Emissions factor

An emissions factor is a standard value that converts a unit of activity, a litre of diesel burned, a kilowatt-hour of grid electricity used, a kilogram of a material produced, into the mass of greenhouse gas it is estimated to produce, usually expressed in kilograms or tonnes of CO2-equivalent per unit. Emissions factors are published by bodies such as national governments, the IPCC and industry associations, and are updated periodically as measurement improves and, for electricity, as a grid's generation mix changes.

Calculating an emission is then activity data x emissions factor = emissions; for example, litres of fuel multiplied by the fuel's CO2e-per-litre factor. This differs from a direct, metered emissions measurement, which is rarer and more expensive; most carbon accounting relies on factors and activity data rather than measuring stack or tailpipe output directly.

Emissions factors are the calculation engine behind scope 1, 2 and 3 emissions reporting and product-level life cycle assessment, and their accuracy sets the accuracy of everything built on top of them. The main pitfalls are using a generic, global-average factor when a region- or supplier-specific one exists and would be materially different, for example a country's grid-electricity factor varies enormously by generation mix, and failing to update factors as they are revised, which silently changes reported emissions in ways unrelated to any real change in activity.