Peak solar and conduction heat gain through glazing by orientation, SHGC, and shading.
Free
No sign-up
Runs in your browser
Excel & CSV export
Window & Glazing Inputs
Results
Solar Gain
—
BTU/h
Conduction Gain
—
BTU/h
Total Window Gain
—
BTU/h
Peak Incident Solar
—
BTU/h·ft² on the glass
Solar Peak Occurs
—
clear-sky design day
Orientation Comparison — Peak Solar Heat Gain (BTU/h)
Same window, SHGC, and shading turned to each orientation. Each bar is that orientation’s own peak — they occur at different hours (and, for the Jun–Oct sweep, different months), so they do not add.
Inputs, results, and method — as an Excel workbook or plain CSV.
Doing this for a whole building? See this step inside a connected AIM Works project — loads, ventilation, equipment, and Design Review in one place.
This window solar heat gain calculator estimates the peak heat gain through glazing. Enter window area, SHGC, orientation, latitude, month (or the Jun–Oct peak), exterior and interior shading factors, the window U-value, and the outdoor-to-indoor temperature difference; the tool returns the solar gain, the conduction gain, the total, the peak clear-sky solar intensity on the glass and when it occurs, plus an orientation comparison chart.
Windows are often the single largest peak cooling load in a perimeter space, and orientation drives it strongly. Seeing the solar and conduction components separately helps you weigh glazing selection, shading, and orientation early in design. The result is instantaneous heat gain: room surfaces absorb most of the transmitted solar and release it over the following hours, so the cooling load peaks later and lower (the Radiant Time Series step in a full load calculation).
Formula & Method
Quantity
Equation
Solar gain
Q‑solar = Area × SHGC × I‑peak × F‑ext × IAC
Incident solar
I = DNI·cos θ + C·DNI/2 + ρ·DNI·(sin β + C)/2, DNI = A·e−B/sin β
Conduction gain
Q‑cond = Area × U × ΔT
Total window gain
Q‑total = Q‑solar + Q‑cond
I‑peak is the highest hourly clear-sky irradiance on the glass (beam + sky diffuse + ground-reflected) on the 21st of the month, from the classic ASHRAE clear-sky model with its published monthly A, B, C constants (ASHRAE Fundamentals, 1997–2005 editions), real sun position (Spencer declination, sun at each hour’s midpoint in solar time), an isotropic sky, and ground reflectance ρ = 0.20. “Peak of Jun–Oct” takes the worst of those five months — south-facing glass peaks in the fall. This is the same solar model AIM Works’ RTS load engine uses. SHGC is applied at its rated (normal-incidence) value to the total incident solar, which is conservative when the sun strikes the glass at a steep angle; ASHRAE Fundamentals Ch. 15 uses an angle-dependent SHGC. F‑ext is the unshaded fraction from overhangs, fins, or exterior screens, and IAC is the interior attenuation coefficient of blinds or shades (1.0 = none). Conduction uses U × A × ΔT at the ΔT you enter. Solar and conduction peaks are added as entered even though they usually occur at different hours, so the total is a conservative instantaneous heat gain, not a cooling load.
Frequently Asked Questions
How is window solar heat gain calculated?
Solar gain equals window area times the solar heat gain coefficient (SHGC) times the peak solar irradiance striking the glass for the orientation, latitude, and month, times exterior and interior shading factors. The irradiance comes from a clear-sky solar model with real sun angles; SHGC is the fraction of it that becomes heat inside; the shading factors reduce it further. This tool also adds conduction gain through the glass separately.
What is the difference between SHGC and SHGF?
SHGC, the solar heat gain coefficient, is a property of the glazing between 0 and 1: the fraction of the solar energy striking the window that ends up as heat inside, directly transmitted plus absorbed and re-emitted inward. SHGF, the solar heat gain factor, belongs to the older CLTD/SCL/CLF method: it was the heat gain through a reference single pane of clear glass, used with a shading coefficient (SC = SHGC ÷ 0.87) rather than with SHGC. Current ASHRAE practice, and this tool, multiplies SHGC by the solar irradiance incident on the glass instead.
Which orientation has the highest peak solar gain?
In midsummer at US latitudes, east and west glass see the highest peaks because the low morning or afternoon sun strikes them nearly head-on; south glass is lower then because the sun is high. In September and October the lower sun swings onto the south, so south, southeast, and southwest glass reach their own peaks in the fall, often above the summer east/west peak. North glass is lowest. Choose the Jun–Oct peak to capture this; the orientation chart shows the relative peaks for your inputs.
How do exterior and interior shading factors work?
Both factors range from 0 to 1 and multiply the solar gain. An exterior shading factor below 1 represents overhangs, fins, or external shades that block sun before it reaches the glass and is very effective. The interior shading factor, the interior attenuation coefficient (IAC), accounts for indoor blinds or shades, which help less because much of the heat is already inside. Setting either to 1 means no shading from that source.
Connect the calculation to the rest of the project.
AIM Works brings peak loads, ASHRAE 62.1 ventilation, equipment and plant sizing, compliance checks, Design Review, and engineering deliverables into one connected mechanical design workspace.
Peak loads (RTS or CLTD/CLF) and ASHRAE 62.1-2019 ventilation
Equipment and plant sizing against ASHRAE 90.1-2022 efficiency floors
Design Review that cites the check behind every finding
An AI co-pilot that drafts changes — you approve each one
Results are design estimates for preliminary sizing. Verify final designs against applicable codes and standards — engineering judgment and a licensed professional engineer’s review are required.