Solar gain & shading on a south-facing window
A vertical section through the window, with the sun's rays traced at the chosen time. The sun's position is calculated for every day and hour of the year, and the shadow of the overhang and window reveal follows from the real geometry. Drag the corner of the overhang in the drawing, or move the sliders.
Sun on the glass, by season
Calculated for every day of the year, every quarter of an hour, with the geometry from the drawing above.
The calculation shows how much sun reaches the glass in an average year. It does not show whether a particular building overheats. That depends on thermal mass, ventilation, movable shading, internal gains and each month's climate, and is checked in PHPP for the real building. In passive houses, spring and autumn can be the critical periods, not only summer.
Solar radiation on the facade, by location
A vertical south-facing facade in Romania receives between 1,018 and 1,101 kWh/m² a year, which is 75–81 % of the horizontal radiation. In Australia, where the useful facade faces north, the ratio drops to 62–71 %, because the summer sun stands close to the zenith. Tap a location to open it in the calculator.
| Location | Global horizontal radiation kWh/m² a year | On the vertical equator-facing facade kWh/m² a year | Share of horizontal | Hours of direct sun* | Mean temperature Jan / Jul °C | Days averaging above 20 °C | Noon sun height summer / winter solstice |
|---|---|---|---|---|---|---|---|
| Romania · south-facing facade | |||||||
| 1,431 | 1,087 | 76 % | 2,531 | -0.4 / 24.5 | 103 | 69.0° / 22.1° | |
| 1,311 | 1,038 | 79 % | 2,252 | -2.2 / 19.8 | 0 | 66.7° / 19.8° | |
| 1,375 | 1,072 | 78 % | 2,186 | 1.1 / 23.7 | 94 | 67.7° / 20.8° | |
| 1,316 | 1,028 | 78 % | 2,474 | -1.7 / 22.8 | 82 | 66.3° / 19.4° | |
| 1,269 | 1,018 | 80 % | 2,393 | -3.7 / 18.0 | 0 | 67.8° / 20.9° | |
| 1,453 | 1,084 | 75 % | 2,692 | 2.6 / 23.9 | 99 | 69.3° / 22.4° | |
| 1,303 | 1,045 | 80 % | 2,138 | -2.3 / 19.4 | 0 | 67.7° / 20.8° | |
| 1,452 | 1,101 | 76 % | 2,668 | 0.1 / 24.8 | 104 | 69.1° / 22.3° | |
| 1,358 | 1,096 | 81 % | 2,565 | -1.0 / 21.2 | 57 | 68.3° / 21.4° | |
| Australia · north-facing facade | |||||||
| 1,720 | 1,226 | 71 % | 3,440 | 22.5 / 13.6 | 129 | 79.5° / 32.7° | |
| 1,612 | 1,149 | 71 % | 3,443 | 20.3 / 9.6 | 28 | 75.6° / 28.7° | |
| 1,805 | 1,113 | 62 % | 3,652 | 24.5 / 15.3 | 201 | 85.9° / 39.1° | |
| 1,970 | 1,214 | 62 % | 3,771 | 25.0 / 13.0 | 150 | 81.5° / 34.6° | |
| 1,807 | 1,201 | 66 % | 3,650 | 21.0 / 9.1 | 48 | 78.5° / 31.6° | |
| 1,832 | 1,132 | 62 % | 3,688 | 23.7 / 15.1 | 182 | 85.4° / 38.5° | |
Unshaded vertical facade, with direct, diffuse and ground-reflected radiation (albedo 0.2). Climate data: PVGIS © European Union, 2001–2026. *Hours with direct radiation above 120 W/m², estimated from satellite data; they can be higher than those measured at weather stations.
The sun, the overhang and the window
How much sun does a south-facing facade get in Romania?
On average between 1,018 kWh/m² a year (Brașov) and 1,101 kWh/m² (Craiova) on an unshaded vertical south facade, which is 75–81 % of the radiation falling on a horizontal surface. The values for each location are in the table above.
What is the profile angle Ω?
It is the angle of the sun's ray seen in the vertical section through the window: tan Ω = tan α / cos Δ, where α is the sun's altitude and Δ the difference between the sun's direction and the facade's direction. A continuous overhang fully shades the glass once Ω exceeds the angle between the overhang edge and the bottom of the glass.
Why does heat get in during summer even when the overhang blocks direct sun?
