How to estimate an appliance’s electricity cost
A useful estimate needs three things: the appliance’s power, how long it runs and the energy rate on your electricity account. The result is approximate because real usage and tariff rules may vary.
Reviewed: 10 October 2026
The simple formula
Once you have estimated the kilowatt-hours, calculate the energy-only cost:
If the bill gives the rate in cents per kWh, convert it to rands first: divide by 100. For example, 325 c/kWh is R3.25/kWh. The WattWise calculator can accept either unit.
Worked example: a kettle
Suppose a kettle is rated at 2,000 W, is used for 9 minutes a day, and is used every day for 30 days:
- Convert watts to kilowatts: 2,000 ÷ 1,000 = 2 kW.
- Convert minutes to hours: 9 ÷ 60 = 0.15 hours.
- Estimate monthly use: 2 × 0.15 × 30 = 9 kWh.
- At an example rate of R3.25/kWh, energy cost is 9 × R3.25 = R29.25.
The rate is illustrative only; use the rate and tariff structure from your own bill.
Choose realistic inputs
- Power: use the wattage on the appliance label or manufacturer information. If the rating is in kilowatts, multiply by 1,000 to get watts.
- Quantity: include how many identical appliances run for the stated time.
- Hours: enter the time the appliance is actively drawing power, not always the full time it is plugged in.
- Days: estimate the number of days it is used in the month.
Appliances controlled by a thermostat or automatic cycle may switch on and off. For these, use an average active running time if you know it. A plug-in energy meter or smart meter can help measure actual use where available.
Why the bill can be different
The formula estimates energy at a single rate. Your final bill can differ because the appliance’s real power draw changes, your household use may cross tariff blocks, the time of use can matter, and bills can include fixed or other charges. The calculator does not estimate those extra bill components.
For multiple appliances, estimate each one separately and add the results. This makes it easier to see which assumptions have the biggest effect.