A tumble dryer has a power-rating of 2.5 kW. It is used for 2 hours each day. Explain how a consumer could use the power-rating and the time of use to work out the cost of running the tumble dryer, and why electricity companies use the kilowatt-hour rather than the joule for billing domestic customers.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Electricity companies charge domestic customers based on the number of kilowatt-hours of energy they use each month.
Model answer (4 marks)
The energy used is calculated by multiplying the power‑rating of the dryer (2.5 kW) by the time it runs each day (2 h). This gives 5 kWh per day. The total energy for a month is then 5 kWh × number of days. The cost is found by multiplying this energy (in kWh) by the tariff rate (e.g. £0.15 kWh⁻¹). The kilowatt‑hour is used for billing because domestic consumption is large; the joule would give very small, inconvenient numbers, whereas the kWh gives manageable figures for customers and suppliers.
Examiner tips
- Show the energy calculation (power × time) and then the cost calculation (energy × rate).
- Explain why the kWh is convenient – large numbers, easier to read and understand.
Common mistakes
- Using watts instead of kilowatts in the calculation.
- Failing to multiply the daily energy by the number of days or by the tariff rate.
Mark scheme (4 marks)
- The energy used is found by multiplying the power-rating (in kilowatts) by the time used (in hours)
- This gives the number of kilowatt-hours consumed, which is then multiplied by the cost per kilowatt-hour to find the total cost
- Domestic energy consumption is very large, so the joule is an inconveniently small unit
- The kilowatt-hour gives more manageable/convenient numbers for billing domestic customers
Key terms in this question
power-rating · kilowatt-hour · joule
Related
- All Eduqas GCSE Physics revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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