What Are Emission Factors and Why Do They Matter?
Greenhouse gas inventories rest on two fundamental components: activity data and emission factors. Activity data tells you how many litres of fuel you burned or how many kWh of electricity you consumed. The emission factor is the conversion coefficient used to calculate how much greenhouse gas that activity released into the atmosphere.
Expressed as a simple formula:
Emissions = Activity Data x Emission Factor
According to the IPCC Guidelines for National Greenhouse Gas Inventories, emission factors are defined as "coefficients that represent the amount of greenhouse gas emitted per unit of activity" (IPCC, 2006 IPCC Guidelines for National Greenhouse Gas Inventories, Volume 1, Chapter 1). For example, if burning one litre of diesel releases approximately 2.68 kg CO2 equivalent, that value is the emission factor for that fuel.
Selecting the wrong emission factor can render an entire inventory meaningless. Even if your activity data is one hundred percent accurate, an incorrect factor can skew total emissions calculations by more than thirty percent. This is precisely why ISO 14064-1:2018 requires that the sources, validity dates, and selection rationale for all emission factors be documented (ISO 14064-1:2018, Clause 6.3).
The Emission Factor Hierarchy
The GHG Protocol Corporate Standard defines an accuracy hierarchy for emission factor selection. Factors at the top of the hierarchy provide greater accuracy, while those at lower tiers are more accessible but carry higher uncertainty (GHG Protocol, A Corporate Accounting and Reporting Standard, Revised Edition, Chapter 8).
Supplier-Specific Factors
The highest tier of the hierarchy. These are factors based on measurement or verified calculations at your supplier's own facility. For instance, if the manufacturer of the steel you purchase has calculated CO2 equivalent per tonne based on furnace-level data at their plant, this is the most reliable data source available. This is the most effective way to improve Scope 3 data quality in CSRD and CDP reporting.
Product-Specific Factors
Factors derived through life cycle assessment (LCA) data for a specific product category. Databases such as ecoinvent and GaBi fall into this category. When direct supplier data is unavailable, product-specific factors offer a reliable alternative.
Country-Specific Factors
Factors published by national agencies that reflect country conditions. For Turkey, TURKSTAT's greenhouse gas inventory reports and energy balance tables serve this purpose. For the United Kingdom, the GHG Conversion Factors guide published annually by Defra/DESNZ has become an industry-standard reference (UK Department for Energy Security and Net Zero, "Greenhouse gas reporting: conversion factors 2024," 2024).
Global Default Factors
Default emission factors published by the IPCC serve as a last resort when no other source is accessible. Because these factors reflect global averages, they may not fully capture country- or sector-specific conditions.
Rule of Thumb: Use the highest available hierarchy tier. However, the highest tier is not always practical. What matters is documenting why you stayed at a particular level.
Key Emission Factor Databases
IPCC Emission Factor Database (EFDB)
The IPCC Emission Factor Database serves as the reference point for preparing national inventories. It covers energy, industrial processes, agriculture, waste, and land use sectors. The database is regularly updated and was expanded with the 2019 Refinement (IPCC, 2019 Refinement to the 2006 IPCC Guidelines for National Greenhouse Gas Inventories).
Defra/DESNZ GHG Conversion Factors
Updated annually by the UK Department for Energy Security and Net Zero, this guide is one of the most widely used sources in corporate carbon accounting. With broad coverage spanning fuels, electricity, transportation, material use, and waste management, the factor set is frequently used by companies outside the United Kingdom as well. The 2024 edition was published in June 2024 (UK DESNZ, 2024).
EPA Emission Factors
Emission factors published by the US Environmental Protection Agency are particularly relevant for companies operating in or exporting to the US market. EPA's Emission Factors Hub provides comprehensive factors for stationary combustion, mobile sources, and industrial processes.
TURKSTAT Greenhouse Gas Inventories
The Turkish Statistical Institute publishes annual greenhouse gas inventory reports. These reports contain emission factors reflecting Turkey's national conditions. TURKSTAT data is the primary source for Turkey's electricity grid emission factor in particular. Electricity generation statistics published by EMRA also provide supporting data for indirect emissions calculations.
Factor Selection by Scope
Scope 1: Direct Emissions
Factor selection for direct emissions is generally the most straightforward. Country-specific or IPCC default factors are used based on fuel type (natural gas, diesel, LPG, etc.). ISO 14064-1:2018 clearly defines the distinction between "measurement and calculation methods" for direct emissions quantification (ISO 14064-1:2018, Clause 5.2.3). Continuous emissions monitoring systems (CEMS) yield the most accurate results for large facilities, though most companies use calculation methods based on fuel consumption data.
