How much energy does my PC use?

Discuss energy-efficiency and energy wastage with students, and demonstrate how it relates to them as individuals on a daily basis as they use PCs at school and home.



  • Understand how to measure the electricity used by an electrical item
  • See the difference in electricity consumed by a PC and a laptop, and whether this is affected by the activation of the screensaver
  • Calculate the cost of the energy used by a PC, and how much is wasted by leaving a computer or monitor switched on overnight
  • Think about the implications of this and how individual behaviour affects energy consumption

Time required: 30 minutes

Resources & preparation required

PC, laptop and other small electrical items – e.g. printer, lamp, heater, mobile phone charger, Hand held monitoring device

If possible, work out / estimate the total number of PCs used within the school, and find out the school’s electricity tariff.


pc-energy-savingAsk students to guess the number of PCs they think there are in the school. Do they think that they and other students / teachers always remember to switch them off at the end of the day? Are some accidentally left on overnight? How much do students think it costs to run a PC for 24 hours?

The aim of this exercise is to calculate the running costs of a classroom PC, and the amount of electricity it consumes.

Plug a PC and its monitor into a normal electric socket via the energy monitor. Explain to students that the energy monitor contains a smart meter, which will measure the amount of electricity that is passing through it.

Show them the graph of energy usage, It should show that no power is currently being consumed.

Discuss the other electrical items that might be used in a classroom and predict how much energy they think they would use compared to a PC. You can then test whether they are right by connecting some of these devices, in turn, to the energy monitor and noting the energy used. Students should record the results in a bar chart.

Turn on the PC and watch the graph as it powers up. Record the figure, in Watts, for energy consumption when the PC is fully active.

Compare this figure with the energy consumed when the PC’s screensaver comes on – is there any difference?

Shut down the PC, but leave the monitor switched on. How much energy is still being consumed? Disconnect the PC and plug in and turn on the laptop instead, and record its energy consumption too.

Explain to students that these figures are not an amount of energy used but the rate of energy usage. Electrical power is measured in Watt-hours (Wh) or Kilowatt-hours (kWh).

To measure energy used, you need the formula: Energy = Power (in W or kW) x Time (in hours) e.g. if the PC uses 120W and is switched on for an hour, its energy usage is 120 x 1 = 120Wh or 0.12kWh.

energy-graphAsk students to draw a simple bar chart to compare the energy usage of the PC, the laptop, the PC monitor and the PC with screensaver activated.
Calculate the cost of the energy used by the PC and the laptop, using either your school’s actual electricity tariff or a standard approximate cost which might be 10p per kWh. How much does it cost to run a PC or laptop for an hour / day / week / year?

How much money is wasted if a PC is left accidentally switched on overnight, or if its monitor is not switched off overnight?

Discussion / summing up

Which devices are the biggest users of energy? Where do students think that most energy is being wasted in their school (e.g. whiteboards / PCs left on standby) Suggest that they also consider other things that will use a lot of power – e.g. lighting, heating.

Extension / homework suggestion

  1. Whole classroom – calculate the electricity usage and running costs of all the PCs in the classroom.
  2. If you have some computers that are older than others in the classroom, is there a difference in how much electricity each uses?
  3. Work out the usage and cost figures for the whole school, and how much money is wasted if: a) the PCs are all left switched on outside of school hours, b) the PCs are all left switched on at the weekend?


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