What is an Emissions Factor? A Definition - Last Updated: July 29, 2023

Emissions factors are used to estimate, calculate, or convert the rate an activity releases greenhouse gases (GHG) into the atmosphere. Emissions factors are also called conversion factors, emission intensity, carbon intensity, or abbreviated as "EF". They are typically expressed as an amount of a pollutant emitted per unit of activity, such as kilograms (kg) of CO₂ generated per kilowatt hour (kWh) of electricity purchased from your utility provider.

The United States Environmental Protection Agency (EPA), which publishes a library of U.S. emissions factors, defines an emissions factor as:

"A representative value that attempts to relate the quantity of a pollutant released to the atmosphere with an activity associated with the release of that pollutant"

Here are some examples of real emission factors:

For example, if a power plant generates 100 megawatt-hours (MWh) of electricity per day, and the emission factor for coal-fired power generation is 0.85 kilograms of CO₂ per kilowatt-hour, then the plant will emit 85,000 kilograms of CO2 per day (100 * 1,000 * 0.85).

There are a variety of emission factors available for different countries, activities, processes, transactions, and investments. These factors are typically developed by government agencies, research institutions, scientific studies, or lifecycle assessments (LCAs), and are based on a variety of data sources, such as emissions measurements, fuel consumption data, and engineering models.

How to Use Emissions Factors

Emission factors are used to convert an activity into an equivalent amount of emissions, typically carbon dioxide. This allows companies, governments, investors, and other entities to convert what they do into CO₂. This process of calculating carbon or GHG emissions is called carbon accounting.

Carbon accounting is an accounting method to count, inventory, track, and report your organization's greenhouse gas (GHG) emissions. This is also known as your carbon footprint. For most companies, the established, global accounting unit for carbon is the greenhouse gas carbon dioxide (CO₂), and "carbon equivalents" (CO₂e or CO₂eq), the sum of carbon plus other emissions like methane converted into carbon.

In carbon accounting, emissions factors are used to convert activity like energy usage, transportation mileage, or financial purchases into kilograms or metric tonnes of CO₂e. Once the correct emissions factor is selected, all that needs to be done is multiply the activity (amount) by the emissions factor.

Activity  *  Emissions Factor  =  CO₂e

If you're performing emissions calculations without carbon accounting software like Brightest's which automatically converts units, it’s important to make sure the measurement unit(s) of your activity and emissions factors match. For example, if your emissions factor converts megawatt hours (MWh) of electricity into kilograms of CO₂e, but you need to calculate and report your GHG emissions in metric tonnes, you need to multiply your calculation by 1,000 to convert kg into metric tonnes.

Similarly, if you're trying to measure the GHG emissions from your monthly electricity bill, even if your bill comes in MWh yu might be able to use an emissions factor measured in gigajoules (GJ), because MWh and GJ are both units of energy. However, you likely wouldn't be able to use an emissions factor that converts miles of distance into carbon, because MWh could only convert into distance if it's being used to charge an electric vehicle.

When using emissions factors, it's important to follow this checklist to ensure the most accurate carbon accounting calculations:

For a typical organization, a carbon accounting process using emissions factors involves:

Importantly, it's almost impossible to perform carbon accounting without emissions factors, making them an integral and complex aspect of emissions measurement. With many new proposed and mandated global regulations that are requiring entities to calculate and report their carbon emissions, it’s important for sustainability practitioners and ESG analysts to understand what emissions factors are, how they’re calculated, and where you can find them.

Different Types of Emissions Factors

There are different methods for calculating carbon footprint, the most common methods are activity-based emissions and spend-based emissions or a combination of the two. Each method has its advantages and disadvantages in terms of available data, estimations and accuracy.

Activity-based emissions factors, also known as process-based or source-based emissions factors, are used to estimate the amount of emissions associated with a specific activity or process, such as liters of fuel burned by a car driving from point A to point B.

In a second example, an activity emission factor could be used to calculate the amount of carbon dioxide emitted per kilowatt-hour of electricity generated from a natural gas-fired power plant. This emission factor allows us to estimate the CO₂e emissions based on the amount of electricity consumed.

Spend-based emissions factors, also known as expenditure- or investment-based emissions factors, are used to calculate emissions related to monetary expenditure or economic activity. They provide a way to measure emissions associated with the consumption or spending of goods and services

For example, a spend emission factor might indicate the amount of GHG emissions generated per dollar spent on a specific category, such as transportation, food, or purchased goods and services.

