A Tirade on Energy Models (or, It’s All Relative!)

Published on January 04, 2018

From Predictability of Energy Use in Homes, Lachut, D., et al. (http://www.cs.usfca.edu/~srollins/pubs/igcc14.pdf)

Lots of people complain about energy models — specifically, how they don’t accurately predict how much energy the occupants of a home (or building) will actually use. This complaint misses the point.

The REAL problem with energy models is that there is widespread confusion about what they can/should be expected to do. Predicting actual energy use for one specific household is not on the list of realistic expectations. Approximating the amount of energy a reasonably large aggregation of households might use? Yes, perhaps, in skillful hands.

The main thing that energy models can and should be used for is to compare the relative effects of different variables on a building’s energy performance, holding constant certain assumptions about the occupants’ usage patterns — which, even in the best circumstances, are fundamentally unknown and must be generalized.

It’s important to remember that a home (except in extraordinarily rare instances) outlives any particular set of occupants and behaviors many times over; those are subject to frequent change. Even one set of occupants will change energy-using behaviors after a time — for example, as toddlers outgrow bathtubs and graduate to shower usage … and then avoid bathing at all … and then evolve to multiple changes of clothing and multiple showers a day (think: soccer and muddy shorts and shoes).

Thus it’s a pointless exercise to attempt a precise fit of behavioral assumptions to a specific set of occupants, or think that we can predict their energy use for any but the most fleeting time period. What we should be doing is designing homes to efficiently serve any household that might reasonably be expected to live there over the life span of those homes.

\With that understanding, and the use of standard occupancy assumptions*, we can certainly use models effectively to compare the relative effects of different features on energy use.