Best angle for west facing panels

drjrt

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I'm installing 10 panels on a small cabin I am renovating deep in the woods. There is no open land for a ground based system so I am stuck putting the panels on the porch roof which has a 9 degree slope. It is the roof that gets the most sunlight and is facing due west. I don't plan on changing the angles of the panels seasonally so was going to go with a fixed 37degree (my latitude) but now I'm wondering if that is the best as that angle is really best for south-facing panels. I already purchased Iron Ridge tilt legs but I'm sure I will use them on another project.....
 

snoobler

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Welcome to the forum.

Tough question, and you're right to ask I suspect it may be best to leave them at as shallow an angle as possible, 9°, as that will get you more of the "noon" sun than a 37° panel would.

You can simulate your actual array and various scenarios at the following site:


It also takes average weather into account; however, it can't take into account shading due to terrain or trees.

If your panels are going to be subject to shading may be devastating to production.
 

drjrt

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Thank you. That's a much better app than I had been using.
I just had an epiphany; face them south on the roof (5 rows of 2 panels each) at 37 degrees angled toward the south; they will have a 9 degree tilt to the west but that shouldn't be a big deal. I will need to get more mounting hardware but I should see significant improvement in output....
 

snoobler

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Definitely a better plan than purely west facing. It doesn't matter if the rectangle is perpendicular to the ground or not.

The only concern with that method is you're likely limited to just a single row of panels as the tilted panels would cast shadows on panels in a back row.
 
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JoeHam

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I just had an epiphany; face them south on the roof (5 rows of 2 panels each) at 37 degrees angled toward the south; they will have a 9 degree tilt to the west but that shouldn't be a big deal. I will need to get more mounting hardware but I should see significant improvement in output....

Sounds good, post some pictures of how you made it work when you’re done!
 

drjrt

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My rectangle is about 7' by 28' with the long side facing west.
I will get my triangle calculator out and see how many rows I can get without one set shadowing the other at December solstice...

I will post, thank you!
 

drjrt

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Interesting playing with the numbers. With the panels at a 27 degree angle facing south I have to space them 57" apart to avoid self-shadowing at winter solstice. With that I can fit a total of 7 panels on the roof which could generate 2,747 Kwh/year per PVWatts . If I lay them all flat facing west at 9 degrees I can fit 9 panels with an estimated generation of 3,103 Kwh/year. However, if I lay 7 flat facing west and have the last two facing south at 27 degrees (nothing for them to shadow) it could generate a total of 3,198 Kwh/year. I'm not sure the extra hardware is worth 95 Kwh/year.....

If consider facing them all south, squeeze them closer together to get all 10 panels up, and just accept some shadowing will SAM help me with that estimation? I have been digging into SAM but I don't have a handle on the capability, yet.
 

snoobler

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What's SAM?

Shading can be devastating. If you shade only the bottom row of cells of a typical panel standing on its short side (standing taller than it is wide), you essentially cripple the entire panel.

It's also highly variable. Different panels react better than others, and it's hard to characterize how the shading actually impacts the shaded panel.

Shading should be avoided to the greatest extent possible.
 

drjrt

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SAM is the System Advisor Model from NREL.GOV
Looks extremely powerful but I'm not sure of all the specifications meanings so just now muddling through it.

I think I have decided on 7 flat facing west and 2 angled facing south. The extra $20 in mounting hardware should pay for itself in 2 years.
Give me a chance to experiment with angled panels before I move on to my next project.
 

Isla

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East and west facing roofs are also suitable for solar panels and will still see a good deal of energy generation throughout the course of the day. Maximum Australian homes have a roof pitch of 20° to 30° which is considered optimum for maximizing solar power production. However, the panels can be installed anywhere between 10 to 35 deg with negligible efficiency loss.
 

svetz

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... accept some shadowing will SAM help me with that estimation? I have been digging into SAM but I don't have a handle on the capability, yet.
Yes SAM can help with that, watch the video on it though, it can be a pain to use. They have a number of tutorials on it.

Modeling Shading Losses for PV Systems, Aug 2014​

The Modeling Shading Losses for Photovoltaic Systems in SAM webinar was presented on August 29, 2014 by Aron Dobos of NREL, and describes options for modeling near-object and self shading in SAM, and previews the new 3D shade calculator that will be included in the new version of SAM scheduled for release in the Fall of 2014.
 

chrisski

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Shading losses can be huge.

To give you an idea of efficiency loss of a shadow, I have 100 watt panels that are about 42”X 20” that output 5 or 6 amps each in maximum sunlight. To test the panel, I will put the shadow of my head over the panels, and I see it drop to 1 amp. That’s an 80% efficiency loss. Also, on a cloudy overcast day, my panel output will drop to 1.2 amps per panel, or 75% less energy produced.

In northern latitudes, south facing panels are always a better choice, barring shade. I’ll call them niche production, but there could be other areas that produce more power than you’d expect. For me this “niche” is my 400 watts of portable panels that I point into the sun three times a day. With the better morning and evening production and the 30 degree tilt of these panels, I typically get more production out of these than my other 950 watts of flat panels.

If you start looking for places that get sun year round, that could make some energy. Also, for me for the winter months panels on my RV walls look like it could produce a bit of energy on an east and west facing wall At the beginning or end of the day. To me that means a K-cup coffee earlier in the morning.
 

drjrt

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Unfortunately I'm in a densely wooded area. My choice is to do nothing or take the efficiency hit. I've chosen the latter with some professional tree pruning (which brings up the ethical questions: is it OK to remove trees to improve solar efficiency to reduce reliance on coal?). I put 36 panels on my primary cabin and they are doing better than expected; meeting 80+% of my needs. My next project is to put in a solar array in an open field about 250ft from the main cabin. Lot's of trenching and big gage wire to pull....
 

drjrt

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Finally got some good weather and put up the racking and wiring. Panels will be pushed out closer to the edge of the roof than they should be but I thought it was more important to have space to work around the panels centrally. Just waiting for my kids to help me pack the panels up to the roof, plug in, and secure. I will update with additional pictures when complete.
Solar 1.jpg
Solar 2.jpgSolar 3.jpg
 

Hedges

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Looks like standoffs screwed through sheetmetal roof. Is there a mastic sheet or something that makes it water tight?
 

drjrt

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These are S-5! VERSABRACKET-47 METAL ROOF MOUNTING CLAMP and S-5! SOLARFOOT METAL ROOF MOUNTING CLAMP which are designed for sheet metal roof installs. They have a layer of "butyl co polymeric sealant" on the bottom which I'm pretty confident will be water tight. The installation screws also have a rubber gasket.
 

Substrate

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Unless I missed it, I'm assuming you are using or planning to use LifeP04 as your chemistry of choice, since maintaining lead-acid properly under these conditions would be a real chore. I've seen some guys do it, but that is rare.
 
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