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      Lesson details

      Learning outcome

      I can compare the power of different appliances and work out how much they cost to use.

      Key learning points

      1. Electrical appliances and devices use different amounts of energy each second, as indicated by their power rating.
      2. Power is a measure of the amount of energy used by or supplied to a device in one second.
      3. Power is measured in watts (W).
      4. Multiplying power of a device in kilowatts (kW) by time in hours (h) gives us the energy used in kilowatt–hours (kWh).
      5. Electricity bills are based on the amount of energy units used; 1 unit of energy is equal to 1 kWh.

      Keywords

      • Power - the amount of energy transferred every second

      • Watts (W) - the unit of power

      • Kilowatts (kW) - a larger unit of power; 1 kW = 1000 W

      • Joules (J) - the unit of energy

      • Kilowatt–hours (kWh) - the unit of energy usage; 1 kWh = 1 unit of energy used

      Common misconception

      Most pupils do not discriminate sufficiently between current, voltage, energy and power.

      Provide pupils with opportunities to calculate energy usage with appliances that have different power ratings and are used for different lengths of time.

      Teacher tip

      It is useful to see how a high powered appliance used for a short time and a low powered appliance used for a long time compare. Encourage pupils to use the smart meters in their homes (if they have them). If not, consider setting this as a homework and then analyse the data in class.

      Equipment

      Wattmeter (joulemeter) if available for demonstration and a range of devices to test.

      Licence

      This content is © Oak National Academy Limited (2026), licensed on Open Government Licence version 3.0
      except where otherwise stated. See Oak's terms & conditions
      (Collection 2).

      Lesson video

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      Prior knowledge starter quiz

      6 Questions

      Q1.
      Which of the following types of circuit is used for car headlamps so that if one light stops working, the other stays lit?

      a series circuit
      Correct answer: a parallel circuit
      either a series or a parallel circuit

      Q2.
      Which of the following statements is correct?

      Correct answer: Voltage is measured in parallel, current is measured in series.
      Both voltage and current are measured in parallel.
      Voltage is measured in series, current is measured in parallel.
      Both voltage and current are measured in series.

      Q3.
      Which of the following statements about a circuit where a high current flows through a bulb and the voltage and current are correctly measured is correct?

      The voltmeter and the ammeter both have a high current passing through them.
      The voltmeter and the ammeter both have a low current passing through them.
      The voltmeter has a high current and the ammeter has a very low current passing.
      Correct answer: The ammeter has a high current and the voltmeter has a very low current passing.

      Q4.
      In the circuit shown, what is the voltage across the electric cell if the voltage across the 8 $$\Omega$$ resistor is 2.0 V and the voltage across the 4 $$\Omega$$ resistor is 2.0 V?

      An image in a quiz
      1.0 V
      1.33 V
      Correct answer: 2.0 V
      4.0 V

      Q5.
      In the circuit shown, what will the ammeter read if the current in the resistor is 0.35 A and the current in the lamp is 0.25 A?

      An image in a quiz
      0.25 A
      0.30 A
      0.35 A
      Correct answer: 0.60 A

      Q6.
      Where does our household electricity come from?

      only fossil fuel power stations
      only nuclear power stations
      wind turbines and solar panels
      hydroelectricity plants
      Correct answer: A mix of different sources, depending on weather conditions and demand.

      6 Questions

      Q1.
      Which of the following is the unit of power?

      ohms
      volts
      amps
      Correct answer: watts
      joules

      Q2.
      Which of the following devices or appliances has the greatest power rating?

      lamp: 220-240 V, 50 Hz, 25W
      loudspeaker: 50W
      Correct answer: toaster: 220-240 V, 50/60 Hz, 860-1050W

      Q3.
      How many watts are there in a kilowatt?

      a thousandth watts
      Correct answer: a thousand watts
      a million watts
      a billion watts

      Q4.
      Which of the following values is equal to 0.1 kilowatts?

      1 watt
      10 watts
      Correct answer: 100 watts
      1000 watts

      Q5.
      How many watts are there in a gigawatt?

      100
      1000
      1 000 000
      Correct answer: 1 000 000 000

      Q6.
      What is the cost of using a 1000 W heater for 30 min, if 1 kWh costs £0.30?

      3 pence
      Correct answer: 15 pence
      30 pence
      60 pence

      To help you plan your 9 science lesson on: Mains electricity, download all teaching resources for free and adapt to suit your pupils' needs...