How does the tool decide when panels are shaded?

For each hour of the day it computes the sun's elevation and azimuth, then marks that hour shaded whenever the sun is lower than the obstruction's elevation angle and falls within the obstruction's angular width. Midday hours count more because the sun delivers more energy then.

Why does it use the equinox sun path?

The equinox is a fair middle-of-the-road reference: winter sun runs lower and causes more shading, summer sun runs higher and causes less. Using the equinox gives an annual-average style estimate without needing a full year of weather data.

What is the obstruction elevation angle?

It is the angle from the horizontal up to the top of the obstruction as measured from the array. A tall tree close to the panels has a high elevation angle; a distant ridge on the horizon has a low one. A Solar Pathfinder or inclinometer measures it directly.

Why enter the share of array affected separately?

A shadow that covers a quarter of the modules does not erase a quarter of all output unless wiring matches. With module-level optimizers or microinverters the loss tracks the shaded area closely, so the affected-share input lets you reflect your layout.

Is this accurate enough to design around?

It is a fast planning estimate to compare sites and obstructions, not a bankable production number. For a final design run a TMY-based simulation in tools like PVsyst or SAM and confirm with an on-site shade reading.

What is the Solar Array Shading Loss Estimator?

Takes an obstruction's azimuth, elevation angle, and width plus the share of array affected to estimate the irradiance-weighted fraction of daylight blocked and the resulting annual PV energy loss. Runs in your browser. It runs free in your browser on Gera Tools, with nothing uploaded.

Solar Array Shading Loss Estimator

Name: Solar Array Shading Loss Estimator
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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A single tree or chimney to the south-east can quietly erase a chunk of a solar array’s morning production. This estimator turns an obstruction’s position and size into an irradiance-weighted estimate of how much annual energy it costs, so you can judge a site before committing to a layout.

How it works

The tool samples the sun’s path across the day and tests each moment against the obstruction’s geometry:

for each daylight moment:
  elevation, azimuth = sun position (equinox, your latitude)
  shaded = sun elevation < obstruction elevation
           AND |sun azimuth − obstruction azimuth| ≤ width / 2
  weight = sin(elevation)         (clear-sky irradiance proxy)
shaded fraction = shaded weight / total weight
annual loss ≈ shaded fraction × affected array share

Weighting by the sine of solar elevation means a blocked midday hour, when the sun is strongest, costs far more than a blocked hour just after sunrise.

How to measure an obstruction’s elevation angle

The elevation angle is the angle from horizontal up to the top of the obstruction, measured from the array location. It is the most important single number in this calculation. Practical measurement methods:

Inclinometer or smartphone level app: Stand at the array position, tilt the phone until you are sighting the top of the obstruction, and read the elevation angle.
Solar Pathfinder or SunEye: The traditional installer tool traces the skyline and maps it directly onto a sun-path diagram.
Trigonometry: If you know the distance to the obstruction and its height above the panel height, the elevation angle is arctan(height ÷ distance).

A tree 20 m away and 15 m taller than your panels has an elevation angle of roughly arctan(15 ÷ 20) ≈ 37°. At that elevation, the obstruction blocks significant solar energy from mid-morning to late-morning for a south-facing system in temperate latitudes.

How azimuth and elevation interact

Two obstructions with the same elevation angle can have very different impacts depending on where they sit in the sky:

Due south (azimuth 180° in the northern hemisphere): An obstruction here blocks the highest-value hours — solar noon and the hours either side of it — and causes the maximum loss per degree of angular width.
South-east or south-west (azimuth ~135° or ~225°): An obstruction here blocks morning or afternoon sun respectively, which is lower in the sky and carries less energy per hour.
Due east or west (azimuth ~90° or ~270°): Obstructions here block only very early or very late sun, which is both low in the sky and of low irradiance. Their annual loss contribution is comparatively small.

Example and notes

A 25° tall obstruction at azimuth 135° (south-east), 40° wide, that shadows a quarter of the array yields a few percent of annual loss because it only bites during low-sun morning hours. Move that same obstruction due south at the same height and the loss climbs sharply, because it now blocks the high-value midday sun.

Seasonal note: winter shading losses run significantly higher than this equinox-based estimate, because the sun tracks much lower in the sky from November to January in temperate zones. An obstruction at 20° elevation that the equinox model shows as barely clipping becomes a major blocker through the winter months. For any system where winter performance is critical — home heating support, agricultural operations — scout obstructions on the shortest day of the year, not just the average.