Each tool generally involves the use of ‘emission factors,’ which relate the amounts of greenhouse gases emitted by a business to a set amount of activity performed by that business. Default values are always provided for the emission factors in case businesses cannot develop custom values. So, in many cases companies need only activity data, such as the amount of distance traveled or fuel combusted, to calculate their emissions.
The default emission factors are averages based on the most extensive data sets available and they are largely identical to those used by the Intergovernmental Panel on Climate Chage (IPCC), the premier authority on accounting practices at the national level. However, the GHG Protocol recommends that businesses should use custom values whenever possible. This is because the industrial processes or the composition of fuels used by businesses may differ with time and by region.