Choosing electricity emission factors: grid, supplier, residual mix
The emission factor you pick can move your electricity footprint by a wide margin. Here is how grid, supplier and residual-mix factors differ and how to choose defensibly.
Your electricity footprint is the product of two numbers: how many kilowatt-hours you used, and the emission factor you multiply them by. Teams spend most of their effort on the kilowatt-hours and then reach for whatever factor is handy. That is backwards. The choice of factor can change the answer by a wide margin, and reviewers know it, so the factor deserves as much scrutiny as the consumption.
There are three main families of electricity emission factor: grid average (location-based), supplier or contractual (market-based), and residual mix. Each answers a different question, and using the wrong one is one of the more common and least visible errors in an emissions inventory.
Grid average factors: the location-based default
A grid-average factor describes the emissions per kilowatt-hour of the physical grid serving a location. It is the basis of the location-based Scope 2 method. In the United States, the EPA eGRID database publishes these factors by subregion, and its Power Profiler tool maps a ZIP code to the right subregion. The EPA also compiles them in its GHG Emission Factors Hub for inventory use.
Grid factors vary enormously by region because generation mixes differ. In Canada the national grid intensity was about 100 grams of CO2e per kilowatt-hour in 2022, but that national average hides a wide provincial spread: hydro-heavy provinces sit near zero while others remain far higher, as the provincial energy profiles show. Using a national number for a single site can be wildly wrong in either direction, which is why location-based accounting is done at the grid-subregion or provincial level, not the country level.
Supplier and contractual factors: the market-based path
If you buy a specific electricity product, sign a power purchase agreement, or hold energy attribute certificates, you can reflect that in a market-based factor. This is where a supplier-specific emission rate or a certificate-backed zero-emission claim enters the calculation. The GHG Protocol Scope 2 Guidance sets a hierarchy for which factor to use and eight quality criteria a contractual instrument must meet before you can rely on it. A supplier factor that is not backed by tracked, retired attributes does not qualify.
Residual mix: the factor for everything unclaimed
The residual mix is the piece most inventories get wrong. Once other buyers in a market have claimed the clean generation through certificates, what remains is a dirtier-than-average pool. If you use grid power without a valid instrument, the market-based method requires you to apply the residual-mix factor, not the plain grid average, so that the same clean megawatt-hour is not counted by two different buyers. Green-e publishes annual residual mix emissions rates by eGRID subregion for this exact use.
Using a grid-average factor for consumption in your market-based total quietly double counts clean energy: someone else already claimed those certificates, but you are still enjoying the low grid average that includes them. The residual mix exists to close that gap. Reviewers look for it.
Which factor for which purpose
| Factor type | Method | Use it for | Typical source |
|---|---|---|---|
| Grid average | Location-based | Consumption at a site, physical exposure | EPA eGRID subregion / provincial grid intensity |
| Supplier / contractual | Market-based | Consumption backed by a valid instrument | Supplier disclosure, PPA, energy attribute certificate |
| Residual mix | Market-based | Unclaimed consumption with no valid instrument | Green-e residual mix by subregion |
Practical rules for choosing a factor
- Match the factor to the grid subregion or province where the electricity is consumed, never a national average for a single site.
- Use the most recent published vintage, and document which release year you used, since factors are revised annually.
- Apply supplier or certificate factors only when the instrument meets the GHG Protocol quality criteria.
- Apply the residual mix to any market-based consumption you cannot back with a valid instrument.
- Keep both a location-based and a market-based total, because dual reporting expects both.
Why this depends on clean consumption first
None of these factor choices matter if the underlying kilowatt-hours are wrong or attached to the wrong site. Factor selection is a per-account, per-period decision: which grid, which supplier, which vintage. That only works when consumption is already structured by account, site and billing period. MartinAI produces exactly that: validated consumption tied to the right account, so the correct factor can be applied cleanly and consistently across a portfolio rather than site by site in a spreadsheet.
Frequently asked questions
What is the difference between a grid factor and a residual mix factor?
A grid-average factor reflects all generation on a grid, including clean generation that others may have claimed through certificates. A residual-mix factor removes that claimed clean generation, leaving the emissions rate of the unclaimed pool. Use the grid factor for location-based accounting and the residual mix for unclaimed consumption in the market-based method.
Where do I find the right emission factor for my location?
In the United States, the EPA eGRID database and its Power Profiler map a location to its grid subregion factor. In Canada, provincial grid intensities from federal energy data serve the same purpose. Match the factor to where the electricity is actually consumed.
Can I use a supplier's zero-emission claim?
Only if it is backed by contractual instruments that meet the GHG Protocol Scope 2 quality criteria, meaning tracked, retired attributes conveyed to you. A marketing claim without valid, retired certificates does not qualify for the market-based method.
How often do emission factors change?
Grid and residual-mix factors are typically revised each year as generation mixes and data are updated. Record the release vintage you used so your inventory is reproducible and comparable year over year.
Scope 2 accounting: market-based vs location-based
The two Scope 2 methods answer different questions and rarely agree. Here is what dual reporting requires, how each number is built, and where your utility data has to be exact.
RECs, Carbon Offsets, and Real Reductions: What Actually Counts
Renewable energy certificates, carbon offsets, and genuine on-site reductions are three different things that often get blurred into one climate claim. Here is how each works, how market-based and location-based accounting treat them, where the greenwashing risk sits, and why measured utility data is the anchor.