A consumer receives an electricity bill showing that their household used 350 kilowatt-hours of electrical energy last month. The bill also states that the cost per kilowatt-hour is 28p. Explain what the term 'power-rating' means for a household appliance, and describe how the power-rating of an appliance and the time it is used both affect the total number of kilowatt-hours shown on an electricity bill.

Eduqas GCSE Physics — 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 of energy transfer of an appliance, expressed in kilowatts. A higher power‑rating means the appliance uses more kilowatt‑hours for the same period of use. The longer an appliance is used, the more kilowatt‑hours are consumed. Therefore the total kilowatt‑hours on a bill depend on both the power‑rating (kW) and the time of use (h); increasing either value raises the total.

Examiner tips

  • Define power‑rating in kW and link it to energy use
  • Show the relationship Q=Pt (kWh= kW × h)
  • Explain that both higher power and longer use increase kWh
  • Use the word ‘maximum’ to show understanding of rating

Common mistakes

  • Confusing power with energy, e.g. writing kWh instead of kW
  • Forgetting to mention that the rating is a maximum, not the actual power used
  • Not recognising that time of use also affects the kWh

Mark scheme (4 marks)

  1. Power-rating is the maximum rate of energy transfer of an appliance
  2. A higher power-rating means more kilowatt-hours are used (for the same time of use)
  3. The longer an appliance is used, the more kilowatt-hours are used
  4. The number of kilowatt-hours depends on both power-rating (in kilowatts) and time (in hours) — greater values of either increase the total

Key terms in this question

power-rating · kilowatt-hour

Related

More Domestic electricity questions

▶ Try answering this question with AI marking (free) →