Watch Out: When converting between volumetric (m3) and mass (kg) units for natural gas, ensure you apply the correct calorific value (net calorific value or gross calorific value). This is one of the most common calculation errors.
Scope 2: Indirect Energy Emissions
Two approaches exist for emissions from electricity consumption:
- Location-based method: Uses the average emission factor for the grid where electricity is consumed. For Turkey, this value is approximately 0.44-0.48 kg CO2/kWh (based on TURKSTAT 2022 data).
- Market-based method: Applies factors based on the source of the electricity the company purchased. Companies that have purchased renewable energy certificates (I-RECs, GOs) can use near-zero factors under the market-based method.
The GHG Protocol Scope 2 Guidance recommends that companies report using both methods (GHG Protocol, Scope 2 Guidance, 2015).
Scope 3: Value Chain Emissions
Scope 3 is the most complex and most debated area. Covering fifteen distinct categories, emission factor selection in Scope 3 varies significantly by category:
- Category 1 (Purchased Goods and Services): Spend-based, weight-based, or supplier-specific factors can be used. The spend-based approach is the easiest to implement but offers the lowest accuracy.
- Category 3 (Fuel- and Energy-Related): Defra/DESNZ factors are widely used for transmission and distribution losses and upstream emissions.
- Category 6 (Business Travel): Passenger-km based factors are applied according to flight distance and class. ICAO's Carbon Emissions Calculator methodology is accepted as a reference.
Annual Updates and Version Management
Emission factors are not static. As energy mixes shift, technologies advance, and measurement methodologies improve, factors are updated. For enterprise teams, this necessitates an annual update process.
Update Calendar
- Defra/DESNZ: New version published each June
- IPCC EFDB: Irregular updates; major revisions occur in five- to six-year cycles
- EPA: Annual updates, typically in the March-April period
- TURKSTAT: Annual GHG inventory, generally publishing the previous year's data by the end of the following year
Version Control
Recording which factor set was used in which reporting year is critically important during audit processes. Maintaining an emission factor register is recommended:
| Activity | Factor Value | Unit | Source | Version/Year | Last Updated |
|---|---|---|---|---|---|
| Natural gas | 2.02 | kg CO2/m3 | IPCC 2006 | Vol. 2, Table 2.3 | 2006 |
| Electricity (TR grid) | 0.46 | kg CO2/kWh | TURKSTAT | 2022 inventory | 2024 |
| Diesel | 2.68 | kg CO2/litre | Defra | 2024 v1.0 | June 2024 |
Documentation Requirements for Audit
ISO 14064-1:2018 places transparency and traceability principles at the forefront of emission factor selection (ISO 14064-1:2018, Clause 9). During audit processes, verifiers will ask the following questions:
- Source selection rationale: Why did you use a factor from this database? Was a more specific source not available?
- Geographic appropriateness: Does the factor reflect the geography where your activities take place?
- Temporal appropriateness: When was the factor published? Is it aligned with the reporting period?
- Consistency: Were there factor changes compared to previous years? If so, what was the reason?
- Uncertainty assessment: What is the uncertainty margin of the factor set?
The GHG Protocol Corporate Standard similarly emphasises that "the sources, applicability, and any assumptions associated with emission factors must be clearly documented" (GHG Protocol, Corporate Standard, Chapter 8, p.45).
Common Mistakes and How to Avoid Them
Mistake 1: Continuing to Use Outdated Factors
A calculation model set up once can run for years without factor updates. This causes significant drift, particularly for electricity grid emission factors. Turkey's energy mix has changed substantially over the past decade; there are notable differences between 2015 and 2023 factors.
Mistake 2: Unit Mismatches
Confusion between net calorific value (NCV) and gross calorific value (GCV) in heating values, or conversion errors in energy units (MJ, kWh, TJ), are frequently encountered problems.
Mistake 3: Geographic Mismatch
Using a European country's grid emission factor for Turkish operations, or attempting to derive Turkey-specific fuel factors from a US-based database, leads to erroneous results.
Mistake 4: Double Counting in Scope 3
Counting the same emissions in multiple Scope 3 categories inflates the total inventory. Clarifying category boundaries according to the GHG Protocol Value Chain Standard is essential.
Conclusion: A Systematic Approach Is Non-Negotiable
Emission factor selection forms the technical backbone of carbon accounting. It is not a one-time decision but a continuous process that must be reviewed, updated, and documented annually. By following the hierarchy, documenting sources, and maintaining the principle of consistency, it is possible to build a reliable and defensible greenhouse gas inventory for audit processes.
Action Item: Review your current emission factor register. Confirm the source, publication date, and geographic appropriateness of each factor. If documentation gaps exist, close them in this cycle.