A tool for calculating the heat that enters a room through a window. Change the glass type, window area, solar irradiance and shading to see, in real time, the solar gain from the sun, the conduction gain from the indoor/outdoor temperature difference, and the impact on the summer cooling load.
Parameters
Glass type
Sets the solar heat gain coefficient SHGC
Window area A
m²
Solar irradiance I
W/m²
Solar energy density striking the window face
Window U-value U
W/m²K
How readily heat conducts through with a temperature difference
Temperature difference ΔT
K
Outdoor minus indoor temperature (outdoor hotter)
Interior attenuation IAC
Effect of blinds/curtains. 1.0 = no shading
Results
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Solar gain Q_solar (W)
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Conduction gain Q_cond (W)
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Total gain Q_total (W)
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Per unit area (W/m²)
—
Effective SHGC
—
Cooling-load impact
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Window solar heat gain — energy flow
Sunlight striking the window splits into a part that enters the room (transmitted + re-radiated = SHGC) and a reflected part. Conduction heat also flows in, driven by the temperature difference.
Solar heat gain Q_solar [W] and conduction heat gain Q_cond [W]. SHGC: solar heat gain coefficient, IAC: interior attenuation coefficient, A: window area m², I: solar irradiance W/m², U: U-value W/(m²·K), ΔT: temperature difference K.
$$Q_{total}=Q_{solar}+Q_{cond}$$
Total heat gain Q_total is the sum of the solar and conduction components. SHGC ranges from about 0.25 (triple Low-E) to 0.86 (single clear glass); the glazing build-up sets the fraction of solar heat that enters the room.
What is window solar heat gain?
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I always assumed it gets hot by a window in summer just because the outside heat "soaks in". Is that not the whole story?
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Good question. Heat actually enters a window in two ways. One is what you described — conduction gain, the heat that flows through the wall and glass because the outdoor air is hotter than the room. The other is solar heat gain, where the sunlight itself passes through the window and warms the room directly. And on a summer afternoon, through a large window, the solar part is usually by far the bigger of the two. A window is a hole in the wall and a hole that light pours straight through.
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How much sunlight gets through a window — what decides that? Isn't glass just glass?
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That is where the SHGC, the solar heat gain coefficient, comes in. It is a number from 0 to 1 for the fraction of incident solar that ends up as heat in the room. Clear single glass has an SHGC of about 0.86 — 86% of the sun that hits it gets in. Double Low-E glass drops to 0.40. Switch the "Glass type" on the left and you will see that for the same irradiance, the solar gain Q_solar entering the room changes sharply.
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I hear about Low-E glass a lot. Does it make the room dark and tinted?
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That is the clever part of Low-E. The name is short for "low emissivity" — there is a thin metallic coating on the glass surface. That coating lets visible light through well, but reflects the infrared that carries heat. So the room stays bright while the solar heat is cut. It works completely differently from heat-absorbing tinted glass, which gets darker to block the sun. Low-E is exactly the answer to "I want a lower cooling load but I don't want a dim room".
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We just close the curtains to block the sun. Is that not good enough?
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It is not bad, but interior curtains are far less effective than exterior shading. A curtain only blocks the sun after it has already passed through the glass and entered the room — most of that absorbed heat stays inside. An overhang, an exterior blind or a screen blocks the sun before it reaches the glass, so the heat never enters at all. The IAC slider in this tool represents interior blinds, but if you are serious about cutting the cooling load, external shading is the way to go.
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I see. So in winter the opposite is true — you'd be glad the sun comes in, right?
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Exactly, and that is the hard part of SHGC design. In winter the solar gain is "free heating" and very welcome. So in cold climates a south-facing window is given a deliberately higher SHGC to capture winter sun and cut the heating load. A hot climate or a window taking strong west sun gets a low SHGC instead. The trump card for "block in summer, admit in winter" is an overhang: in summer the sun is high and the overhang blocks it, while in winter the sun is low and slips in under the overhang.
Frequently Asked Questions
The SHGC (solar heat gain coefficient) is a dimensionless number from 0 to 1 that gives the fraction of solar radiation striking a window that ends up as heat inside the room. It is the sum of the part transmitted directly through the glass plus the part absorbed by the glass and then re-radiated and convected inward. An SHGC of 0.40 means 40% of the incident solar energy becomes heat in the room. It varies strongly with glazing: about 0.86 for single clear glass, 0.40 for double Low-E and 0.28 for triple Low-E.
The total heat gain through a window is the sum of the solar gain and the conduction gain. Solar gain is Q_solar = SHGC × IAC × A × I, where A is the window area, I is the solar irradiance and IAC is the interior attenuation coefficient (blinds or curtains). Conduction gain comes from the indoor/outdoor temperature difference: Q_cond = U × A × ΔT, where U is the U-value. The total Q_total = Q_solar + Q_cond is what this tool computes to rate the impact on the cooling load.
Through large windows the solar gain Q_solar is far greater than the conduction gain Q_cond and is usually the single biggest summer cooling load. The most effective change is to lower the SHGC, that is, switch to Low-E glass. A Low-E coating cuts the solar gain dramatically while keeping the glass clear and the room bright. Even more effective is external shading (overhangs, exterior blinds, screens), which blocks the sun before it enters the glass and so beats interior blinds at reducing the cooling load.
