An emissions calculation usually consists of activity data multiplied by an emission factor. Sustainability teams often spend significant time collecting and validating activity data, while treating the emission factor as a number that can simply be selected from a database.
In practice, selecting an emission factor also requires judgment.
Two practitioners can use the same activity data and calculate different emissions if they select different emission factors. Both factors may come from credible sources, but they may represent different regions, years, system boundaries, or methodologies.
For example, an electricity factor for one country may be different from a factor for another country because the electricity generation mix is different. A factor published in 2021 may also differ from one published in 2025 because the grid has changed. A fuel factor that includes upstream production and transportation will also produce a different result from one that only includes direct combustion.
This means that emission factor selection should be based on the specific activity being measured and the reporting requirements being followed. The reason for selecting a factor should also be documented so that the calculation can be reviewed and understood later.
What the library holds
The Hydrus Emission Factor Library provides access to more than 790,000 emission factors, continuously updated and drawn from authoritative providers including BEIS, EPA, and IEA.
Users can filter the library by category, sector, region, year, scope, and source. Opening a factor provides more detailed information, including its description, value, unit, category, year, and other relevant attributes.
A large factor library provides broad coverage across different activities, regions, and reporting needs. However, it also means that several factors may appear suitable for the same activity.
The filters help users narrow the available options and identify the factor that best matches the activity being measured.
Five filters, in the order they should be applied
Start with the factors that determine whether an emission factor is suitable for the activity, then review the remaining details before making a final selection.
- Region. Select a factor that matches the location of the activity as closely as possible. Electricity grids, fuel compositions, transport systems, and waste treatment methods can vary significantly between countries and regions. Using a factor from the wrong location may reduce the accuracy of the calculation.
- Year. Use a factor that matches the reporting year where possible. If a factor for the exact year is not available, use the most recent suitable factor and record the difference. This is especially important for electricity factors, which can change from year to year as the electricity generation mix changes.
- Activity category and unit. Confirm that the factor matches the activity being measured and that the unit is compatible with the activity data. A factor expressed per litre should not be applied directly to data recorded in kilograms. A passenger-kilometre factor is also different from a vehicle-kilometre factor. If unit conversion is required, it should be done clearly and documented.
- Scope and boundary. Check what the factor includes. Some factors cover only direct combustion, while others also include upstream activities such as fuel extraction, processing, and transportation. Some factors include only CO₂, while others include several greenhouse gases expressed as CO₂e. The factor boundary should match the scope and category in which the emissions are being reported.
- Source. When several credible sources provide factors for the same activity, select the source that best matches the geography, methodology, and system boundary of the activity. Once a source has been selected, use it consistently where appropriate. Changing sources between reporting periods can create differences in reported emissions that are caused by methodology rather than actual operational changes.
When sources disagree
Different sources often publish different emission factors for the same activity.
A national government agency, an international organization, and a lifecycle database may all publish factors for the same fuel or activity, but the values may not be the same.
These differences can result from several factors, including different datasets, geographic coverage, system boundaries, greenhouse gas assumptions, and calculation methods.
When this happens, the factors should not simply be averaged, and the lowest value should not be selected without justification.
The appropriate factor should be the one that best matches the activity being measured and the reporting method being used. The reason for selecting that factor should be documented, and the same approach should be used consistently for similar calculations.
Consistency is especially important when comparing emissions between reporting periods.
If an emission factor changes because the source updates its methodology or because the organization changes providers, the reported emissions may change even when the underlying activity has not changed.
These methodological changes should be recorded separately so that users can distinguish between changes caused by operational performance and changes caused by the emission factor itself.
What to capture with every selection
The information associated with an emission factor should be recorded so that the calculation can be reviewed and reproduced.
At minimum, each calculation should include:
- Factor value, unit, and the exact source or publication from which it was obtained
- Publication year and the reporting period in which the factor was used
- Country, region, or electricity grid represented by the factor
- Greenhouse gases covered and the global warming potential values applied
- System boundary, including the emissions that are included and excluded
- Reason for selecting the factor when several suitable options were available, together with the date the decision was reviewed
In Hydrus, the selected emission factor can remain linked to the calculation it supports. This means the factor can be viewed together with the activity data, the relevant facility or supplier, and the review or approval associated with the calculation.
When the emission factor library is updated, the calculation history can show which factor was used for each reporting period.
This makes it easier to review previous calculations and explain changes if a reported figure needs to be updated or restated.
For each reported emissions figure, the organization should be able to identify the factor used, the source and year of that factor, and the reason it was selected.
Emission factors should therefore be selected carefully based on the activity, location, reporting year, methodology, and system boundary. Clear documentation makes the resulting calculation easier to understand, review, and support during assurance.
This guide is educational and not legal advice.