Oh, you’re talking about how realistic it is. No, a HERS rating doesn’t track well with that either.

Allison, it looks more sophisticated than a rule of thumb, but it doesn’t necessarily point you in the right direction. HERS scores don’t give you any credit for building a smaller house that has the same useable floor area (e.g., less hallways and other dead spaces). They also don’t give you any credit for building a house with less exterior surface area for a given floor area (e.g., two-story square houses vs. one-story long narrow houses). Thus, a lower HERS doesn’t necessarily mean lower energy use for the same basic function.

Roy, it’s been a while since I’ve thought about how HERS models the reference home, but there’s more to it than just floor area. It also has the same orientation, 18% window-to-floor area with windows equally distributed in the four cardinal directions… So it’s not anything like a rule of thumb.

Norman, yes, rules of thumb like that are OK just to get a sense of whether the loads are reasonable or not but shouldn’t be used for sizing or HVAC design.

The same is true when the power company or my Sense energy monitor makes comparisons to other homes: it is always to sq ft and maybe the decade of construction

Of course in my PGH, I did not rely on such. Instead I ran my own BEopt calcs and compared those results to the HVAC Manual J and the calcs run by my energy evaluator. The fact that they were in line with each other with minor variations gave me confidence in the result. Oh, and the 1 ton per 1000 “validated” these, too. 🙂