Rooftop solar panels only work when they are in direct sunlight. So if you have 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 on Renewable Energy World for practical, economic advice for homeowners who are thinking about solar on a partially shaded roof.
About The Energy Show
As energy costs consume more and more of our hard-earned dollars, we as consumers really start to pay attention. But we don't have to resign ourselves to $5/gallon gas prices, $200/month electric bills and $500 heating bills. There are literally hundreds of products, tricks and techniques that we can use to dramatically reduce these costs — very affordably.
The Energy Show on Renewable Energy World is a weekly 20-minute podcast that provides tips and advice to reduce your home and business energy consumption. Every week we'll cover topics that will help cut your energy bill, explain new products and technologies in plain English, and cut through the hype so that you can make smart and cost-effective energy choices.
About Your Host
Barry Cinnamon is a long-time advocate of renewable energy and is a widely recognized solar power expert. In 2001 he founded Akeena Solar — which grew to become the largest national residential solar installer by the middle of the last decade with over 10,000 rooftop customers coast to coast. He partnered with Westinghouse to create Westinghouse Solar in 2010, and sold the company in 2012.
His pioneering work on reducing costs of rooftop solar power systems include Andalay, the first solar panel with integrated racking, grounding and wiring; the first UL listed AC solar panel; and the first fully “plug and play” AC solar panel. His current efforts are focused on reducing the soft costs for solar power systems, which cause system prices in the U.S. to be double those of Germany.
Although Barry may be known for his outspoken work in the solar industry, he has hands-on experience with a wide range of energy saving technologies. He's been doing residential energy audits since the punch card days, developed one of the first ground-source heat pumps in the early ‘80s, and always abides by the Laws of Thermodynamics.
Lead image: Green microphone via Shutterstock