Window SHGC Heat Gain Calculator

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From the NFRC label (whole window, normal incidence). Clear single glass ≈ 0.86, clear double ≈ 0.76 center-of-glass, spectrally selective low-E commonly 0.2–0.4.
Total window area receiving this irradiance — group windows by orientation and run each group separately.
ft²
On the glazing plane. Clear-sky peaks: roughly 150–250 Btu/h·ft² (470–790 W/m²) on a sun-facing window, ~320 Btu/h·ft² (1,000 W/m²) on a horizontal skylight.
Btu/h·ft²

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Default result
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Solar heat gain(q)
2,400Btu/h
Pass

The solar term only — add conductive U·A·ΔT separately for the full window load.

Also computed

Cooling load equivalent0.2tons

Through-glass flux(q/A)80Btu/h·ft²

SHGC × I — what each unit of glass admits at this sun condition.

Method notes 4 notes
  • Solar term of the fenestration load only (q = SHGC × A × I); conductive gain U·A·ΔT and infiltration are separate terms in the full load calculation.
  • SHGC from the NFRC label is the whole-window value at normal incidence — high sun angles, overhangs, screens and interior shades all reduce the real gain.
  • Hot-climate energy codes commonly cap prescriptive SHGC near 0.25 for new glazing; the adopted code and the orientation-weighted trade-offs govern.
  • Peak irradiance hits each orientation at a different hour — west glass peaks with the afternoon temperature peak, which is why it dominates residential cooling sizing.

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