Green desktop computing

1. Estimate

To work out how much electricity you can save we consider two policies for managing desktop computers:

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

We will calculate financial costs and greenhouse gas emissions over a year. For the purposes of the calculations below, we assume:

  • A typical desktop computer (80 Watts) and LCD monitor (25 Watts) together run at 0.105 kilowatts (or 105 Watts)
  • Electricity is currently (2009 in UK) 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 not needed between 5pm and 9am, at weekends, and 25 days of holidays.
  • Desktop computers normally consume between 1-5 Watts when switched off, and in standby or hibernate modes. To keep calculations simple we will exclude this information.
  • There is no automatic power management configured for the computer or monitor i.e. computers and monitors never go into sleep or hibernate modes when they are 'idle'.

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
1000 920,000 110,000 494,000
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)
1 198 24 106 87 387
100 19,800 2,400 10,600 8,660 38,800
500 99,000 11,900 53,200 43,300 194,000
1000 198,000 23,800 106,000 86,600 388,000
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 run at is to use a plug-in power meter.

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