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Science 7–10 · Year 9

Energy audit: power and energy use of household appliances on a plug-in meter

Physical sciences — Energy, content group Electrical energy (NSW Stage 5 focus area)

PracticalMedium risk

School laboratory, not for home

In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

The idea

An appliance transforms electrical energy at a rate given by its power, so energy used equals power multiplied by time and can be measured, costed and compared with the rating label.

Safety card

Medium riskA teacher supervises

Setting: In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

Hazards

  • mains electricity
  • boiling water and steam
  • worn appliance leads

Controls

  • only appliances tagged and tested under the school's electrical safety program
  • no wet hands near the meter
  • teacher handles the kettle
  • power boards not overloaded

Note

Heat or electrical energy is involved. Complete the school's risk assessment for the activity before the lesson, using CSIS 1.7 (Risk assessment – a pre-requisite for risk control) from the department's Chemical Safety in Schools package (2021 Technical Update), which the NSW Department of Education Science safety and compliance page names for risk assessment advice.

What you need

  • plug-in power and energy meter (reads W and kWh), 1 per group
  • appliances with rating labels: 2 kW kettle, desk lamp with an LED globe, laptop charger, small fan, 1 each
  • 1 L measuring jug, 1
  • stopwatch, 1
  • a recent electricity bill or the retailer's published tariff for the cost per kWh

How to do it

  1. Read each appliance's rating label and record the stated power in watts.
  2. Plug the kettle into the meter, add exactly 1.0 L of cold water, switch on and record the meter's power reading every 20 s until it switches off; record the total energy in kWh (or Wh) and the time taken.
  3. Repeat for the lamp, charger (with the laptop connected and charging) and fan, running each for exactly 5 minutes and recording the steady power reading.
  4. Check the kettle's measured energy against power x time (in joules: watts x seconds; in kWh: kilowatts x hours). For the lamp, charger and fan, work out the energy as power x time, because 5 minutes of a low-power appliance uses less energy than the meter's kWh display can show.
  5. Cost each appliance's measured energy at the tariff on the bill, then estimate a week's cost from a realistic daily use time.
  6. Rank the appliances by energy used per typical day, justify the ranking with the measurements, and identify which label figure was furthest from the measurement.

What you should see

The kettle's power reading is close to its label. Heating 1.0 kg of water by 80 degrees C needs at least 1.0 x 4186 x 80 = 334,880 J, which is 0.093 kWh, so a 2.0 kW kettle cannot boil 1.0 L from 20 degrees C in less than 167 s and its measured energy is above 0.093 kWh because some energy heats the kettle and the air. For the kettle, power x time agrees with the meter's energy reading to within the display's last digit when the power reading is steady. The LED lamp, charger and fan read far lower powers than the kettle, and their 5 minute energies are too small for the meter's kWh display: a 10 W lamp uses 3000 J, only 0.0008 kWh, in 5 minutes, so their energy comes from power x time. The learner knows it worked when the kettle's measured energy is above the 0.093 kWh minimum and every other appliance's power x time energy is far below it.

What changes

What you change
appliance
What you measure
measured power (W) and energy used (kWh) over a fixed time
What you keep the same
  • same meter
  • fixed run time for the low-power appliances
  • same volume of cold water for the kettle

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

  • Leaving a charger plugged in uses as much energy as running a heater.
  • A 2 kW label means the kettle always uses 2 kWh.
  • Energy and power are the same quantity.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Sources

The pages the author read to write this activity.

  1. curriculum.nsw.edu.au/learning-areas/science/science-7-10-2023/outcomes
  2. curriculum.nsw.edu.au/learning-areas/science/science-7-10-2023/content/stage-5/fab404b99e
  3. education.nsw.gov.au/teaching-and-learning/curriculum/science/science-curriculum-resources-k-12/science-7-10-curriculum-resources/science-s5-energy
  4. spark.iop.org/using-energymeter-measure-efficiency-energy-transfer

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