The Importance of Documenting Emission Factor Assumptions

The Importance of Documenting Emission Factor Assumptions

Emission factor selection is one of the cornerstones of carbon inventory. No matter how accurate the activity data is, using wrong or incompatible emission factors completely distorts the result. But most companies don't document factor selection. This gap creates problems during the verification process, disrupts year-over-year consistency, and leads to information loss during team changes.

In this article, we cover in detail why emission factor documentation is critical, what information should be recorded, and a systematic documentation structure.

Why is documentation important?

Verification requirements

Verification teams question the source, validity, and selection rationale for each emission factor. ISO 14064-3 and ISAE 3410 standards require the verifier to be able to reproduce calculations. If the factor source is uncertain, the verifier cannot verify the calculation and this is reported as a finding.

Typical questions asked by verifiers:

• Where was this factor obtained from?
• Why was this source preferred?
• What year is the factor from?
• Is the geographic scope appropriate?
• Does the technological scope reflect the current situation?

Year-over-year consistency

Carbon inventory should be comparable across years. Using different factors in different years hides actual performance changes. Documentation enables tracking of which factors changed when and why.

Example: A company used 0.48 kg CO2e/kWh as the Turkey grid electricity factor in 2023. With the IEA update in 2024, this factor became 0.42 kg CO2e/kWh. If this change is not documented, it cannot be understood whether the emission reduction is from actual efficiency or factor change.

Team continuity

When the person preparing the carbon inventory changes, documentation is the foundation of knowledge transfer. If the rationale for previous factor selections is unknown, the new team member may make different decisions and inconsistency occurs.

Reproducibility

The same calculation should produce the same result by different people. If the factor value is specified but the source is unclear, the calculation cannot be reproduced.

Documentation template structure

A comprehensive record should be kept for each emission factor. Recommended template fields:

Basic information

Scope information

WTT, TTW, WTW distinction

There are three different scopes in fuel emission factors. Understanding this distinction is critical:

Well-to-Tank (WTT): Emissions from fuel extraction, refining, and transportation stages. These are upstream emissions occurring before the fuel is used. Falls under Scope 3 Category 3.

Tank-to-Wheel (TTW): Emissions from combustion of fuel at the point of use. Falls under Scope 1.

Well-to-Wheel (WTW): WTT + TTW total. All life cycle emissions.

Common mistake: Using WTW factor when TTW factor should be used, or vice versa. This error leads to approximately 15-25% miscalculation of emissions.

Source hierarchy

GHG Protocol recommends a hierarchy for emission factor selection. Starting from the most reliable source:

1. Primary data (supplier specific)

The highest quality factor is the product or service-specific value from the supplier. For example:

• Contract-based emission factor from electricity supplier
• Specific fuel analysis from fuel supplier
• PCF value from raw material supplier

2. National factors

Turkey National Greenhouse Gas Inventory (NIR) is the reference source for energy and fuel factors. Official institution data should be preferred:

• TurkStat/Ministry of Environment and Urbanization data
• National energy agency data
• Industry association data

3. Regional factors

If national factor is not available, regional sources are used:

• EU JRC (Joint Research Centre) data
• IEA (International Energy Agency) country data
• Ecoinvent regional data sets

4. International factors

If regional data is not available, international sources are consulted:

• DEFRA (UK) emission factors
• EPA (US) emission factors
• GaBi/Ecoinvent global data sets

5. Default values

As a last resort, IPCC default values are used. These values represent global averages and have the lowest accuracy level.

Selection justification: At each stage, why a higher-tier source could not be used should be explained.

Year matching requirements

The emission factor year should be compatible with the activity data year. General rules:

Ideal situation: Factor year = reporting year

• 2024 factors should be used for 2024 inventory

Acceptable: Factor year = reporting year ± 2 years

• 2022-2026 factors can be used for 2024 inventory
• Difference should be explained

Careful use: Factor year > 5 years old

• Adds significant uncertainty
• Justification and impact analysis should be conducted
• Improvement plan should be stated

Special cases

Electricity grid factors: Grid composition changes annually. Most current year factor should be used when possible.

Fuel factors: Although relatively stable, there are differences between refineries. Prefer national averages.

Material factors: More stable unless production technology changes. 5-year factors are acceptable.

Geographic scope assessment

The geographic scope of the emission factor should be compatible with the region where the activity takes place.

Compatibility matrix

Special note for electricity: Turkey has a single grid. The grid factor is the same in different regions. However, if there is a green energy contract, the supplier factor is used for market-based.

Version control

Emission factors are periodically updated. Which version was used when should be tracked.

Version control table

Change management procedure

Steps to follow when a factor is updated:

1. Change detection: Source update announcements are monitored
2. Impact analysis: Impact of new factor on inventory is calculated
3. Decision: Will update be made, from which period?
4. Implementation: Calculation files are updated
5. Documentation: Change is recorded, rationale is stated
6. Communication: Relevant stakeholders are informed

Base year recalculation triggers

Factor change may require base year recalculation. GHG Protocol criteria:

• If materiality threshold (usually 5% or 10%) is exceeded
• If methodology fundamentally changed
• If new data significantly affects previous calculation

Audit trail requirements

Full traceability should be provided for verification.

Audit trail elements

Source document reference:

• Full name of document from which factor was obtained
• Page or table number
• Access date (for online sources)
• Archived copy (source may change)

Calculation link:

• Which calculation the factor was used in
• Formula reference
• Intermediate calculations

Change record:

• When factor was added to system
• Who added it
• Any updates made

File retention

Emission factor documentation should be stored together with inventory data:

• Minimum retention period: 7 years (or according to company policy)
• Accessible format (Excel, PDF, database)
• Backup and version control

Practical implementation recommendations

Central factor library

Create a central library that collects all factors in one place:

• Excel spreadsheet or database
• All fields above for each factor
• Easy search and filtering
• Regular update procedure

Factor selection checklist

When adding each new factor:

• Is source reliable?
• Is geographic scope appropriate?
• Is temporal scope acceptable?
• Is technological scope compatible?
• Is WTT/TTW/WTW distinction correct?
• Is unit correct?
• Is selection justification written?
• Have alternatives been evaluated?

Annual review

Factor library should be reviewed at least once a year:

• Have sources been updated?
• Are better alternatives available?
• Unused factors should be archived
• Are factors needed for new activities?

Conclusion

Emission factor documentation is the cornerstone of carbon inventory reliability. A systematic approach speeds up the verification process, ensures year-over-year consistency, and prevents information loss during team changes.

This information should be kept in a separate tab or document. Creating a central factor library saves time in the long run and reduces error risk. Factor selection is not just finding a number, but understanding and recording all the context behind that number.