Shading Impact
Monthly Shading Detail
| Month | Obstruction Angle | Noon Sun Elevation | Shading Fraction | Energy Lost (kWh/m²) |
|---|
How to Use the Solar Shading Calculator
Choose the type of obstruction: an adjacent building, trees or vegetation, a roof overhang, or a distant ridge or hill on the horizon. Each type uses slightly different geometry. Select the nearest Canadian city for the correct solar angle calculations at your latitude.
For buildings and trees, enter the height of the obstruction above the collector level and the horizontal distance to the obstruction. These two measurements define the obstruction angle - the elevation angle from the collector to the top of the obstruction. The wider the obstruction, the more sky arc it blocks.
Leave tilt blank to use your city's latitude as default. Enter the unshaded annual irradiance for your collector from the solar irradiance calculator, or use 1,300 to 1,500 kWh/m²/yr as a typical south-facing Canadian value at latitude tilt.
The monthly chart and table show when shading is worst. Winter months typically show the most shading because the sun is low. The annual shading loss in kWh quantifies the real impact on your solar system output, which you can subtract from the unshaded output calculated by the other solar tools.
Understanding Solar Shading in Canadian Installations
Shading is the single biggest cause of underperforming solar installations in urban and suburban Canadian settings. A system that looks well-placed on a south-facing roof can lose 15 to 40% of its expected annual output from obstructions that the installer didn't account for or that grew over time. Understanding how the sun's position changes through the year is essential to assessing shading risk accurately.
Sun angle geometry at Canadian latitudes
The sun's noon elevation angle at any Canadian location follows a sinusoidal pattern through the year. At the summer solstice, noon solar elevation equals 90° minus latitude plus 23.45° (the solar declination at solstice). At the winter solstice it equals 90° minus latitude minus 23.45°. In Toronto at 43.7°N, the noon sun elevation ranges from 69.8° in June to 22.9° in December. In Edmonton at 53.5°N, that range is 60.0° to 13.1°. Lower latitudes get a higher winter sun, which explains why northern cities are more vulnerable to obstruction shading in winter - the sun stays closer to the horizon all day, so even relatively short obstructions can cast long shadows across the entire heating season.
The obstruction angle: the key shading parameter
Whether an obstruction shades a south-facing collector at solar noon depends on whether the obstruction's angular height above the horizon - the arc-tangent of its height divided by its horizontal distance - exceeds the sun's elevation at that moment. A 5-metre-tall building 10 metres away presents an obstruction angle of atan(5/10) = 26.6°. In Toronto in December, noon sun elevation is about 22.9°, so this building shades the collector completely at noon in winter. In June, noon sun elevation is 69.8°, so the same building causes no noon shading at all. The seasonal transition between shaded and unshaded months occurs when the sun's elevation passes through the obstruction angle - around the equinoxes in this example.
How width of obstruction affects annual loss
An obstruction's angular height determines which months see noon shading, but the azimuthal width determines what fraction of the morning and afternoon production is also affected. A narrow tree trunk might block only 15 minutes of direct sun per day even when the sun passes behind it. A solid row of buildings spanning the entire southern horizon from southeast to southwest blocks direct sun for all hours when the sun is anywhere near south. This calculator applies a width factor that scales the shading fraction by the fraction of the relevant sky arc the obstruction occupies.
Roof overhangs: intentional vs. accidental shading
Roof overhangs are a special case: they're often intentionally designed to shade south-facing windows in summer (when the sun is high) while admitting winter sun (when the sun is low). A properly designed overhang on a south window in Toronto would project about 55 to 65% of the window height to fully shade the window at the summer solstice while leaving it fully unshaded from October through March. This calculator models overhang shading geometry to confirm whether an existing or proposed overhang achieves this seasonal selectivity. The overhang sizing calculator sizes overhangs specifically for this purpose.
Trees: seasonal shading and growth planning
Deciduous trees lose their leaves in autumn and regain them in spring, which means their shading pattern is the inverse of what a solar system needs: they shade in summer (when shading is least harmful to collectors) and are largely transparent in winter (when shading is most damaging). This makes deciduous trees on the south side of a solar installation far less damaging than evergreen trees in the same position. However, even leafless deciduous branches can reduce diffuse sky irradiance by 20 to 40% due to branch density, so a "leafless" tree still causes measurable loss. Tree growth also needs to be projected: a 3-metre sapling today could be an 8-metre obstruction in 15 years, which matters for a 25-year PV system design life.
Frequently Asked Questions
Even partial shading of a solar panel can have an outsized effect on output. In a standard string inverter system, shading one panel in a series string reduces the current through all panels in that string, not just the shaded one. This means a single tree branch shading 10% of one panel could reduce output of the entire string by 50% or more. Micro-inverters and power optimisers eliminate this problem by letting each panel operate independently. For solar thermal collectors, shading directly reduces the irradiance hitting the absorber, reducing heat output proportionally without the electrical stringing issue. The solar thermal calculator uses monthly irradiance that you can reduce by this calculator's shading fraction.
The solar elevation angle is the angle of the sun above the horizon at any given time and location. It varies throughout the day and across seasons. In Canadian cities, the sun's maximum noon elevation in June ranges from about 61° in Toronto to 50° in Edmonton. In December, that drops to 22° in Toronto and 13° in Edmonton. Whether an object shades a solar collector depends entirely on whether the object's angular height above the horizon exceeds the sun's elevation angle at that moment. Low winter sun angles mean objects that don't shade a collector in summer can completely block it in December and January - exactly when solar thermal heating is most needed.
The minimum clear distance between a building and a solar collector depends on the building height and the lowest sun angle that matters for your application. For a south-facing collector in Toronto (noon elevation 22.9° in December), a 5-metre-tall obstruction due south needs to be at least 11.7 metres away to avoid noon shading in winter (5 / tan(22.9°) = 11.7 m). The rule: minimum distance = obstruction height / tan(minimum acceptable solar elevation). This calculator computes this for your specific inputs and shows the monthly profile. Use the overhang sizing calculator for intentional south-window shading design.
Snow coverage of a solar collector is an interruption of solar energy collection, but it's distinct from geometric shading by obstructions. Snow typically clears from steeply tilted collectors within a day or two of snowfall due to panel heat and gravity, and most Canadian solar energy models account for snow losses as a separate factor from shading - typically 2 to 5% annual energy loss for panels tilted above 35°. Tilting collectors steeper than 45° significantly improves snow shedding. This calculator models geometric shading from physical obstructions. Use the solar irradiance calculator for location-specific annual irradiance that already incorporates typical Canadian climate factors.
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