Emissions factors are also commonly grouped into carbon accounting categories for measuring emissions from different sources and sectors. A few common categories are:

Emissions Factors for Different GHG Emission Types

While we've primarily focused on carbon dioxide and CO₂e, the international standard unit for carbon accounting measurement, it's important to remember that CO₂e isn't the only type of GHG. Similarly, emissions factors may be designed to convert activity not just into CO₂ or CO₂e, but also into the amount(s) of emitted methane (CH₄), nitrous oxide (N₂O), or other GHGs like refrigerant gases.

As CO₂ and CO₂e act as the base unit in science and carbon accounting, the global warming potential (GWP) for CO₂ is 1. 100 year GWPs for other common GHGs are provided by the Intergovernmental Panel on Climate Change (IPCC) in periodic reports. Examples of GHG's GWP values are show in the table below, based on the IPCC's 5th (2013) and 6th (2021) Assessment Reports:

From a math perspective, that means we can convert 1 gram of methane into 28 grams of CO₂e, using CH₄'s AR5 GWP. That's how carbon accounting normalizes different types of GHG emissions into one standard unit.

Calculating Emissions with Emissions Factors

Activity-based emissions factors are often deemed as the more accurate and precise way of measuring emissions. Since activity-based emission factors are based off of direct measurements and rigorous scientific research, they can provide sector-specific insight on specific activities or processes. One of the downsides of activity-based emissions factors is that they’re a bit more difficult to calculate and can require a lot of time and resources to collect accurate input data. Additionally, activity-based emission factors focus solely on emissions from a specific activity, so associated emissions with other parts of the activity may risk be over-looked.

Spend-based emission factors are often considered the next-best method for calculating carbon emissions. They are estimated using a mixture of economic models and analytical frameworks including Input-Output Models and Environmentally Extended economic models. For organizations who already track their financial accounting data, spend-based emissions calculations can be easier from a data collection perspective, but may not be quite as precise as activity-based estimates.

Even though many experts recommend using activity-based over spend-based emissions factors, spend-based emission factors is often a more feasible or practical solution for calculating a company’s carbon emissions. Using spend-based emission factors allows a company to convert their financial spend, for general items like fuel or more complex items like hired services, into carbon equivalent emissions. Because spend data is readily available, these emissions factors are often easier to use than activity-based emissions factors. This is especially true for certain Scope 3 GHG categories like purchased goods and services.

Where to Find Emissions Factors - Databases and Other Sources

Generally, when it comes to finding good sources for emissions factors, you have three options:

1. Research and gather your own emissions factors from reputable databases and government sources
2. Create your own emissions factors using LCA studies and lifecycle analysis
3. Use a carbon accounting and sustainability software tool like Brightest with build-in emissions factors

The right approach will depend on a variety of factors, like your level carbon accounting experience, available budget and resources, and the amount of time you have to dedicate to emissions factor research.

Depending on your location, organization type, business model, value chain, and what emissions factors you're looking for, here are some trusted databases for finding emissions factors:

Your Next Steps Working With Emissions Factors

Even with various government emissions factors databases at our disposal, finding the correct emission factors from a reputable source can sometimes be challenging, if not downright frustrating. Not every type of activity and transaction your organization relies on may have an established, documented emissions factor.

CO₂e emissions are the global standard unit for measuring and benchmarking our progress towards net zero and mitigating climate change. Yet, even today, emission factors remain scattered across different databases, many of which require paid subscriptions for access. Updates are often infrequent. Moreover, there’s a lack of consistency for specific activities due to variations in data sources and assumptions in calculations. It can make calculating emissions feel like an administrative burden instead of a productive, strategic task for the organization.

We're listening (and highly sympathetic to making sustainability professional's lives easier). We don't believe access to emission factors and other carbon accounting resources should be a barrier for organizations wanting to measure and reduce their emissions. Freely sharing carbon accounting resources, calculations, experiences, successes, and challenges is a win-win solution for everyone. If organizations don’t have the tools to calculate their environmental footprint, how can we expect them to collect meaningful and accurate data and set realistic carbon reduction strategies?

Recently, we've started working on steps to open source our growing emissions factor library to make it publicly available - work we plan to complete by the end of 2023. In the meantime, if your organization is in need of carbon accounting support, please get in touch. A central part of our mission and work here at Brightest is enabling better data-driven decision-making (and actions) that lead to a better future for us all.