Emission Factor Selection: IPCC, DEFRA, or EPA? When to Use Which?

Emission Factor Selection: IPCC, DEFRA, or EPA? When to Use Which?

One of the most debated topics in carbon inventory is the emission factor. Two different teams work with the same activity data and produce two different results. This often turns into a "which is correct" debate. In practice, the right question is different: For what purpose is this study being prepared and in what context will it be used?

In this article, we compare emission factor sources and provide a practical framework for selection criteria.

What is an emission factor?

An emission factor is a coefficient that converts an activity into emissions. It is multiplied by activity data (e.g., fuel consumption, electricity use, travel distance) to calculate emissions.

The basic formula is: Emissions = Activity data × Emission factor

For example: 1,000 liters of diesel × 2.68 kg CO2e/liter = 2,680 kg CO2e

The factor is determined by the fuel's combustion characteristics, electricity generation mix, and transport mode efficiency. Different factor sources can give different values for the same activity.

Basic source types

Primary data (supplier data)

The most reliable factor type is measurement-based data from the supplier or manufacturer. For example, an emission factor calculated by an electricity supplier based on their own generation mix.

This data type is generally used in market-based Scope 2 calculations or product carbon footprint studies.

Regional and national databases

Factors compiled at country or regional level. For Turkey, the TEİAŞ (Turkish Electricity Transmission Corporation) grid emission factor is the main reference. TEİAŞ publishes the emission intensity of Turkey's electricity grid annually. This factor should be used for location-based Scope 2 calculations. For EU countries, AIB residual mix factors fall into this category.

Other sources in Turkey include Ministry of Energy and Natural Resources data and TÜİK (Turkish Statistical Institute) energy statistics.

International databases

Factor sources that can be used at global scale. Factors published by organizations like IPCC, DEFRA, and EPA fall into this category.

Detailed main sources

IPCC (Intergovernmental Panel on Climate Change)

IPCC publishes default emission factors for use in national greenhouse gas inventories. The 2006 IPCC Guidelines and subsequent updates are reference documents.

Advantages:

• Scientifically accepted reference
• Broad coverage for all gases and sectors
• Cross-country comparability

Disadvantages:

• Default values don't fully reflect regional differences
• Low update frequency
• Can be too general for corporate inventory

When to prefer: In national inventories, when consistency is needed for global companies, when no regional factor exists.

DEFRA (UK Government)

DEFRA publishes a comprehensive emission factor set updated annually. It provides detailed factors for Scope 1, 2, and 3 categories.

Advantages:

• Currency with annual updates
• Wide options for Scope 3 categories (transport, waste, procurement, etc.)
• Well-to-tank and tank-to-wheel distinction
• Free access

Disadvantages:

• Optimized for UK context
• Direct use in other countries may create inconsistency

When to prefer: In Scope 3 calculations, for business travel and logistics, in well-to-tank calculations.

EPA (US Environmental Protection Agency)

EPA provides emission factors for the US context. eGRID (electricity grid) and GHG Emission Factors Hub are the main sources.

Advantages:

• Detailed regional factors for US electricity grid
• Regular updates

Disadvantages:

• Limited applicability outside the US

When to prefer: For US operations, in US grid electricity calculations.

LCA databases (Ecoinvent, GaBi, OpenLCA)

Databases designed for product life cycle assessment. They provide process-based detailed factors.

Advantages:

• Product and process-based detail
• Suitable for PCF and LCA studies
• Geographic and technological variations

Disadvantages:

• License cost (Ecoinvent, GaBi)
• Too detailed for corporate inventory

When to prefer: In PCF calculations, EPD preparation, product comparisons.

Selection criteria framework

Geographic appropriateness

The factor should be compatible with the geography where the activity takes place. For electricity consumed in Turkey, the Turkey grid factor is preferred. Global factors are the last option.

Temporal appropriateness

The factor should be current. Old factors don't reflect technology changes and fuel mix updates. Ideally, factors close to the reporting period should be used.

Scope appropriateness

The factor should be compatible with the emission scope being calculated. Well-to-tank (fuel production chain), tank-to-wheel (combustion), and well-to-wheel (total) differences are important. For Scope 1, tank-to-wheel is generally used; for Scope 3 Category 3, well-to-tank is used.

Technological appropriateness

The factor should be compatible with the technology or fuel type used. Diesel and biodiesel require different factors. Electric and fossil fuel vehicles should be evaluated differently.

Hierarchy approach

GHG Protocol recommends a hierarchy for factor selection:

1. Primary data (supplier data, measurement)
2. Regional or national factors
3. International databases
4. Global default values

The higher level should be preferred when possible.

Documentation requirements

Emission factor selection should be documented:

• Factor value used
• Source (database, publication, version)
• Year
• Geographic scope
• Selection rationale

This documentation is questioned during the verification process. Factor selections without clear rationale can be a source of findings.

Consistency principle

It may be necessary to use factors from different sources within the same inventory. But consistency should be maintained as much as possible. Using different factor sources for the same activity type weakens comparability.

Practical emission factor selection workflow

Follow these steps in emission factor selection:

1. Define the activity type: Fuel combustion, electricity consumption, transportation, waste, etc.
2. Determine the scope: Scope 1, 2, or 3? Well-to-tank or tank-to-wheel?
3. Check geography: Where does the activity take place? For Turkey, first look for national sources
4. Evaluate currency: Use the most current year's factor
5. Document the source: Record factor value, source name, version, and year
6. Escalate if no alternative: If no national factor exists, move to regional or global factors

Conclusion

Emission factor selection is as much a communication decision as a technical one. Being able to explain why a factor was selected is as important as the calculation itself. Following the hierarchy approach, documenting, and maintaining consistency are the cornerstones of a reliable inventory.