My Green ICT students are doing estimates of carbon emissions cased by computer and telecommunications equipment. I get them to do the calculations for real organisations (their work place or parts of the unviersity). The calculations are simple in theory: measure how much electricity is used by the equipment and then multiply by a conversion factor to turn kilowatt hours into kilograms of CO2 equivalent. However, getting the data is very difficult and knowing which conversion factor to use is also not easy.
There are numerous calculators available on the web, where you energy how many computers you have and it calculates the emissions. The mistake many make is to use a calculator from another country. Electricity in different places is generated differently. The emission factor for coal fired electricity is about 1 kg per kwh. That is about one kilogram of CO2 is created for each kilowatt of electricity produced. In some places, such as Tasmania, hydroelectricity creates relatively little CO2 and so the emission factor is lower. In other places, such as Victoria, dirty brown coal is used and so the factor is higher.
One example is the CoSN Energy Usage Calculator. This appears to use an emission factor of 0.5898, which is a very low figure, perhaps because the use of US nuclear power is assumed. If used to estimate CO2 emissions in Australia it will underestimate emissions by about one third (the official figure for the ACT is 0.89 kg CO2e/kWh in National Greenhouse Accounts (NGA) factors).
The other issue is that the calculations assume a particular behaviour from computer users, as to how much they use their equipment each day and if they turn it off. The result is that different calculations making difference assumptions give different results. If the students figures are within an order of magnitude I consider they are doing well.