A common moment in building a Scope 3 inventory is when you have estimated a category from spend and then a supplier sends you their own measured emissions for the same purchases, and the two figures differ considerably. In most cases the supplier's figure is the better one to use, because it reflects measured activity rather than an industry average applied to dollars. It is not automatically the right number for your report, though, and relying on it without checking the method behind it can import errors of its own. This guide covers where supplier-specific data sits in the GHG Protocol hierarchy, when it is worth the effort to collect, what to ask a supplier for, and how to check a figure before you rely on it.
Where it sits in the hierarchy
The GHG Protocol ranks Scope 3 calculation methods by how specific the data is, placing supplier-specific data at the top, above average-data factors, which sit above spend-based estimates. More specific data is not automatically more accurate, and the Protocol is explicit about that, which is one reason a supplier figure still has to be checked. Even so, a supplier measuring its own operations can capture things an industry average cannot, such as whether its trucks are electric or its warehouses run on solar. A spend factor treats every provider in a sector as identical, whereas a supplier-specific factor reflects the particular provider you used and how it actually runs.
When it's worth chasing
Supplier data costs effort to collect, so match the effort to the emissions. Where a supplier is a large share of your footprint, the accuracy is worth the engagement, and it's often where a real reduction will eventually show up. Where a line is small, a spend or average factor is fine and chasing primary data isn't a good use of your time. The practical filter is materiality: go supplier-specific on the handful of vendors that move your total, and leave the long tail on estimates.
“A supplier that has measured its own emissions for several years can provide a figure a spend-based estimate cannot reach, because the estimate has no way to see how that particular supplier operates. On your largest suppliers, collecting that data is usually worth the effort involved.”
Where supplier data is worth the effortWhat to ask for
A number on its own isn't enough. Ask for the figure and the method behind it, so you can judge it and an assurer can too:
What to request from a supplier:
The allocated figure
The emissions attributed to your purchases over the period, not the supplier's whole-company total.
The boundary
What operations and activities are included, so you can see it lines up with your reporting.
Base year and factors
The base year of any factors used, and whether the electricity figure is location- or market-based.
The method or policy
If they won't share the underlying factor, ask for the methodology behind the number so you can document it.
How to check it before you rely on it
Supplier data is higher quality, but it isn't automatically right for you. Two things to watch. A figure can be lower than a location-based comparison would give, for example if it rests on a market-based electricity claim backed by certificates rather than the physical grid. And it can be scoped differently from your report, so it isn't directly comparable until you reconcile the boundaries. When you do rely on it, keep the supplier's methodology on file. The figure has to survive the same assurance questions as any other: where did it come from, and how was it built?
Deciding method by category?
Supplier-specific is the most specific method, but not every line needs it. Start with spend-based vs activity-based accounting, and mind the inflation trap in spend factors for the lines you leave on spend.
Turn supplier data into a factor you can defend
If you'd like to see a supplier's own numbers turned into a documented, activity-based factor on your footprint, book a check-up. No obligation.
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