Ben’s Garage: Winter Solar Access

by Ben N on December 14, 2016

The rough shell of the garage is now up! We got the roof boards and felt on just before the first major winter storm of the season. Since then, a cold front has moved through. While that DOES drop the temperature (single digits F. right now,) it also means CLEAR skies! It’s cold, but sunny!

So, how much sunlight will I ACTUALLY get on the solar panels on the roof of the garage in the winter? Not only are there fewer hours in the day, but the sun is SO low in the sky that shadows stretch unnaturally long.

The winter solstice is only a week away, and it was sunny out, so I thought I would test the ACTUAL solar conditions by filming a time-lapse video of the roof of the garage. I set up a step-ladder about 20 feet back from the garage and put a camera on it to shoot a still image every thirty seconds. What resulted was a video showing EXACTLY how shadows will fall on the garage roof.

DCIM100GOPROMost notable was the shadow of a pine tree in my front yard. That clearly sweeps across the garage roof. The upside of this is that it looks like it only swings past the lower third of the roof. I’m planning on installing 21 solar panels, in three rows of seven panels across. That shadow shouldn’t effect the top two rows of solar panels.

Secondly, I also noticed some deciduous tree branch shadows. I’m afraid to say that I really don’t know how much of a negative effect those shadows will have. They are thin and spread out.

Lastly, the largest and most distinct shading is very late in the day, when the shadow of the peak of my house infringes on the garage solar access. At first, just the peak of the roof reaches the corner, but then most of the garage roof ends up getting covered. The one upside of this is that it IS late in the day. It’s only about the last hour of daylight that the house shadow blocks the garage. By this time, the sun is so low in the sky that the solar panels would be producing minimal power anyways.

The main thing that I would like to point out is that this shading is the WORST CASE SCENARIO. Essentially, we are seeing how much sunlight shines on the garage during one of the SHORTEST DAYS, when the sun is about as LOW IN THE SKY as it ever is, and SHADOWS are LONGEST.

The rest of the year, there are many more hours of daylight, and the sun is much higher in the sky.

IMG_2418My plan for solar is to have 21 panels of the 60-cell variety in a grid of 3 rows of 7 in a portrait orientation. Each panel will have it’s own grid-tie micro-inverter. This will produce a faceplate value of over 5,000 watts. In my area, we average 4 to 4.5 hours of peak sunlight per day. This should produce a year-round average of about 600 kWhs of electrical energy. My running 12-month average for electrical use is 563 kWh. So, on paper at least, installing 5,000 watts of solar on my roof SHOULD bring me an annual NET ZERO electrical use! That’s for all the power of running my house AND charging an electric car.

Of course, if I can fit a little MORE solar than that on the roof, I’d certainly like to. That would help produce additional energy to offset converting the natural gas furnace and water heating to electric or adding an additional plug-in car in the future.


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