The Long Shadow – Winter Solar Shading

by Ben N on December 17, 2018

We’re coming up fast on the winter solstice!
It’s been sunny the last few days, a nice change to all the cloudy weather we have had for so long!

However, the sunny weather also reveals the LONG shadows cast this time of year!

My garage does NOT have IDEAL solar access, but it’s pretty good. It’s the only place on my property that’s appropriate for solar. Lot lines, trees, and the neighbors house all limit where I would have been able to install solar. I knew I would have issues with late day shading. As the sun moves to the west in the afternoon, the neighbors trees begin to shadow my solar panels.

With that in mind, I installed “Micro-Inverters”. Micro inverters are small electronic devices which convert the DC power at the solar panel to AC power. The micro-inverters are installed (one each) directly behind each solar panel. One of the advantages of this system is that each solar panel is INDEPENDENT from all the others. If one panel is shaded, the rest are unaffected. In traditional series-connection solar panels going to a central inverter, even a single small shadow can greatly reduce the power output of ALL the panels!

Besides the westerly shading, I have one pine tree in my front yard. It’s pretty far away from the garage. Far enough that you might not think about it casting a shadow on the solar. That’s easy to do in the summer when the sun is high in the sky. In the winter, the sun is so much lower that it casts a MUCH longer shadow. While the angle of the sun at the winter solstice CAN be calculated with just a little trigonometry, it’s much easy just to use an app or web solar calculator. (Here’s one I thought was pretty cool!) At this time of year, the HIGHEST the sun gets in the day is 23 degrees above the southern horizon!

Solar calculators can tell you all sorts of information, such as sunrise and sunset, angle of the sun (altitude and azimuth) and all sorts of other information for ANY DAY OF THE YEAR and ANYWHERE ON EARTH! They can also tell you how long a shadow will be on any day and time. In my part of the world, a winter shadow can be 2.3 times as long as a tree, telephone pole, or other object is tall! (Solar installers in my area use a “three times the height” as a rule of thumb to avoid shading issues.)

The good news is that the pine tree only shades the solar panels for a few weeks a year. For the most part, it’s just the bottom of the three rows of panels. Even better is that, because I am using micro-inverters, ONLY the shaded panels have reduced output, instead of all of them.

A wide open field would be ideal for solar, but residential solar often has a few limitations. Even so, careful planning, using micro-inverters, and a few other tricks can help maximize electric energy production year round!

For more in general about the garage and solar system, visit Ben’s Solar Garage.

Stay charged up!
-Ben Nelson

Take a look to see what my solar system is doing, LIVE at:


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{ 4 comments… read them below or add one }

1 Don December 25, 2018 at 11:14 am

Ben – Your best day this year was 28 April when you made 41.5 Kw, so obviously your 30 degree angle isn’t optimized for summer – I’m sure you did that on purpose so as to make more power in winter, when the sun is lower. Only 9 hours of daylight on the winter solstice – Your northern latitude is killing you more ways than just the low angle. Here on the Gulf Coast, we had more than 10 hours on that same date – Don

2 admin December 25, 2018 at 11:23 am

The angle of the solar panels are NOT optimized for any particular time of year. It’s simply based on the panels being flat to the roof. The exact angle of the roof was based on a few different limitations, but particularly by a local zoning ordinance on the maximum height of garages.
I’ve actually found that the biggest limitation in production is simply CLOUDS in November and December. The sky is commonly just solid white clouds, and production might be 1/10th to 1/20th what it could otherwise be!
The production does well in the spring due to the combination of clear skies and still relatively cool temperatures. PV produces higher voltage (and thus higher total power) when the panels are colder than when they are warmer.
While we have very short days in the winter where I live, the days in the middle of summer are very long. It’s nice to do something like going camping and still having light until almost 10 at night!

3 Don December 25, 2018 at 4:24 pm

Still, you would expect the maximum production to be sometime in June or July, wouldn’t you? Longer days and a higher sun angle, but your max day was April 28th. I found that more than a bit surprising and figured maybe you had chosen your roof angle to improve fall and winter performance at a small expense of summer output. If I was designing a system, that’s what I would do, but you say you didn’t. It’s a mystery, I tell you!

4 admin December 26, 2018 at 12:01 pm

I ran some software predictions. The most interesting part of it was a big bump in production in MARCH. March has the spring equinox, so we actually end up with 12 hrs of sunlight by the end of the month, but it’s still winter, so it’s COLD. The combination or decent sun angle, cold air, and relatively long days produces more power than you would actually think! We produced 824kWh this past March!
You also have to remember that a single day of maximum production doesn’t mean much. Long term averages are really much more important. Temperature and weather can vary quite a bit from one day to another. For ideal production, a person wants to think long term.

There’s a great solar calculator called PV WATTS. It’s free to use and pretty straight-forward. You can enter information such as your location and roof angle, and it will predict how much solar energy you can expect to produce. Change the variables and you can see how much more or less power you would produce based on your roof angle, angle away from true south, array size, etc. You might be surprised when peak production is expected to be!
So far, my real-world production has matched up fairly well with the PV WATTS prediction, after factoring shading.

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