Shading analysis methods exist because the sun traces a different path every day of the year, and the obstruction that means nothing in June can cover half an array in December. A visual check at noon on the day of the site visit captures one point out of several thousand. A proper study evaluates the sky dome from each part of the array across the whole year and turns that into an energy number.
What a real study is measuring
From any point on a roof, the sun occupies a band of the sky that shifts north and south with the season and sweeps east to west each day. A shading study maps the obstructions surrounding that point, chimneys, trees, parapets, neighboring buildings, onto the same coordinate system as the sun path, then calculates for every hour of a typical year whether the sun is behind an obstruction. The output is an access factor: the fraction of annual available irradiance that actually reaches that spot. Because the sun sits low in winter, obstructions to the south, southeast and southwest in the northern hemisphere matter far more than their apparent size suggests, and the hours they take are the hours a winter-constrained system needs most.
The tools, from simple to rigorous
The traditional instrument is a handheld device with a reflective dome or a fisheye camera that captures the horizon from a specific point and overlays the sun path, producing an access percentage on the spot. Modern equivalents use a phone camera with the same mathematics. Above that sit software approaches: aerial imagery combined with lidar elevation data to build a three-dimensional model of the site and its surroundings, or a manually constructed model in design software where the surveyor enters obstruction heights and distances measured on site. Each approach has failure modes. Handheld readings are only valid at the exact point taken, so several readings across the array are needed. Lidar models can be years out of date on vegetation. Manually built models are only as good as the measurements entered.
Where studies commonly go wrong
Trees grow. A survey that shows acceptable access today can be wrong in eight years, and the cost of that error lands on the owner. A competent survey notes vegetation and estimates future growth, or at least flags it. Obstructions that are not there yet are another gap: a neighbor’s planned addition, a rooftop mechanical unit scheduled for replacement with a taller one, a new stack from a furnace upgrade. Self-shading within the array is the third, where one row shades the next in winter on a flat roof or ground mount, which is a row-spacing calculation rather than an external obstruction. And a single whole-array access number averages over modules that may have very different exposures, which is exactly the information a designer needs to group strings sensibly.
Turning access into energy and architecture
An access factor is not directly an energy loss. Design software combines the shading profile with the sun path, the module layout, the string wiring and the electronics architecture to model how much energy is actually lost, because a shadow crossing cells within one bypass-diode group costs differently than one crossing several modules in a string. This is where the shade study earns its keep: it tells the designer whether to use module-level electronics, how to group strings so that shaded and unshaded modules are separated, and whether a particular roof plane is worth using at all. A study delivered only as a single percentage on a sales sheet has not been used for any of that.
Questions worth asking about your own study
Ask how many measurement points were taken and where. Ask whether tree growth was considered. Ask whether the model used current imagery or archived data. Ask whether the string layout in the design reflects the shade groups the study found, or whether it was drawn for convenience. And ask to see the production model both with and without shading, so you can see how much energy the obstructions are costing and judge whether trimming, removing or relocating is worth discussing. Those five questions separate a design from a sales estimate quickly.
What to check before you act
- Ask how many points on the array were surveyed, not just the average.
- Confirm tree growth and planned construction were considered.
- Check that string grouping matches the shade zones found.
- Request the production model with and without shading losses.
Common questions
Can I do a shade study with a phone app?
Consumer apps can give a useful first indication, but their accuracy depends on calibration and technique. Use one to screen a site, not to size a system you are paying for.
Does a small vent pipe shadow really matter?
It can. A narrow shadow falling across cells in series can cost far more than its area, especially if it lingers during high-production hours. Relocating the pipe is often cheap.
The practical takeaway
Insist on a shade study that covers the whole year at several points across the array, accounts for growth and future obstructions, and actually informs the string layout. Then look at the modeled production with and without shading before you sign anything.
References and further reading
- NREL PVWatts: models annual production and lets you compare configurations
- NREL: photovoltaics research including solar resource assessment
- NREL: a technical report from the laboratory’s photovoltaics publication library
Numerical scenarios are illustrative unless identified otherwise. Follow the exact product instructions; component ratings and local installation requirements can differ.
