While in pure definitional form, an Asset Rating excludes any and all effects of occupancy, domestic hot water in particular cannot be meaningfully modeled without considering the number of occupants in a home. Holding the number of occupants constant, results in an insensitivity of hot water energy use across a wide range of conditions, leading to over-predictions for homes with low occupancy (but mitigated by the fact that a default of, say, three people can only overestimate the actual conditions by two people) and significant under-predictions for homes with high occupancy.
We take number of bedrooms as a proxy of occupancy, using Building America calculation method as defined on Page 63, Equation 28 of the "Building American House Simulation Protocols". Since our DOE2 inputs require an integer and the formula produces a fractional result, we added a rounding function to the nearest integer. The implemented calculation and results relationship is the following:
occupants = ROUND(0.59 * numberBedrooms + 0.87)
Using the Building America Research Benchmark Definition (Hendron, Engebrecht, 2010), the number of bedrooms directly affects the amount of calculated domestic hot water consumption for the household. That includes regular fixtures, dishwasher and clothes washer hot water use. Additional documentation of our hot water draw methods is here.
In addition, we allow Miscellaneous Electric Loads (MELS) and lighting to be proportional to floor area or numbers of bedrooms. We used Tables 4a and 4c on Page 44 of Parker, Fairey, and Hendron, "Updated Miscellaneous Electricity Loads and Appliance Energy Usage Profiles for Use in Home Energy Ratings, the Building America Benchmark Procedures and Related Calculations", as follows:
Residual misc. elec kWh = 0.91 * (conditioned floor area)
and
TV kWh = -3 * (number Bedrooms)^2 + 89 * (number of Bedrooms) + 390
Scaling televisions by number of bedrooms captures the effects of occupancy more effectively than would simply linking television use to house size. The remaining plug loads are scaled by floor area.
We employ the following algorithms to estimate lighting use, based on Parker, Fairey, and Hendron:
Interior lighting kWh = 455 + 0.8 * conditioned floor area
Exterior lighting kWh = 50 + 0.05 * conditioned floor area
A secondary impact from the bedrooms/occupancy calculation is a better scaling of the internal gains (which, in turn, effect heating and cooling energy use) due to the number of occupants. This impact is very small with respect to the aforementioned DHW, MELS, and lighting adjustments.
Testing of these refinements in the model (in comparison to measured energy use for large numbers of homes) showed much better predictions for total home energy than was the case with fixed occupancy and no dependence of MELs or lighting on floor area.