For the same shading ratio, external shading wins by a wide margin. Interior blinds only act after the sunlight has already passed through the glass and entered the room, so most of the absorbed heat stays inside. Overhangs and exterior blinds, by contrast, reflect or block the sunlight before it reaches the glass, so the heat never enters the building. The IAC (interior attenuation coefficient) in this tool represents interior blinds; a low value (around 0.3) means strong shading, but in real window design combining external shading is recommended.
Real-World Applications
Energy-efficient housing: Windows in houses and apartments strongly influence both the summer cooling load and the winter heating load. In energy codes, window performance — the U-value and the solar heat gain coefficient η (SHGC) — is a key metric. Making a large south-facing patio door double Low-E and adding a summer overhang can cut air-conditioner electricity by a noticeable margin. Comparing glass types and window areas in this tool and watching the cooling-load impact is the first step.
Office buildings and glass facades: In fully glazed buildings the solar heat gain through windows is the dominant driver of HVAC energy. Designers use different SHGC glazing per orientation and add external louvers on the west and east faces. Knowing the order of magnitude of "window area × SHGC × irradiance" early in design improves both the sizing of the air-conditioning plant and the accuracy of annual energy simulations.
Pre-study for building-energy simulation: Before running a full thermal-load tool such as EnergyPlus or TRNSYS, a simple formula like this tool gives a first read on whether solar gain or conduction gain dominates. If solar dominates, the glazing SHGC and shading matter most; if conduction dominates, insulation (U-value) does — so the design variables to focus on become clear. If a detailed analysis differs from this estimate by an order of magnitude, it is a useful sanity check for an input error.
Retrofit decisions for existing buildings: When choosing how to retrofit the windows of an old building or house — adding an interior window, replacing with Low-E double glazing, or fitting external shades — this gives the material for a cost-effectiveness judgement. Lowering only the U-value does not reduce solar heat, so if summer overheating is the problem, target shading and SHGC; if winter cold is the problem, target the U-value. This tool lets you separate the two.
Common Misconceptions and Pitfalls
The most common mistake is the belief that "improving the window insulation (U-value) also makes summer cooler". The U-value only affects the conduction gain Q_cond, driven by the temperature difference; it has almost nothing to do with the solar gain Q_solar, the sunlight itself coming in. With the default settings (double Low-E) this tool shows Q_solar at 1680 W against Q_cond at just 168 W — a tenfold difference. The main reason it is hot by a window on a summer afternoon is solar heat, and lowering that requires improving the SHGC and shading, not the U-value. High insulation does not "solve everything".
Next, assuming the solar irradiance is always constant. The irradiance I striking the window face varies enormously with time of day, season, orientation and weather. At noon in midsummer, with clear-sky direct sun on a horizontal surface, it can approach 1000 W/m²; under overcast skies it is 100-200 W/m². For the same window, the south and west faces have different peak times and intensities. The I in this tool is a single-point input for "the solar hitting that window face at that instant" — getting an annual cooling load needs hourly, per-orientation solar data. Do not confuse a single-point result with an annual figure.
Finally, "the lower the SHGC, the better" is not true. In a cooling-dominated hot climate, a lower SHGC is an advantage, but in a cold, heating-dominated climate the solar heat gain is welcomed as "free heating". Pushing the SHGC of a south-facing window too low can actually raise the winter heating load. The ideal is "block in summer, admit in winter", and what makes that possible is external shading such as an overhang, whose effect changes with season. Keep in mind that the optimum SHGC depends on the local climate (cooling- or heating-dominated) and the window orientation.
How to Use
Enter window area in m² (typical range 1–50 m² for residential/commercial facades)
Set solar irradiance in W/m² (standard 1000 W/m² for peak summer conditions, or 200–400 W/m² for winter/cloudy days)
Input U-value in W/m²K (double-glazed: 2.8–3.0, triple-glazed: 1.5–1.8, high-performance: 0.5–1.0)
Define temperature difference (indoor–outdoor) in K; typical summer: 5–10 K, winter: 15–25 K
Review output: solar gain Q_solar, conduction gain Q_cond, total gain Q_total in watts, and effective SHGC percentage
Worked Example
A commercial office with triple-glazed window (U=1.6 W/m²K, area=8 m², SHGC=0.35): Summer peak irradiance 850 W/m², outdoor temp 32°C, indoor setpoint 22°C (ΔT=10 K). Solar gain: 8×850×0.35=2380 W. Conduction gain: 8×1.6×10=128 W. Total: 2508 W cooling load. Winter at 150 W/m² irradiance, ΔT=20 K: Solar 420 W, conduction 256 W, total 676 W helps offset heating demand by 420 W.
Practical Notes
SHGC mismatch: high-transmittance glass (SHGC 0.6+) suits cold climates; low-SHGC coated glass (0.25–0.35) cuts summer cooling 30–40% in hot zones
Orientation matters: south-facing windows in 32°C peak summer accumulate 20% more heat than horizontal skylights; east/west facades spike at ±3 hours from solar noon
Dynamic shading (external blinds lowering effective SHGC from 0.35 to 0.12) reduces total gain by 1500+ W on 8 m² south exposure
U-value degradation: aged seals can increase U from 1.6 to 2.2 W/m²K over 15 years, raising winter conduction loss by 37%