About the Passive Solar Heat Gain
This calculator estimates how much solar heat passes through a window and into a room, based on the window's size, its Solar Heat Gain Coefficient (SHGC), and how strong the sunlight hitting it is. It is useful for anyone evaluating whether a window helps or hurts a home's heating and cooling load.
How It Works
You enter the window's area, its SHGC (a 0 to 1 rating found on the window's NFRC label describing what fraction of solar radiation becomes usable heat), and the solar irradiance hitting the glass in watts per square meter. The calculator converts the window area to square meters and multiplies the three values together to get heat gain in watts, then converts that figure into BTU per hour, the unit more commonly used in HVAC load calculations.
Formula & Methodology
SHGC represents the share of incoming solar radiation striking the glass that actually makes it through as usable heat, so multiplying irradiance, the raw solar energy hitting each square meter, by SHGC gives the effective heat flux per square meter of glass. Multiplying that by the window's area in square meters gives total heat gain in watts for the whole window at that moment. Because HVAC sizing in the US is conventionally done in BTU per hour rather than watts, the calculator also reports the same figure multiplied by the standard watts-to-BTU/hr conversion factor of 3.41214.
Examples
South-facing window, average sun
A 20 sq ft window with SHGC 0.4 under 500 W/m2 of irradiance converts to about 1.86 m2 of glass, producing roughly 372 W of heat gain, or about 1,268 BTU/hr.
Low-SHGC window in peak summer sun
A 15 sq ft window with a low-e coated SHGC of 0.25 under peak irradiance of 900 W/m2 works out to about 1.39 m2 of glass and roughly 314 W of heat gain, or about 1,070 BTU/hr, showing how a lower SHGC blunts even strong direct sun.
Advantages
- Turns an NFRC label spec, SHGC, into a concrete watts or BTU/hr figure you can compare against a room's heating or cooling needs.
- Makes it easy to see the tradeoff between window size and SHGC rating when comparing replacement window options.
- Provides output in both metric watts and the BTU/hr units used in most US HVAC sizing calculations.
Common Mistakes
- Using a rough guess for solar irradiance instead of a value appropriate to the window's orientation, time of day, and climate, which can swing the result substantially.
- Treating a single instantaneous result as a full-day or full-season heat gain total, rather than a snapshot at the specified irradiance level.
- Forgetting that a lower SHGC is desirable for reducing unwanted cooling load in hot climates, while a higher SHGC is desirable for passive heating on cold-climate south-facing windows - the better number depends on the goal.
Edge Cases to Watch For
- The result is an instantaneous estimate for the irradiance value entered, not a daily or seasonal total - actual heat gain varies through the day and year as the sun's angle and cloud cover change.
- SHGC is meant to stay between 0 and 1, matching the field's labeled min and max; a value outside that range would produce a physically meaningless result.
- The calculation does not account for window orientation, shading, tinting films applied after installation, or interior blinds, all of which change the effective heat gain even for the same rated SHGC.
Common Use Cases
- Homeowners comparing replacement window SHGC ratings for a specific window's contribution to heating or cooling load.
- Passive solar design enthusiasts sizing south-facing glazing to capture winter heat.
- Anyone trying to understand why a particular room overheats in direct sun and wants to quantify the solar contribution.