Choosing emission factors

What are emission factors?

An emission factor expresses the greenhouse gases released per unit of activity, such as kilograms of CO2 per litre of diesel or tonnes of CO2 equivalent per megawatt-hour of electricity. Emissions equal activity data times the factor, so the factor is as important as the data. Some factors cover only carbon dioxide; others include methane and nitrous oxide, already converted to CO2 equivalent. Some are combustion-only; well-to-tank factors add the upstream emissions of producing and transporting the fuel, which belong in scope 3, category 3. See calculating scope 1 and 2 emissions.

Where do emission factors come from?

• Government publications: the US EPA's GHG Emission Factors Hub, last updated in January 2025, and its eGRID data for US grid regions; the UK government's conversion factors, updated each June, most recently on 11 June 2026; and national inventory factors elsewhere.
• The IPCC: default factors in its national inventory guidelines, and the global warming potentials in its assessment reports.
• Electricity data: grid operators and agencies for location-based factors, and residual mix publishers, such as the Association of Issuing Bodies in Europe, for market-based ones.
• Suppliers and measurement: utilities' supplier-specific rates, fuel suppliers' data, and direct measurement such as continuous emissions monitoring.

How should a company choose emission factors?

The GHG Protocol's principles of accuracy, relevance and consistency point to a simple order: measured or supplier-specific data first, then factors specific to the fuel, technology and country or grid region, then national averages, then international defaults. Factors should relate to the reporting year, or the most recent year available, and come from a credible, public source. A company should use the same source and approach every year and explain any change. Spend-based factors for scope 3 follow the same logic, from supplier data down to industry averages. See the spend-based method.

Emission factors in practice: one plant, two sets of choices

A food plant has three sources. A first draft used a national average grid factor and global warming potentials from the IPCC's Fifth Assessment Report (AR5); the reviewed inventory uses the grid factor for the plant's region and AR6 values, as IFRS S2 requires. The grid factors are illustrative; the global warming potentials are the IPCC's.

The regional grid factor makes the biggest difference: the plant sits in a region with more coal-fired generation than the national average, adding 1,200 tonnes. The newer global warming potentials raise the refrigerant figure and slightly lower the methane figure, because AR6 gives non-fossil methane a value of 27.0, below the AR5 value of 28 that most factor sets used. The company discloses the sources of its factors and global warming potentials, and explains the change from the draft basis in its methodology note.

Which global warming potentials should be used?

Global warming potentials convert each gas into CO2 equivalent over a 100-year horizon. IFRS S2 requires the values from the latest IPCC assessment available at the reporting date, currently the Sixth Assessment Report, and its December 2025 amendments let a company use other values where a law or exchange requires them, for the part of the business affected. Many government factor sets and emissions trading schemes still use AR5 or earlier values, so a company may need to recalculate rather than take published CO2-equivalent factors as they are; refrigerant blends are a common case, see refrigerants and fugitive emissions. See also the December 2025 IFRS S2 amendments.

How often should emission factors be updated?

Each year, as new factors are published, a company should use those that best match the reporting year. A change in factors is normally applied from that year forward. The base year is recalculated only if a change in methods or a significant improvement in accuracy would distort the trend, under the company's recalculation policy. Electricity factors move most from year to year, as grids add renewable power.

What do IFRS S2, the ESRS and SB 253 require?

IFRS S2 asks companies to disclose their measurement approach, inputs and assumptions, including emission factors. ESRS E1 asks for the methods and significant assumptions behind emissions figures. CARB's proposals for SB 253 from 2027 require companies to document any missing emission factors and the substitutes used. Assurance providers test that factors match the activity, year and location, so a clear record of each factor's source saves time. See the GHG Protocol explained and sustainability assurance.

This guide is general information. It is not tax or legal advice for your situation.