Guide
How to Calculate Electricity Usage
From the wattage on a label to the number on your bill, step by step.
Watts, kilowatts and kilowatt-hours
Watts measure power — the rate at which a device draws energy. A kilowatt is 1,000 watts. A kilowatt-hour is one kilowatt drawn for one hour, and it is the unit your supplier bills you in.
The conversion is simple: watts times hours divided by 1,000 gives kilowatt-hours. A 1,500 W heater running two hours uses 3 kWh.
Find your real rate per kWh
Take the total on a recent bill and divide it by the units consumed. That includes standing charges and taxes, so it is usually higher than the headline tariff rate — and it is the figure that reflects what you actually pay.
If your tariff is tiered or time-of-use, the rate that matters for extra consumption is the top tier or the peak rate, not the average.
Handle devices that cycle
Fridges, freezers, air conditioners and water heaters do not run continuously; they cycle with a thermostat. Using 24 hours a day overstates their consumption badly. A common approximation is a third of the period for a fridge, adjusted upward in hot weather.
Target the big three
Heating, cooling and water heating dominate most bills because they combine high wattage with long runtimes. Calculate those first. Lighting and electronics are worth attention only after the large loads are understood.
Frequently asked questions
Why is my bill higher than my appliances add up to?
Standing charges, taxes and always-on devices are not captured when you calculate appliances individually. Divide your total bill by units consumed to get the real rate per kWh.
How do I handle a fridge that runs all day?
It cycles with a thermostat rather than running continuously. Estimate effective hours at roughly a third of the day, adjusted upward in hot weather.
Which appliances should I calculate first?
Heating, cooling and water heating. They combine high wattage with long runtimes and usually dominate the bill. Lighting and electronics come after.
Conclusion
Watts times hours divided by 1,000 gives kilowatt-hours; multiplied by your real rate per kWh that is the cost. Calculate the big three first, account for cycling devices, and use your actual bill to find the rate that matters.