hour (kWh) | The unit electricity is billed in: one kilowatt drawn for one hour. A 1,000 watt appliance running one hour uses exactly one. |
| Duty cycle | The share of time an appliance actually draws its rated power. Fridges and heaters cycle on and off, so their average draw is well below the label figure. |
The inputs explained
| Field | What to enter |
|---|---|
| Power draw (W) | The power draw in watts, usually printed on the appliance or its plate. |
| Hours used per day | How many hours a day it actually runs, not how many hours it is switched on at the wall. |
| Price per kWh ($) | Your electricity price per kilowatt hour, from your bill. Use the usage rate rather than the total divided by units, unless you want the supply charge included. |
| Days per year used | How many days a year it is used. A heater used only in winter might be 120 rather than 365. |
When to use it
Deciding whether an appliance is worth replacing
Comparing the annual running cost of an old appliance against a more efficient replacement shows how long the difference takes to pay for the new one.
Finding what is driving a bill
Running the main appliances through the same calculation usually shows that one or two dominate, and they are rarely the ones with the highest wattage.
Costing a seasonal appliance
A heater or air conditioner used for part of the year needs the days figure adjusted, which changes the annual cost dramatically.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
What do different power draws cost to run for 3 hours a day?
A range of appliance power draws at the same usage pattern.
| Power draw | Cost per year | Energy per year |
|---|---|---|
| 100 W | $35.04 | 109.5 kWh |
| 500 W | $175.20 | 547.5 kWh |
| 1,200 W | $420.48 | 1,314.0 kWh |
| 2,400 W | $840.96 | 2,628.0 kWh |
Questions
Where do I find the wattage?
Usually on a plate on the back or underneath, or in the manual. Where only volts and amps are given, multiplying them gives watts. A plug-in energy meter is more accurate still, since it measures what the appliance actually draws rather than its maximum.
Why is my fridge cheaper than this suggests?
Because a fridge does not run continuously. It cycles on and off to hold temperature, so its average draw over a day is much lower than its rated wattage. For anything that cycles, use an estimate of actual running hours rather than 24.
Should I include the daily supply charge?
No. The supply charge is fixed and is paid whether or not the appliance runs, so it does not belong in a per-appliance figure. It matters for the bill as a whole, which is a separate calculation.
What about standby power?
This calculates active running cost. Standby draw is small per device but continuous, so for a device left plugged in permanently it is worth running a second calculation at the standby wattage across 24 hours.
For the whole bill including the daily supply charge, see the electricity bill calculator. For the lighting case specifically, see the LED savings calculator.