1. Estimate

Here we consider energy consumption with respect to two policies for managing desktop computers:

  1. Leave switched on all the time
  2. Switch off at the end of each working day

We calculate financial costs and greenhouse gas emissions over a year and make the following assumptions:

  • A typical desktop computer (80 Watts) and LCD monitor (25 Watts) together run at 0.105 kilowatts (or 105 Watts)
  • Electricity is charged at £0.12 kWh (12 pence per kilowatt-hour)
  • One kWh of UK Grid electricity causes the emission of 0.537 kg CO2eq. (a measure of greenhouse gas emissions)
  • To calculate the number of hours a computer will be switched on over a year with Policy B we assume people use their computer for 8 hours a day, and 226 days over the year i.e. computers are powered down overnight, weekends and 25 days of holidays.
  • Desktop computers normally consume between 1-5 Watts when switched off, in standby (ACPI S3) or hibernate modes. To keep calculations simple we ignore this energy consumption.
  • There is no automatic power management e.g. monitors do not go to sleep during the day.

To calculate the total electrical energy consumed over a year with policy 1 we simply multiply the power the computer and monitor run at by the total time they are used over a year. So for our typical computer the sum would be: 0.105 kW x 24 hours x 365 days = 920 kWh. To estimate the financial costs we simply do 920kWh x 12p/kWh = £110. To estimate greenhouse gas emissions 920kWh x 0.537 kg CO2/kWh = 494 kg CO2.

The tables below use this calculation to show costs and greenhouse gas emissions for different stocks of computers. Table 2 also shows savings that might be achievable with a shift from policy 1 to 2:

Table 1. Policy 1: Leave computers switched on
ComputersElectricity
(kWh)
Costs
(£)
Greenhouse gas
(kg CO2)
1 920 110 494
100 92,000 11,000 49,400
500 460,000 55,200 247,000
1,470,000 1,350,000,000 162,000,000 726,000,000

Table 2. Policy 2: Switch computers off at the end of each working day
ComputersElectricity
(kWh)
Costs
(£)
Greenhouse gas
(kg CO2)
Policy 1 to 2
(£)
Policy 1 to 2
(kg CO2)
1 190 23 102 86 392
100 19,000 2,300 10,200 8,000 39,200
500 95,000 11,500 51,000 43,000 196,000
1,470,000 291,000,000 34,900,000 156,000,000 127,000,000 570,000,000

The figure of 1 470 000 computers is included as this is the estimate for the total number of desktop computers in UK Further and Higher Education, according to the SustainIT Report, P. James et al, 2009

These calculations are meant to illustrate the differences between the two policies. The actual savings that an organisation can achieve will be dependent on local conditions:

  • The power that desktop computers run at
  • The cost of a unit (kWh) of electricity
  • The amount of time computers are actually used over a year
  • Whether power management is currently implemented e.g. policies may already be in place to switch computers into sleep/standby (ACPI-S3), hibernate and off.

It is therefore important to use local measurements to estimate how much electricity consumption can be reduced. (The easiest way to estimate the power your computers when switched on is to use a plug-in power meter).

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