Energy Modeling and Savings Analysis
Models as decision tools
Energy modeling in multifamily work supports measure selection, savings estimates, and program reporting. Candidates should understand the difference between calibrated models (adjusted to match measured utility data) and engineering estimates (calculated from assumptions). Know the major inputs: envelope characteristics, HVAC system types and efficiencies, domestic hot water, plug and common-area loads, and occupancy schedules. In multifamily, domestic hot water and common-area loads often carry more weight than in single-family, and central systems complicate allocation of savings to units.
Savings calculations
Be comfortable with baseline vs. proposed comparisons, normalization for weather and occupancy, and the concept of interactive effects - a measure that reduces heating load may also reduce cooling savings or change ventilation needs. Understand measure life and simple payback, and why first-cost payback is not the same as lifecycle cost-effectiveness.
Traps to watch for
A frequent trap is double-counting savings when two measures overlap, such as air sealing and insulation both reducing infiltration losses. Another is using a model outside its calibration range or applying single-family schedules to multifamily occupancy. Questions may also test whether you understand gross vs. net savings and the effect of free-ridership and market effects on reported results.
Worked example
If a model predicts 12 percent heating savings and 4 percent cooling savings from air sealing, do not add them as 16 percent of total energy unless the model's end-use breakdown supports it. Report per end use, and check for interactive effects with the ventilation strategy.