Why do you not recommend using a custom fuel mix for purchased electricity?
I’m so glad you asked this question, since it highlights an important changes in “best practices” and methodological guidance in the carbon accounting world. This question of whether it is more accurate/more useful/better to have emissions factors for electricity that are more or less customized has been one that many organizations have wrestled with for years. That is why when we first designed the Excel spreadsheet that became the Campus Carbon Calculator (and later CarbonMAP and now SIMAP), we included an option to enter your fuel mix to do the customized calculation based on the disclosure label; at the time it seemed logical to assume that supplier-specific data would be more accurate and relevant than regional data.
Over time, what has developed in higher ed, and in other sectors in which organizations track and report their electricity-related carbon footprint, is that there was no one “right answer” as to the best way to do this, but the values that were considered most credible for comparative purposes were the ones relying on regional eGrid data (because it was using one standardized approach—and an approach that recognizes the inherently regional nature of the grid and the way the infrastructure and markets combine to supply power throughout).
In 2017, the GHG Protocol, which is the international arbiter of “best practices” in carbon accounting across all sectors, issued an updated guidance document for Scope 2 emissions calculation. That document recognized that there are inherent strengths and weaknesses in both approaches (using supplier-specific versus regional grid factors), and recommended that organizations do, and understand the results and implications, of both types of calculations.
Thus, the recommendation is now that schools should do a Location-based calculation, using regional eGrid factors; and Market-based calculation, using a supplier specific emissions factor if you have one, or a regionally averaged Residual factor if you don’t. One important note in the updated guidance, however, is that organizations should move away from calculating emissions factors based on their utilities’ fuel mixes. Their supplier-specific factors used to do the Market-based calculations should be published emissions rates (which is what the regional eGrid factors are based on: emission outputs divided by total generation. If a published supplier-specific emissions rate is not available, the default Residual factor should be used. You can read more about this in Chapter 6 of the updated Scope 2 Protocol: specifically, consult Table 6.3 and section 6.11.3 (pp55-56) for more on the criteria for Market-based emission factor choice. https://ghgprotocol.org/sites/default/files/standards/Scope%202%20Guidance_Final_0.pdf
This is all a really long way of saying, I definitely would not move to a Custom Fuel Mix approach for doing your Scope 2 footprint at this point. We still have it as in option in SIMAP only because there were schools who started doing it that way long before the latest guidance was published, and we wanted them to be able to see and show some degree of consistency in methodology and results as they were moving from other tools (including the CA-CP Campus Carbon Calculator and/or CarbonMAP) to SIMAP. But what we are advising all of our users, based on the consensus being established in the broader global carbon accounting community, is that in the long term they should consider moving away from the “Custom Fuel Mix” method and, in addition to the Location-based calculation done automatically using eGrid data, use the Market-based method, which allows you to input a customized supplier-specific emission rate (rather than a custom fuel mix) into SIMAP but supplies the regional Residual mix for the many utility customers to whom this data is still available.