Grit Blogs > Mosquito Mountain Montana Homestead

Solar Panels need Full Exposure to the Sun

We get a lot of questions about solar power 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.

100 watt solar panel 

This is the 100 watt solar panel. It’s divided into 72 sections or "grids."

solar panel b 

This is the panel 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.

 solar panel c 

I used my hand to cover two of the "squares."

 solar panel d 

Note that covering two sections out of seventy-two reduced charging amps by thirty-percent. The panel is now putting out 53 watts.

 solar panel e 

I covered 15 squares (approximately 20 percent) with a hand towel.

 olar panel f 

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.

 solar panel g 

I covered half of the panel with a blanket.

 solar panel h 

…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.