A tumble dryer has a power-rating of 2.5 kW. Explain what is meant by the term power-rating and describe how the amount of energy transferred by the tumble dryer in kilowatt-hours could be determined for a single drying cycle.

WJEC GCSE Physics (Wales) — 1.4 Domestic electricity · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Power‑rating is the maximum rate at which the appliance can transfer energy – in this case 2.5 kW.

The energy used in one drying cycle can be found by:
1. Measuring the running time of the dryer for that cycle (in hours).
2. Multiplying the power rating (2.5 kW) by the running time (h) to give the energy in kilowatt‑hours (kWh).

Examiner tips

  • Use the exact definition of power‑rating as a rate of energy transfer.
  • Show the calculation: energy (kWh) = power (kW) × time (h).
  • Mention that the kilowatt‑hour is the unit of energy recorded on bills.

Common mistakes

  • Confusing power with energy or using watts instead of kilowatts.
  • Failing to convert the running time to hours before multiplying.
  • Not recognising that the kilowatt‑hour is a unit of energy, not power.

Mark scheme (4 marks)

  1. Power-rating is the maximum rate of energy transfer of an appliance
  2. The kilowatt-hour is a unit of energy used for recording domestic energy consumption
  3. Measure the time the tumble dryer runs for one drying cycle (in hours)
  4. Multiply the power in kilowatts by the time in hours to give energy in kilowatt-hours

Key terms in this question

power-rating · kilowatt-hour

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