What is an emission factor?
An emission factor is the bridge between a record you have and the greenhouse gas emissions you need to report. Its unit is a quantity of emissions divided by a unit of activity, such as kg CO2e/kWh, kg CO2e/litre, kg CO2e/km, or kg CO2e/USD. CO2e means carbon dioxide equivalent, which puts different greenhouse gases on one comparable basis.
Why does your factor choice matter?
A customer, reporting standard, assurance provider, or platform may test whether someone else can reproduce your number. Keep a methodology note with the activity data, factor name, publisher, dataset version, factor year, geography, unit, and whether the result is location-based or market-based. Retain the source file or supplier document as evidence.
Year and geography matter most for electricity because grid generation changes across places and over time. Match the factor to where the activity occurred and, where available, to the reporting year. If you use an older year or a broader geography as a proxy, say so.
How does the calculation work?
The basic formula is: activity data x emission factor = emissions. If a site uses 1,000 kWh and the factor is 0.25 kg CO2e/kWh, the kWh units cancel and the result is 250 kg CO2e. The denominator of the factor must match the unit in your source data.
Choose the source that matches the activity. For location-based electricity reporting, use a current grid factor for the site's geography. For market-based reporting, use eligible supplier or contract data and apply the residual mix, which represents unclaimed electricity, to any uncovered use. For fuel and process emissions, use the applicable national inventory or IPCC method. If you only have spend, use a factor per currency unit for screening and record the category, currency conversion, price year and limitation.
What mistakes should you avoid?
- Using an old factor year without checking for a newer value or explaining the proxy.
- Using a national average when a relevant subnational grid factor is available, or applying the factor for the wrong country.
- Mixing kg CO2e with tCO2e. Divide kilograms by 1,000 once, at the end.
- Labelling a grid-average location-based result as market-based without eligible contractual data, a residual mix, or a documented fallback.
Is a supplier-specific factor always better?
No. Prefer it when it covers the product or service you bought, the same reporting period and geography, and the same emissions stages you intend to report. A current, well-matched average can be more defensible than an incomplete supplier figure.
Can I use a spend-based factor?
Yes, for screening when quantity, distance, energy, or supplier-specific data is unavailable. Record the spend category, currency conversion, and price year. Replace it with physical or supplier data for material categories when better data becomes available.
How can someone check my factor?
Give them the methodology note, the factor source and year, the exact value and unit, the geography, and the activity-data record. Show any conversion from kg to tonnes and explain every proxy or substitution.