Rooftop solar panels only work when they are in direct sunlight. So if you install solar on a partially shaded roof, the output of your system will be lower than if there were no shading. And if your roof is heavily shaded, then rooftop solar is probably not going to be cost effective.

The reduction in output that shading causes depends on the type of shading, the orientation of your roof, and the use of microinverters or optimizers (these module electronics can help reduce shading problems). The most common examples of rooftop shading include trees, chimneys, neighboring structures, and utility poles. A small amount of shading — perhaps from a vent pipe or utility pole — will not have much impact on overall system output. But trees or other structures can have a big impact. A good rule of thumb is the object creating shading should be twice as far away as it is higher than the panels.

There is a home around the corner from me with a solar system on the west roof, and a big Magnolia tree right next to the house shading all the panels in the afternoon (when the sun should be shining directly on the panels). These west-facing panels are illuminated by the sun in the morning, but at a very oblique angle (about 20 degrees). Because of the shading, the panels never get direct sunlight perpendicular to the panels; the 20 degree sun angle means that these panels are generating only about 34% of what they could generate if there were no shading.

I hope the solar company that installed this system gave the homeowner an accurate estimate of the output (or lack thereof) of these shaded panels. Otherwise this homeowner will be unhappy when they get their annual utility true up statement and realize that their savings were not what they were led to believe.

Installing solar on a partially shaded roof is really an economic question. Will the resulting payback from the system — after factoring in lower energy output from shaded panels — still be acceptable? It might make sense to install panels in a partially shaded location if the installation costs for these incremental panels are low and the electric rates are high. Please Listen Up to this week’s Energy Show for practical, economic advice for homeowners who are thinking about solar on a partially shaded roof.