Tuesday, February 26, 2013

Solar Panels Need Full Exposure To The Sun - Guest post by Steven Gregersen


We get a lot of questions about solar installation and one of the most vexing problems people have to deal with is finding an area with full exposure to the sun. We had one person agonize for days about two trees that would shade the panels for part of the day. She, like a lot of people, thought that if half of the panels were still in the sun she’d only lose half of her generating power during that time. It just isn’t so!

We’re currently taking life easy in the desert of Southern Nevada (in a “no-snow” zone), living in our U-Haul truck that we’ve converted to a motorhome. We’ve installed a 100 watt solar panel to supply our electrical needs (to power a couple of lights, our notebook computers and watch DVD’s on our portable DVD player).
We have an ancient analog charge controller that we use to show the importance of full sun on a solar panel so I took a few photos to illustrate what happens when solar panels are shaded from direct sunlight.

This is the 100 watt solar panel. It’s divided into 72 sections or “grids.”
This is the charge controller showing the panel's performance at its maximum output of 6 amps or 79.5 watts (take 13.25 volts times 6 amps to get 79.5 watts). Very few solar panels actually put out their rated watts in a real-world setting. The reasons why will be covered in a future post.
I used my hand to cover two of the “squares.”
Note that covering two sections out of seventy-two reduced charging amps by thirty-percent. The panel is now putting out 53 watts.
I covered 15 squares (approximately 20 percent) with a hand towel.
That reduced the amps to 3 and the charging voltage to 12.75 (38.25 watts). Twenty percent shade resulted in a fifty-percent drop in power.
I covered half of the panel with a blanket.
…and reduced the power to approximately ½ amp and 12.5 volts (which was battery voltage).

We should note that this is not a scientific experiment. The battery we’re using is old and marginal which affects some of our numbers but the percentage the charge rate is reduced in relation to the amount of shading is accurate.

So if you’re considering a solar array the most important aspect of siting it is to insure that it will be in full sunlight for the most hours possible during the day.


  1. Covering a panel is not the same as shading a panel. Covering is 100% blockage of the light. Shade is not. You can still see when you are in shade; some light is still coming through. The density of the tree cover would affect how much is blocked, and that also could vary seasonally if the tree is deciduous.

    1. If you put that same blanket over your head you would see that there is not 100% blockage of the sun. Enough light would come through that you would be able to see, just like heavy tree cover. Same with other 'covering' objects.

      On the same note, we've watched the needle on the charge controler when a cloud passes over the sun. The drop is dramatic, just for a cloud passing by. Same thing at home when, at certain times of the day, a tall pine tree over 100' away from our solar panel tower, blocks the sun for a short while as the sun passes behind it during the time of year when the sun is lower on the horizon. The needles drops to almost nothing, even though there is bright sunlight all around, including reflective light from the bright white snow.

      The amount of reduction in solar charge, even when shaded by a tree/trees or clouds is substantial. You can't do anything about cloudy days, but you can do something about where you place your solar array. Try to keep it in the sun as much of the day as possible.

  2. A blanket over the panel might not be like cloud cover but it does illustrate the point. I see people talking about the negatives of solar panels, wind generators ect. I would like to remind people that nothing is perfect and solar power is just on part a preparedness plan, I use a gas generator a wind generator and solar panels. It all adds up to a lot of money and time but it is worth it. Its not even something I suggest for the newby. When someone gets to the point they are looking at back-up power systems learn as much as you can (like this articular here at Poverty Prepping) so you can avoid those costly mistakes. Thanks for the great article.