How the electricity usage calculator works
Last updated 10 November 2026
Electricity costs are made up of many small loads and a few big ones. Seeing them side by side shows where saving effort pays off.
How it works
- Energy in kWh = watts × number of appliances × hours a day × days in a month ÷ 1 000.
- Cost = energy × your tariff.
- A month is taken as 30.4 days (365 ÷ 12).
Worked example
With the starting figures:
- The starting appliances use about 449 kWh a month.
- At the example tariff, that costs R 1 346 a month.
- The biggest user is Electric geyser, at 61% of the total.
How to read your result
The headline is the energy your list of appliances uses in a month, in kilowatt-hours (kWh), and what that costs at your tariff. With the starting figures, that is about 449 kWh and R 1 346.
The table ranks the appliances by cost, and the chart shows each one’s share. Look at the top few rows first. A small cut in a big load saves more than a large cut in a small one.
What a kWh is
A kilowatt-hour is the energy used by a thousand watts running for one hour. A kettle rated at two thousand watts that runs for half an hour uses one kWh. Your electricity bill and prepaid tokens are measured in these units, which is why the tool works in kWh.
Finding the watts and the hours
The two numbers that decide each row are the watts and the hours.
- Watts are on the label on the back or bottom of the appliance, or in its manual. If you only find kilowatts, multiply by a thousand.
- Hours a day should be the time the appliance is actually drawing power, not the time it is plugged in.
- Fridges and freezers cycle on and off. Enter the hours the motor really runs, as the starting example does.
- Standby loads from decoders, chargers and routers are small, but they run all day. Add them as rows if you want them counted.
Why your bill may differ
The result is an estimate built from the figures you enter. Your bill can differ for several reasons: appliances missing from the list, hours that vary through the year, a tariff with steps or time-of-use prices, and fixed monthly charges that do not depend on use.
To check the estimate, compare it with the kWh figure on a recent bill or with readings from your meter. If the two are far apart, look for appliances you left out.
Common questions
Does a bigger appliance always cost more? Not always. A heater with high watts that runs for a few minutes can cost less than a small device that runs all day. Cost depends on watts and hours together.
Why is the geyser so high? Heating water takes a lot of energy, and a geyser can switch on several times a day to keep the tank warm. The hours you enter should reflect how long its element is really on.
Can I save my list? Yes. Download your data to keep it, and load the file later. Nothing you enter is stored on this site.
Things to watch
- Heating appliances, such as geysers, ovens and kettles, use a lot because their watts are high.
- Fridges and freezers run all day but only part of the time. Use the hours they are actually on.
- Your tariff may have steps or time-of-use prices that a single rate cannot show.
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