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Window Solar Heat Gain Calculator

Calculate peak solar heat gain and conduction gain through windows by orientation, SHGC, latitude, and shading — based on ASHRAE peak SHGF values.

Window & Glazing Inputs
Results
Solar Gain
—
BTU/h
Conduction Gain
—
BTU/h
Total Window Gain
—
BTU/h
Peak SHGF Used
—
BTU/h·ft²
Orientation Comparison — Peak Solar Heat Gain (BTU/h)
SHGF values from ASHRAE HOF Table 26 (July, no shading). Actual gain = Area × SHGC × SHGF × shading factors.
Inputs, results, and method — as an Excel workbook or plain CSV.
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About This Calculator

This window solar heat gain calculator estimates the peak cooling load through glazing. Enter window area, SHGC, orientation, latitude, month, exterior and interior shading factors, the window U-value, and the outdoor-to-indoor temperature difference; the tool returns the solar gain, the conduction gain, the total, and the peak solar heat gain factor it used, plus an orientation comparison chart.

Windows are often the single largest peak cooling load in a perimeter space, and orientation drives it strongly. Seeing the solar and conduction components separately helps you weigh glazing selection, shading, and orientation early in design.

Formula & Method
QuantityEquation
Solar gainQ‑solar = Area × SHGC × SHGF × SC‑ext × SC‑int
Conduction gainQ‑cond = Area × U × ΔT
Total window gainQ‑total = Q‑solar + Q‑cond
Month adjustmentSHGF × month factor (Jul = 1.0)

The peak solar heat gain factor (SHGF) values are from ASHRAE Handbook of Fundamentals Table 26 by orientation and latitude (July baseline, with month factors for June and August). SHGC is the glazing solar transmittance, and the exterior and interior shading coefficients further reduce the gain. Conduction through the glass uses the standard U × A × ΔT relationship. The result is a peak-hour estimate, not an hour-by-hour profile.

Frequently Asked Questions
How is window solar heat gain calculated?
Solar gain equals window area times the solar heat gain coefficient (SHGC) times the peak solar heat gain factor (SHGF) for the orientation and latitude, times exterior and interior shading factors. The SHGF is the solar energy striking the glass; SHGC is the fraction transmitted; the shading factors reduce it further. This tool also adds conduction gain through the glass separately.
What is the difference between SHGC and SHGF?
SHGC, the solar heat gain coefficient, is a property of the glazing between 0 and 1 that tells you what fraction of incident solar energy passes through. SHGF, the solar heat gain factor, is the incident solar intensity in BTU per hour per square foot for a given orientation, latitude, and month. Multiplying them, with the window area, gives the solar load through the glass.
Which orientation has the highest peak solar gain?
In the northern hemisphere summer, east, west, southeast, and southwest glass see the highest peak intensities because the sun is low and strikes the glass nearly head-on in the morning and afternoon. South glass peaks lower in summer because of the high sun angle, and north glass is lowest. The orientation comparison chart on this page shows the relative peaks for your inputs.
How do exterior and interior shading factors work?
Both factors range from 0 to 1 and multiply the solar gain. An exterior shading factor below 1 represents overhangs, fins, or external shades that block sun before it reaches the glass and is very effective. An interior blind factor accounts for indoor blinds or shades, which help less because the heat is already inside. Setting either to 1 means no shading from that source.
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AIM Works · launches Oct 15, 2026

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How the calculations work →

Results are design estimates for preliminary sizing. Verify final designs against applicable codes and standards — engineering judgment and a licensed professional engineer’s review are required.