Solar Heat Gain Calculator

This calculator estimates peak solar heat gain through a facade's glazing by multiplying Window Area by the Solar Heat Gain Coefficient (SHGC), an approximate peak solar intensity for the chosen Orientation, and the Shading Coefficient. Orientation is the single biggest lever in the whole calculation: because south-facing glass is modeled at roughly five times the peak intensity of north-facing glass (with east and west in between), switching Orientation moves Peak Solar Gain further than an equivalent percentage change to Window Area does. Cooling Load and Annual Heat Gain are both fixed multiples of Peak Solar Gain rather than independent calculations -- Cooling Load applies an assumed 8 peak sun hours per day to turn the instantaneous BTU/hr rate into a daily BTU total, and Annual Heat Gain applies an assumed 2,000 cooling hours per year -- so both always track Peak Solar Gain directly and never diverge from it on their own. Shading Reduction works differently from those three -- it is the percentage of unshaded gain that SHGC and the Shading Coefficient block together, and because Window Area, Orientation, and Latitude scale both the shaded and unshaded gain by the exact same factor, all three cancel out of that ratio for any Window Area greater than zero.

At a Window Area of exactly zero there is no window to have any solar gain through in the first place, so the ratio's guard against dividing by zero reports full (100%) reduction instead of continuing that cancellation -- a degenerate edge case, not a real shading result. Only SHGC and Shading Coefficient move Shading Reduction across the normal, non-zero range, and raising either one lowers it, since a higher SHGC or a shading coefficient closer to 1.0 (no shading) both mean less of the incoming solar energy gets blocked. This model does not account for window frame heat transfer, interior thermal mass, or seasonal sun-angle changes beyond the single latitude-based adjustment factor.