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Solar Production Question (lemmy.world)
submitted 2 months ago by Serinus@lemmy.world to c/solarpunk@slrpnk.net
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This is my setup

I'd expect A and B production to be pretty similar. And C times 5/9 should be about the same as D. That's not really what I'm getting.

Snow, of course, makes this more complicated.

These are the numbers I'm being told from my installer/Tesla.

The installer's theory is that: (MPPT B) is quite a bit lower than (MPPT A) for the same array size on the same south facing side of the roof but could be related snow melt on the peak prior to snow melt on the lower roof. This theory seems to be the case on the north facing roof, with the lower array being lower than would be expected.

Seems kind of odd in November though. I can't say the exact date that tree had no leaves, but it's not exactly hanging over the panels. When the sun gets to a low angle, production dies regardless of the tree, so I don't think it's making a huge difference, particularly in the winter. Plus periods of snow are marked by 0 or near 0 production. They're not that hard to spot.

I want to say that a 20-30% difference in A's production vs B's production probably means something's wrong. Any thoughts would be welcome.

I also don't know why their numbers are so far off of what Home Assistant is showing (other than watts vs kilowatts).

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[-] eleitl@lemmy.zip 2 points 2 months ago

Your post has no pictures.

[-] perestroika@slrpnk.net 2 points 2 months ago

Short answer: snow lingers longer on the lower part of your roof. The flatter the roof, the bigger the difference.

[-] Serinus@lemmy.world 3 points 2 months ago

It didn't snow every week of November though, and this seems pretty constant.

[-] perestroika@slrpnk.net 2 points 2 months ago* (last edited 2 months ago)

Then I would suspect partial shadow. Is there anything near the lower string that could cast a partial shadow on the panels?

I notice these green curved lines on your diagram. They resemble what people usually draw to denote tree cover on a map. If they are trees, how tall, how far, and how high up are your panels?

[-] Serinus@lemmy.world 2 points 2 months ago

It's maybe 20 feet away, no leaves, and I expect would need a quite low direct south angle from the sun to have much of an effect.

I don't expect that it would account for 15-25% lower production.

Maybe I can get the installer to come out and test the connection to each of the panels. I don't know what they're gonna want to charge for that.

[-] Dogyote@slrpnk.net 2 points 2 months ago

As others have said, just a bit of shade on one panel can impact the entire string. Another weird idea is temperature. Are string B's panels warmer than string A? The warmer they are the less they'll produce.

[-] pdqcp@lemmy.dbzer0.com 1 points 2 months ago* (last edited 2 months ago)

I'm not a specialist, so don't take my word for it, but as far as I could understand, those are not microinverters, right?

In that case, even if only one panel on string B is being partially shaded by snow/tree/leafs/debris/etc, the whole string will output the same amount as that one panel, which might explain the difference

Whereas on String A you probably don't have anything shading any of the panels

See this image for reference:

edit: How was it last year? or did you just install this system?

[-] Serinus@lemmy.world 2 points 2 months ago

It's a new system this spring. There is a tree, but it's not much higher than the roof, maybe 20 feet away, and likely doesn't shade much of anything in November with no leaves. Especially when it seems that the sun at a low angle doesn't seem to generate anything, anywhere.

Yeah, Microinverters would have been much better, but the price was also 50% higher. Seeing as how this was quite an investment limb in the first place, I didn't have the disposable luxury income to go with the microinverters. Sure would have been nice for this situation though.

Overall with this system, minimal regrets. If I had to do it again, I wouldn't have Tesla anything. But for this project I had jerked around the installer enough, and changing plans in the last month wasn't really feasible. On the other hand, this installer (Kokosing) tends to do more commercial stuff, and it sure seems like they've been tired of my shit every time something comes up.

this post was submitted on 19 Dec 2025
17 points (94.7% liked)

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