HSC · Topic explainer
How to calculate electricity cost from kWh
Multiply power in kilowatts by running time in hours to get energy in kilowatt-hours. Multiply that energy by the tariff per kilowatt-hour. Divide watts by 1,000 to obtain kilowatts, and convert a tariff in cents to dollars before multiplying if the answer is required in dollars.
Worked example
A 1,800 W heater runs at rated power for 2.5 hours. Convert 1,800 W to 1.8 kW, then multiply by 2.5 h to get 4.5 kWh.
At 32 cents per kWh, the energy costs 4.5 × $0.32 = $1.44. If the heater cycles on and off, use its actual running time for this calculation. A daily supply charge is a separate item on a bill.
Official exam practice
Find other years and marking guidance in the Mathematics Standard 2 official exam archive.