A household uses several electrical appliances every day. Explain why the kilowatt-hour is used as the unit of energy for recording domestic energy consumption rather than the joule.

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).

A family receives an electricity bill showing the number of units of energy used each month. Each unit on the bill is one kilowatt-hour.

Model answer (4 marks)

The joule is a very small unit of energy. Domestic appliances use large amounts of energy, so the number of joules would be very large and difficult to work with. The kilowatt‑hour is a larger, more convenient unit for measuring energy at a domestic scale. It links directly to the power rating of an appliance and the time it is used, making billing straightforward.

Examiner tips

  • Use the word ‘kilowatt‑hour’ correctly; explain its relation to power and time.
  • Show the reason for using a larger unit – ‘large numbers’ and ‘convenience’.”]
  • common_mistakes
  • :

Mark scheme (4 marks)

  1. The joule is a very small unit (of energy)
  2. Domestic appliances transfer large amounts of energy, so the number of joules would be very large / difficult to work with
  3. The kilowatt-hour is a larger / more convenient unit for measuring energy at domestic scale
  4. The kilowatt-hour links directly to the power-rating of an appliance and the time it is used, making billing straightforward

Key terms in this question

kilowatt-hour · domestic energy consumption

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