What is an emission factor, and why versioning it matters

Draft — pending editorial review.

Every carbon figure you publish ultimately comes down to one calculation: activity data multiplied by an emission factor. Understanding what that factor actually is — and isn’t — matters more than most people expect.

The basic idea

An emission factor converts a unit of activity (litres of fuel, kilometres travelled, kilowatt-hours consumed) into an equivalent mass of greenhouse gas, expressed as CO2e — carbon dioxide equivalent, which normalises different gases like methane and nitrous oxide onto one comparable scale.

Where factors come from

Factors are published by recognised bodies — government agencies like DEFRA, international frameworks like the GHG Protocol, and industry-specific databases — based on measured or modelled emissions for a given activity, region and year.

Why the version matters

Emission factors are updated regularly as measurement methods improve and grids change (a kilowatt-hour today is cleaner than the same kilowatt-hour five years ago, in most grids). That means the same activity recalculated with a newer factor can produce a different number — which is exactly why a defensible carbon report needs to record which version of a factor was used for every calculation, not just the current one.

What this means in practice

If an auditor asks “how did you get this number two years ago,” the honest answer needs to be reproducible: the activity data, the factor, and the factor’s version, all traceable together. That traceability — not just the calculation itself — is what turns a number into something you can stand behind.