Design, installation and performance
Shading
Also known as: shade, shadowing.
Definition
Shading is the shadow cast by trees, buildings or other obstacles over the panels, and it reduces their output. A good design avoids shaded areas or uses microinverters or optimizers to minimise the impact.
The impact of shade is disproportionate to its size. In a string of panels connected in series, the current is set by the most limited module, so shade over a fraction of one panel can cut the output of its whole string. The module's bypass diodes mitigate the effect by letting the current route around the affected area, but they do not cancel it out.
There are two kinds of shade with different treatments. Permanent shade — a water tank, a chimney, a party wall, a tree that will not be pruned — is resolved in the design: that area of the roof is avoided or electrically isolated. Temporary, moving shade, which sweeps across different parts of the roof according to the hour and the season, is better handled with microinverters or optimizers, which decouple each panel from the rest.
A serious shading study is part of the technical visit: it consists of analysing the sun's path across the roof through the day and the year, not looking at the roof on one clear midday. If a proposal does not mention the shading on a roof that clearly has some, that is a signal to ask.
The essentials of Shading
- Partial shade can penalise a whole series-connected string of panels.
- Bypass diodes reduce the damage, but do not eliminate it.
- Permanent shade: avoided in the design. Moving shade: microinverters or optimizers.
- The shading study must cover different hours and seasons of the year.
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Frequently asked questions about Shading
Does a nearby tree ruin my solar installation?
Not necessarily, but it shapes the design. You have to assess at which hours and seasons its shade reaches the roof and decide between pruning, relocating the panels or using microinverters or optimizers. Ignoring it does ruin the expected output.
How much output is lost to shading?
It varies with the extent, the duration and the electrical architecture of the system. Brief shade over a panel with a microinverter barely affects the whole; the same shade on a string can cost a notable fraction of that string's output for as long as it lasts.
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