Solar Obstruction Shade Loss Calculator
Enter a tree, chimney, or wall’s size and bearing to estimate when it blocks direct sun on one selected day. The result separates the shade duration from the energy-weighted loss on your panel plane.
Estimated total irradiance loss0%
- Obstacle elevation angle (°)
- 26.57
- Obstacle angular width (°)
- 18.92
- Shade interval in solar time
- No usable direct-beam shade interval
- Direct-beam shaded time (minutes)
- 0
- Direct-beam exposure loss
- 0%
- Solar-noon clearance distance
- 4.88 m
Each sample is one minute of local solar time; clock time requires longitude, time zone, and the equation of time.
Direct-beam loss is weighted by the sun angle on the entered panel plane, while entered diffuse light is treated as unshaded.
How to use this calculator
- Enter latitude and the day of year you want to check, such as 355 for about December 21.
- Measure the obstacle height above the panel reference point, its horizontal distance, width, and compass bearing.
- Enter the panel tilt and the direction its front surface faces using bearings clockwise from north.
- Choose an opacity for the obstacle and an estimate of the diffuse share of sunlight.
- Read the solar-time shade interval and the direct-beam and total irradiance loss estimates.
How the shade loss is calculated
The calculator represents one obstacle as a rectangular silhouette. Its top edge has an elevation angle of theta_o = atan(h / d), where h is height above the panel and d is horizontal distance. Its horizontal angular width is W = 2 atan(w / (2d)), where w is the obstacle width.
For every minute of local solar time, it uses NOAA’s Meeus-based solar-position approach to find solar elevation alpha and compass azimuth A. A minute is geometrically blocked when the sun is at or below the obstacle’s top edge and its bearing falls within half the obstacle width: alpha <= theta_o and |wrap180(A - A_o)| <= W / 2.
Not every shaded minute has the same production value. The direct-beam weight on the panel is the front-side cosine-of-incidence term:
b = max(0, sin(alpha) cos(beta) + cos(alpha) sin(beta) cos(A - A_p))
Here beta is panel tilt and A_p is panel azimuth. The direct-beam exposure loss is L_b = q x sum(s x b) / sum(b), where s marks blocked minutes and q is shade opacity as a decimal. The total irradiance estimate is L_t = L_b x (1 - f), with f as the entered diffuse-light share.
What changes the result most
Height divided by distance is the main geometric driver: a nearby chimney can occupy a larger vertical angle than a much taller distant tree. Width and bearing determine whether the obstruction overlaps the morning, noon, or afternoon sun path. Panel azimuth and tilt matter because the calculator weights sun that lands more directly on the panel more heavily. A south-facing panel in the northern hemisphere generally gives a south obstacle more weight than the same obstacle at the north.
Shade time is not energy loss. A long period near sunrise can have little plane-of-array beam weight, while a short interval near the panel’s best angle can matter more. Opacity then scales that direct-beam loss: a solid wall is close to 100%, while leaf-off or sparse foliage should be entered lower only if you have a defensible estimate.
What this tool does not model
This is clear-geometry planning for one day, not an annual production model. It does not model clouds, terrain, reflected light, seasonal foliage, snow, bypass diodes, string mismatch, optimizers, microinverters, inverter clipping, or several overlapping obstacles. Diffuse irradiance is assumed to remain unshaded, and the result is in solar time rather than local clock time. Use a professional shade survey or an hourly PV model before making an installation decision.
Worked example
Suppose a south-facing, 30 degree panel is at latitude 35 degrees north on day 355. A solid obstacle is 3 m above the panel reference point, 6 m away, 2 m wide, and due south. Its elevation angle is atan(3 / 6) = 26.57 degrees, and its angular width is 2 atan(2 / (2 x 6)) = 18.92 degrees.
With 20% diffuse light, the calculator evaluates the one-minute solar path, identifies the direct-beam minutes within that silhouette, and reduces the direct-beam loss by 20% for the total irradiance estimate. The displayed interval is solar time, so it is centered near 12:00 solar time rather than necessarily 12:00 on a clock.
Common questions
How do I measure the height and direction of a tree or chimney?
Measure the horizontal ground distance from the panel reference point to the obstacle, then estimate the vertical difference between that point and the obstacle top. Use a compass or mapping app for the bearing from the panel to the obstacle, correcting for local magnetic declination if your compass reports magnetic north. Width is the visible width facing the array, not trunk diameter alone for a tree.
What is solar time, and why does it differ from clock time?
Solar time puts local solar noon at 12:00, when the sun is highest for that date. Clock time also depends on longitude within the time zone, daylight saving time, and the equation of time, so a 12:00 solar-time result can occur earlier or later on a watch. Solar time lets this calculator work without requiring a location longitude.
How should I estimate shade opacity for a deciduous tree?
Use 100% for an opaque structure. For a tree, opacity changes with species, canopy density, season, and solar angle, so a single percentage is only a planning assumption. Check both leaf-on and leaf-off cases if winter production matters, and use field observations or a professional survey for a design decision.
Why is shaded time different from energy-weighted shade loss?
Panel output from direct sun changes through the day with solar elevation and the angle between the sun and the panel. A minute near sunrise generally contributes less than a minute when the sun is more square to the panel. The calculator sums those plane-of-array beam weights instead of treating every shaded minute equally.
Do microinverters or power optimizers eliminate shading losses?
No. They can reduce mismatch losses from unevenly illuminated modules compared with some string configurations, but they cannot replace sunlight blocked by an obstacle. This calculator estimates only lost irradiance and does not model electrical mismatch or device behavior.
Why does annual shade loss require more than this calculator?
Annual loss depends on the complete horizon, changing solar path, weather, diffuse conditions, leaf cover, and the PV system’s electrical layout throughout the year. One obstacle on one selected clear day is useful for screening a location, but it is not a production forecast. An hourly PV model or shade survey can represent the full year and multiple obstructions.