The two engines
Every per home figure is the conservative of two physics engines. Both start from the same envelope model and diverge on how they treat the weather and the house's own heat.
Degree day
Envelope UA is built from geometry and assemblies: floor area and storeys give footprint and gross wall area, a fixed window fraction covers glazing, and volume drives infiltration at 0.018 Btu per cubic foot per degree F. Annual thermal load is UA times 24 times heating and cooling degree days at a fixed 65F base. Loads convert to electricity through SEER2 for cooling and HSPF2 for heating. Fast, transparent, and blind to internal gains.
Modified bin
Same UA, but load is summed across the climate's 5F bin hours rather than one annual degree day total, and the balance point is derived per home from an internal gains model: appliance and plug load at 3.1 kWh per square foot per year, the sensible fraction of which offsets heating and adds to cooling. A tighter or more occupied house starts needing heat at a lower outdoor temperature, so this engine separates from the degree day result exactly where that matters.
How demand is claimed
Full load hours are computed once on the pre upgrade home and reused for every measure, because they describe the baseline load shape rather than anything the retrofit changes. Each measure's kWh is divided by those hours to give a summer and a winter demand figure, and the larger of the two is claimed. Smart thermostats are given no demand credit, by program convention.
Why a package saves less than its parts
Measures apply in a canonical order, and each one is credited only the savings left after the ones before it. Insulate the attic and the house needs less heating and cooling, so the same equipment upgrade then saves fewer kilowatt hours than it would have alone. Adding standalone estimates counts that overlap twice. For the full five measure package here, the parts add to 10,218 kWh while the engine returns 8,040, which is 21.3 percent lower. Peak follows at 12.4 percent lower.
The same overlap shows up in price. HVAC replacement alone costs $11,239 per kW removed and attic insulation costs $1,476. Bundled, the package carrying that same HVAC job blends to $4,851 per kW.