Through diffuse light from the sky and light reflected from the ground, which an overhang blocks only in part. For the example window (4.0 × 1.6 m, 1.5 m overhang, Cornu de Jos), 99 % of the solar gain in June–August is diffuse light.
Why do spring and autumn matter in passive houses?
At the equinoxes the noon sun reaches about 45° at Romanian latitudes and passes under overhangs sized for the summer sun. In the calculator's example, autumn brings 498 kWh through the glass, almost twice as much as summer (261 kWh). Whether this gain leads to overheating depends on the building and is checked in PHPP.
Does the overhang's length matter, not just its depth?
Yes. In the morning and afternoon the sun comes from the side and passes around the ends of a short overhang. For a 1.6 × 2.0 m window with a 0.8 m overhang in Bucharest, an overhang as wide as the window lets 68 % of the summer sun through, a continuous one 56 %.
What changes in the southern hemisphere?
The window that collects the sun faces north, summer falls in December–February and the solstices swap. In Sydney the noon sun climbs to 79.5° on 21 December and drops to 32.7° on 21 June. In Australia a vertical north facade receives only 62–71 % of the horizontal radiation, because the summer sun is close to the zenith.
Where does the data come from and how accurate is it?
From PVGIS, the European Commission's service: the 2005–2023 average, hour by hour, for each location. Compared with PVGIS values for a vertical facade, the calculation differs by at most 6–7 % per month (Bucharest, Cluj-Napoca, Sydney). The calculator is a preliminary estimate and does not replace a PHPP calculation.
How the calculation works
The calculator follows the sun every 15 minutes, on every day of the year, projects the shadow of the overhang and window reveals onto the glass, and uses each location's 2005–2023 average climate. For the locations checked, the result differs by less than 4 % a year from the official PVGIS values for vertical facades.
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Sun position
NOAA's solar position equations, calculated for 2026 at each city's local time, including daylight saving time. The facade faces the equator: south in Romania, north in Australia.
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Local climate
The average day of each month, hour by hour, from the 2005–2023 mean: direct radiation, diffuse radiation and air temperature. For Romania the data comes from satellite observations (SARAH-3), for Australia from the ERA5 reanalysis.
Sources: PVGIS · ERA5, Copernicus
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Shadow on the glass
The glass is divided into 24 horizontal bands. For each band the calculator checks whether the sun's ray is blocked by the overhang, centred on the window and of the chosen length, or by the top and side reveals. The drawing shows the section through the window centre, and the outside view shows the shadow across the whole glass.
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Diffuse light
The Hay–Davies model splits the sky into the bright area around the sun, shaded by the overhang just like the sun itself, and the rest of the sky, of which the glass sees only the part the overhang leaves open. Ground-reflected light uses an albedo of 0.2 (grass, gravel).
Reference: Hay and Davies (1980), described in the pvlib documentation
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Unwanted heat
Solar gain on days when the mean outdoor temperature exceeds 20 °C. The daily temperature is interpolated from the PVGIS monthly means. The calculation shows the amount of heat, not whether a particular building overheats.
How accurate it is
Compared with PVGIS values for an unshaded vertical facade facing the equator:
| Location | Difference per year | Largest per month |
|---|---|---|
| Bucharest, south facade | −3.5 % | 6 % |
| Cluj-Napoca, south facade | −3.1 % | 5 % |
| Sydney, north facade | −3.3 % | 7 % |
Limitations
- The g-value is taken as constant at every angle and for diffuse light.
- The share of sky the glass sees is calculated as for a continuous overhang, so diffuse light is slightly underestimated for short overhangs.
- Sunshine hours are estimated from satellite data and can be higher than those measured at weather stations.
- Custom coordinates have no climate data: the calculation uses a clear sky (ASHRAE model), so it is an upper limit.
- This is a preliminary design estimate, not a PHPP calculation.
Sources
- PVGIS, European Commission Joint Research Centre
- NOAA Solar Calculator, Global Monitoring Laboratory
- SARAH-3, EUMETSAT CM SAF
- ERA5, Copernicus Climate Change Service
- ASHRAE Handbook — Fundamentals, clear-sky model
- Passive House Institute, Darmstadt
Calculation and text: SEMPER SUPRA INITIATIVE S.R.L. · Climate data: PVGIS © European Union, 2001–2026.