Solar Shading

by Ben N on December 6, 2017

A few weeks ago, I started noticing a shadow creeping up to the solar panels on my garage. Sure enough, in winter the sun would be low enough in the sky for the shadow of a pine tree in my front yard to start blocking solar panels!

What’s a person to do?! Well, other than cutting down trees, (which I also did) the best thing is to PLAN AHEAD!

IMG_7339That started with me originally doing a Site Survey and figuring out how good of a location this would be for solar panels in the first place. Before I even rebuilt the new garage, I was already planning for the solar and used a “Solar Pathfinder” tool to see how much of the garage would be shaded, and at what times in the year. The Solar Pathfinder is essentially a curved plastic dome with a solar chart on a piece of paper below the dome. Looking at the reflection in the dome (sort of like a security mirror) a person can see trees, buildings, and other obstructions that would shade the solar location. Those can then be traced onto the slip of paper, and a little math can be done to figure out what percent of total solar production would be knocked down by the shading.

IMG_7309I knew that one issue with where my garage was is the fact that I have a narrow north-south facing property, and my neighbor has the property line planted with mature Maple trees. Those help keep my house a little cooler on hot summer afternoons, but they also cast a huge late afternoon shadow over my garage. Likewise, in the winter, with the extra-low sun in the sky the shadow of a tree in my front yard may actually be long enough to start blocking out the bottom row of my solar panels. And that’s exactly what I started seeing recently.

IMG_4271So, when I designed the solar array, I planned on using MICRO-INVERTERS. A micro-inverter is a relatively small device which mounts behind a solar panel and converts the DC power of the panel directly to AC power. That AC power is then combined with that created by the other panels, and routed to my breaker panel in the garage. Each panel produces power independent of the others. By contrast, a traditional “Series-String” inverter has many solar panels connected in series, one to the next to the next, and then to the inverter to be converted to AC power. One problem with that design is that a shadow over part of just one panel will reduce the amount of power made by ALL the panels in that string. Imagine the old Christmas lights where when one bulb would go out, they all go out. Only in this case, if one panel isn’t producing power, none of them do. Because of that, even a small amount of shading can have LARGE consequences in terms of photovoltaic energy production.

Another nice feature of using micro-inverters is that the system can provide information about each individual solar panel! Now that’s actually far more information than anybody would want to look at on a daily basis, but it’s very nice to have access to for things like troubleshooting. I’m using Enphase brand micro-inverters and can access all of the data about the panels and micro-inverters through a web page interface. It also allows me to give public access to a streamlined version of that information. If you want to see what my solar panels are doing right now, you can take a look at:…

Looking at the “Enlighten” software graphical interface, it’s very interesting to see how the pine tree shadow sweeps across the bottom row of solar panels. But the best part is that it’s ONLY that one or two panels that takes a hit to power production. Any panel not shaded is completely uneffected.

PineTreeShadingDec2017I also used the software to run an animation of the solar panels late in the day during the summer. Sure enough, it’s very easy to see the shade of my neighbor’s leafed-out Maple trees sweep across the solar array. However, the panels on the east side of the garage just keep producing power, even while the ones on the west side of the garage are shaded. This gives the more easterly panels up to another hour and a half of production time. All that time adds up when it comes to producing energy, and keeping my electric bill low (or NEGATIVE all summer!)

The graphing features in the micro-inverter web display are also interesting. Looking at a week in summer versus a week in winter, I can see how skinny the curves are on winter days. I can also see how the production curve of a typical day is steeper in the PM than the AM. That’s because of that late afternoon shading. However, I’d still much rather have solar panels than not, even if my solar access isn’t perfect!

If you have a location that might be good, but not perfect, for solar production, don’t let it stop you. Tricks like using micro-inverters can help maximize solar resources. That doesn’t mean you don’t do your homework or put panels just anywhere. Careful planning, researching solar, and using tools like a Solar Pathfinder can allow you maximize your resource and start making renewable energy!

Until next time, stay charged up!

PS: In the video, there’s a number of times where I use the terms “Power” and “Energy” almost interchangeably. To be clear, the two are different. Power is how much work is being done right now, whereas Energy is a total amount of power over some unit of time, typically an hour. Power measured in Watts (W) or Kilowatts (kW) and energy usually in Kilowatt-Hours (kWh). Think of it this way, an old fashioned light bulb might use 60 watts of POWER. You will use it for some number of hours over a month, and you will get an electric bill for your ENERGY use in kWh.

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