GHG Protocol Emission Factors
Emission factors are the conversion coefficients that translate activity data (litres of fuel, kWh of electricity, tonnes of waste) into greenhouse gas emissions. Choosing the right emission factor source — and using the correct version for your reporting year — is one of the most consequential methodological decisions in GHG accounting.
Emission factors are the conversion coefficients that translate activity data (litres of fuel, kWh of electricity, tonnes of waste) into greenhouse gas emissions. Scope 1 emission factors (fuel combustion): DEFRA GHG Conversion Factors (UK) — the most comprehensive and widely used global reference, updated annually each June.
Emission factor sources by scope
Scope 1 emission factors (fuel combustion): DEFRA GHG Conversion Factors (UK) — the most comprehensive and widely used global reference, updated annually each June. ADEME Base Carbone (France) — French national factors. US EPA Emission Factors for Greenhouse Gas Inventories. For multinational companies, use country-specific national factors where available and DEFRA as a fallback for countries without published national factors.
Scope 2 emission factors (purchased electricity): Location-based — IEA Electricity Emission Factors (annual publication) or national grid operator factors. The European Environment Agency publishes EU country-specific grid factors annually. Market-based — supplier-specific residual mix factors from AIB (Association of Issuing Bodies) for European companies using GOOs.
Scope 3 emission factors: Activity-based categories (travel, freight, waste) — DEFRA conversion factors cover the most common Scope 3 activity categories. Spend-based categories (Cat 1 purchased goods) — EEIO (Environmentally Extended Input-Output) factors from EXIOBASE, US EPA USEEIO, or commercial providers (Sphera, SimaPro).
The annual update problem — why version matters
Emission factors change annually as energy mixes shift, methodology improves, and GWP values are updated. Using the wrong year's factors introduces errors that assurers will identify.
For DEFRA factors: the 2025 edition (published June 2025) must be used for FY2025 reporting. Using the 2024 edition for FY2025 data is a methodology error — document why if you have operational reasons for using a prior version.
For grid electricity factors: the IEA Electricity Emission Factors typically have a 2-year lag — 2025 factors are based on 2023 grid data. This lag is acknowledged in GHG Protocol guidance; use the most recent published factors and disclose the data year.
For GWP values: IPCC AR6 GWP100 values (published 2021) are now the standard for corporate GHG reporting. CH4 GWP100 = 27.9 (was 25 under AR4). N2O GWP100 = 273 (was 298 under AR4). The switch from AR4 to AR6 increases reported emissions for methane-intensive businesses — disclose the switch and restate prior year figures where the impact is material.
Spend-based vs activity-based factors — the accuracy tradeoff
For Scope 3 Category 1 (purchased goods and services), two main factor types are used:
Spend-based EEIO factors: Apply a monetary input-output emission factor (kg CO2e per € or $ of spend) to your procurement expenditure by category. Accuracy: ±50–100%. No supplier data required. Good for initial screening.
Activity-based factors: Apply a physical emission factor (kg CO2e per kg of material, per tonne-km of freight, per kWh of energy) to physical activity data. Accuracy: ±20–40%. Requires operational data from suppliers. Preferred for material categories.
Supplier-specific factors: Use actual GHG emissions reported by individual suppliers, divided by the quantity of product or service purchased. Accuracy: ±5–10%. Requires supplier disclosure programme. Required for material Category 1 suppliers under ESRS E1-6 best practice.
The ESRS E1-6 requirement to disclose the percentage of Scope 3 calculated using primary data vs secondary data creates a direct incentive to increase supplier-specific factor coverage over time.
Frequently asked questions
Where do I download the DEFRA emission factors?
From the UK Government website — search 'DEFRA GHG Conversion Factors' and select the current year's publication. The file is a multi-tab Excel workbook covering fuel combustion, electricity, freight, travel, waste, and water. New factors are typically published in June each year for the preceding calendar year.
Can we use a different emission factor source for different scopes?
Yes — it is common to use DEFRA for Scope 1 fuel combustion, IEA for Scope 2 grid electricity, and a combination of DEFRA and EEIO providers for Scope 3. What matters is disclosure — your methodology note must specify which factor source and version was used for each emission category.
What is a residual mix factor and when do we use it?
A residual mix factor is the emission factor for grid electricity after removing the electricity covered by renewable energy certificates (GOOs). It is used for market-based Scope 2 calculation when a company has not purchased GOOs — it is typically higher than the average grid factor because renewable generation has been 'claimed' by companies purchasing certificates. The AIB (Association of Issuing Bodies) publishes EU residual mix factors annually.