Energy Modeling Fundamentals and Theory
Core physics
Every annual simulation is an accounting exercise built on the first law of thermodynamics and on heat transfer by conduction, convection, and radiation. Be able to explain the difference between steady-state load calculations (used to size equipment on a design day) and dynamic energy simulation (used to predict annual consumption with a weather file). Know the major solution methods: the heat balance method, radiant time series (RTS), and weighting factors, and understand why thermal mass and time lag make a design-day block load fundamentally different from an annual result.
Psychrometrics and equipment
Expect questions on dry-bulb, wet-bulb, dew point, humidity ratio, and enthalpy, and on how coil loads split into sensible and latent parts. Know part-load metrics: EER/COP are rating-point values, while IPLV and SEER are weighted part-load figures, so they are not interchangeable. Fan and pump affinity laws (flow varies linearly with speed, power with the cube) appear often.
Weather and schedules
Simulations use typical meteorological year data, not design-day data; design days belong to sizing runs. Internal loads and their schedules drive annual energy more than envelope details in many commercial models.
Traps
The most common trap is mixing sizing and simulation: using design-day weather for an annual run, or reporting design loads as annual energy. A second trap is misreading part-load metrics or forgetting that infiltration can be wind/stack driven rather than a fixed schedule value. A third is ignoring thermal bridging or solar gain through shading.
Worked rule of thumb
A fan drawing 10 kW at full speed run at 90% speed draws about 10 × 0.9³ = 7.3 kW - roughly a 27% power reduction for a 10% speed reduction.