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ASHRAE Fundamentals Ch.15SHGC × irradianceFree

Solar Heat Gain
Calculator

Multiply verified glazing area, SHGC, incident irradiance and attenuation for one declared condition. The result is not a cooling load or equipment size.

Who it's for: architects and engineers sizing peak cooling loads — bring the glazing area, the SHGC from the window spec sheet, and your shading setup at design development.

Q = A · SHGC · E · entered attenuation
Use values from the selected project method and exact product/condition. No irradiance, shade performance, climate, hour, orientation, conversion or coincidence factor is embedded.
Solar Gain
BTU/h

How to use this calculator

1
Enter glazing area
Type the net glass area of the window in square feet — glass only, not the frame.
2
Enter SHGC
The Solar Heat Gain Coefficient is on the window spec sheet or NFRC label; it ranges from 0 to 1, where lower numbers mean less solar heat transmitted.
3
Enter verified incident irradiance
Use the value supplied by the selected climate and solar method for the exact orientation, date, hour and design condition.
4
Enter verified attenuation
Use an applicable product- and method-specific factor; enter 1 only when no additional attenuation is being modeled.
5
Read the peak solar gain
The result is the peak instantaneous solar heat gain in BTU/h and an equivalent tons-of-cooling figure for sizing the cooling system.

The formula

Q = A × SHGC × E × attenuation. Every multiplier is entered explicitly. The worksheet does not select irradiance, attenuation, a peak hour, or a cooling-load conversion method.

Worked example

Example
40 ft² south-facing window, SHGC 0.35, light blinds (IAC 0.7): Q = 40 × 0.35 × 165 × 0.7 = 1,617 BTU/h (0.13 tons). At roughly 40 BTU/h·ft² effective load, three such windows add nearly 0.4 tons to the peak cooling requirement.

When this estimate will be off

  • The user must obtain incident irradiance for the exact latitude, date, hour, orientation and selected analysis method.
  • Peak instantaneous BTU/h overstates the cooling-system load; the ASHRAE Radiant Time Series method spreads gain over several hours of thermal lag before it reaches the air.
  • SHGC already accounts for all glazing layers — do not multiply by a separate shading coefficient, which would double-count glazing attenuation.
  • Does not model external shading (overhangs, fins, adjacent buildings) or inter-reflections — add those reductions to the inputs manually.

Frequently asked questions

A common mistake: The user must obtain incident irradiance for the exact latitude, date, hour, orientation and selected analysis method.

SHGC (Solar Heat Gain Coefficient) is a number from 0 to 1 measuring what fraction of incident solar radiation enters the space as heat. A value of 0.35 means 35% of solar energy becomes internal gain. Find it on the window manufacturer's data sheet, the NFRC label, or the product's energy performance certificate. If you only have the older Shading Coefficient (SC), use SHGC = SC × 0.87.

Solar geometry determines how directly the sun strikes each face. A south-facing window (165 BTU/h·ft²) intercepts full mid-day sun while north (38 BTU/h·ft²) receives mostly diffuse sky radiation. East and west peak similarly at about 185 BTU/h·ft² in morning and afternoon. Skylights face the steepest sun angle and peak highest at roughly 250 BTU/h·ft². These are ASHRAE peak design figures, not annual averages.

Not directly. Peak instantaneous solar gain is an input to a cooling-load method — typically ASHRAE's Radiant Time Series or the older CLTD/CLF approach — which accounts for thermal mass absorbing and re-radiating heat over time. The actual cooling-load contribution is typically 60–80% of the peak gain. ASHRAE Fundamentals Chapter 18 covers the conversion in detail.

Yes. SC and SHGC are related by SHGC ≈ SC × 0.87, as documented in ASHRAE Fundamentals Chapter 15. So an SC of 0.40 equals approximately SHGC 0.35. Enter the converted value in the SHGC field. SC was the older metric and has largely been replaced by SHGC on modern window labels and energy codes.

Sources

  • ASHRAE Fundamentals Handbook, Ch.15 — Fenestration — solar heat gain equation Q = A · SHGC · E · IAC and peak irradiance design values by orientation.

This free Solar Heat Gain Calculator is built and maintained by DataDrivenAEC, using the relevant codes and standards. It does not substitute for professional judgment.