Window SHGC Heat Gain Calculator

Solar gain through glazing from the NFRC SHGC rating, glass area and incident irradiance: q = SHGC × A × I, with cooling tons and per-area flux.

HVAC 3 inputs 3 results

Inputs

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Solar heat gain coefficient (SHGC)

Glazing area (A)

ft²

Incident solar irradiance (I)

Btu/h·ft²

Results

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.

Solar gain through glazing is the product the SHGC rating was built for: q = SHGC × A × I — the NFRC whole-window coefficient times glass area times the irradiance actually hitting the pane (peak clear-sky: ~150–250 Btu/h·ft² on a sun-facing window). 30 ft² of SHGC-0.40 glass under a 200 Btu/h·ft² afternoon sun admits 2,400 Btu/h (0.2 tons). This calculator returns the gain, its cooling-ton equivalent and the per-area flux; conduction U·A·ΔT is a separate term.

How to use this calculator

Read the SHGC. From the NFRC label or the glazing schedule — the whole-window value.
Group glass by orientation. Each orientation peaks at a different hour; run each group at its own design irradiance.
Pick the irradiance. Peak clear-sky values: ~150–250 Btu/h·ft² on a sun-facing window, ~320 on a horizontal skylight; shading and overhangs cut it.
Add the conductive term separately. U × A × ΔT joins this solar term in the full load calculation.

How it works

Fenestration load splits into two independent terms — solar and conductive. The solar term is the product the SHGC rating was built for:

q_solar = SHGC × A × I

SHGC compresses everything optical about the window — transmittance, absorptance, the inward-flowing fraction — into one 0–1 number, so the designer's job reduces to picking the irradiance honestly for each orientation and hour. The result lands straight in the sensible load: convert it to airflow with the airflow & BTU load calculator, and weigh it against the whole-building picture from the diversity factor screen — west-glass solar peaks are exactly the non-coincident loads diversity is about.

Worked example

Verified against the live calculator

A living room with 30 ft² of west glass rated SHGC 0.40, at a 200 Btu/h·ft² late-afternoon clear-sky peak:

q = 0.40 × 30 × 200 = 2,400 Btu/h ≈ 0.2 tons

Each square foot of that glass admits 80 Btu/h. Had the same opening been clear double glazing (center-of-glass SHGC ≈ 0.76), the peak gain would be 4,560 Btu/h — the coating choice alone nearly halves the cooling load this room dumps on the system at the worst hour of the day.

Frequently asked questions

How do you calculate solar heat gain through a window?

Multiply three numbers: q = SHGC × A × I — the rated solar heat gain coefficient, the glazing area, and the solar irradiance hitting the glass. 30 ft² of SHGC-0.40 glass under a 200 Btu/h·ft² afternoon sun admits 0.40 × 30 × 200 = 2,400 Btu/h.

What is a good SHGC value?

Depends on the climate fight: hot-climate energy codes commonly cap new glazing near 0.25 to shed cooling load, while passive-solar designs in heating climates deliberately go high. Spectrally selective low-E typically rates 0.2–0.4; clear double glazing is about 0.76 center-of-glass.

Is SHGC the whole window or just the glass?

The NFRC label rates the whole window — glass, frame and spacers — at normal incidence. Center-of-glass values run higher, which is why a "0.76 clear double pane" window usually labels in the 0.5–0.6s once the frame is averaged in.

Does this include heat conducted through the window?

No — this is the solar term only. The full fenestration load adds conduction U × A × ΔT (and infiltration), which flows whenever there is a temperature difference, sun or not.

Method & assumptions

Standard fenestration solar term (ASHRAE Fundamentals fenestration chapter): q = SHGC × A × I with user-entered irradiance — no solar position or weather tables are embedded.
SHGC is treated as the constant normal-incidence rating; real gain falls off at high incidence angles and under screens, overhangs and interior shading.
Conduction (U × A × ΔT) and infiltration are separate load terms and are not included here.
Sizing-grade load calculations (ACCA Manual J, ASHRAE heat balance) handle hourly solar geometry properly — this screen is the transparent first-principles check.