Heat Gain Calculator (Instantaneous Estimate)

This tool estimates instantaneous heat gain from entered envelope components, window sunshine, outdoor air and internal sources. It separates temperature-related and moisture-related gains and shows solar changes during the day. Heat gain at one instant is not the same as the peak cooling load: building storage, weather, controls and ventilation can change timing and capacity requirements.

How to use this tool

1. Enter outdoor and indoor design conditions, including the outdoor dew point.
2. Enter your latitude, the month and the hour (solar time) to evaluate.
3. Enter wall and ceiling areas with their R-values and the temperature above the ceiling.
4. Enter window areas for each side with their U-factor, SHGC and any interior shading.
5. Add the air-leakage rate, occupants and appliance watts, then read the totals and the hourly table.

Formula

Sensible = max(0, conduction + window sun + 1.08·CFM·Δ°F + 230·people + 3.412·W); Latent = max(0, 0.68·CFM·Δgr + 200·people)

CFM = natural ACH × volume ÷ 60; Δgr is the humidity-ratio difference in grains per pound. Sea-level pressure and fixed air heat factors are assumed. Solar irradiance uses a clear-sky beam fit with an assumed horizontal diffuse term of 10% of beam-normal irradiance and 20% ground reflectance. These are model assumptions, not a local weather prediction. Correction rows show any zero floor applied to negative net gains.

Example

At latitude 40° on July 21 at 4 pm solar time, the simplified sun position is about 36° high and nearly west. The model estimates about 752 W/m² on west glass. With 100 sq ft of glass, SHGC 0.5 and no extra interior shading, that is about 11,926 BTU/h of instantaneous solar gain. Stored heat and release timing are not modeled, so this number alone cannot select an air conditioner.