Regarding solar panels: Here’s a new mystery…

Sun’s out: Yesterday afternoon got warm enough to melt off quite a bit of remaining snow cover, and last night got plenty cold enough to freeze all the resulting moisture back out of the atmosphere. The result was a nice hard frozen ground for my morning walkie…

Coming home over the ridge overlooking the Lair, I noted that the rooftop solar panels had already shed their frost but the groundmount panels hadn’t even started. This was no surprise, since they’re aimed in different directions* and of course the rooftop panels had been out of the shade longer.

But there was a sort of smudge on one panel I thought rated a closer look…

The hell?

I have no explanation for this. These panels are over 15 – probably closer to 20 – years old and this might be a sign of some sort of malfunction. But I can’t imagine what it might be. The system overall seems to be working fine.

*When I built the cabin I deliberately oriented it such that the roof slope would face due south, thinking that was the right thing to do. In a classic example of not knowing what you don’t know, I had completely forgotten/ignored all that talk about “declination” in my long-ago ground nav courses. In the SW desert, the difference between magnetic south and true south is substantial.

In a classic example of what we’ll call a Reverse Murphy, this later provided the unintended advantage that my rooftop panels function better in early morning while my groundmount panels function better in latish afternoon. Also they’re set at different angles, with the groundmount panels (deliberately – not everything I do is a mistake) permanently angled to take better advantage of winter sun, but that’s not really relevant to this post.

About Joel

You shouldn't ask these questions of a paranoid recluse, you know.
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14 Responses to Regarding solar panels: Here’s a new mystery…

  1. According to Ian’s blog, he used to work for a solar power place. I’ll bet he knows.

  2. Zack says:

    My next-door neighbor had a bunch of panels installed on his roof this spring; I noticed that his panels developed the same thing that you have on yours but his blotches are on a wholesale basis. I have no idea why. They looked perfectly uniform when brand new but after the heat of the summer months they looked all scroungy.

  3. Ben says:

    As long as your system is supplying your needs without frequent drama, all is well. That said, it’s likely safe to assume that your black smudge isn’t good news.

    Some nice sunny morning, and before your battery gets to float, you may want to go to the trouble of measuring the current contribution from your individual panels.

  4. Robert says:

    Practical joker sneaking over and wiping off just one square to perplex Joel? Cuz that’s the kinda thing I might do.

  5. Mike says:

    From the photo, it looks like one of two things. At the three o’clock position of the area that has become discoloured, it looks like something may have hit it (ice pellet?) and moisture has got in between the glass and the photovoltaic part that makes the electricity.


    Considering how old the panels are and the conditions, they endure (very hot, very cold, wind, rain, snow), the sealant between the glass and the photovoltaic part may be beginning to fail.

    Now having written the above, I will be the first to admit that I’m just guessing.

  6. Joel says:

    No, no, all the apparent discoloration is just because those are oldfashioned “multicrystalline” PV panels. All the cells look like that. What I can’t figure out is how/why that one single cell was warm enough to melt the frost.

  7. Perhaps a short circuit or damage to the wiring for that particular cell has occurred and as a result the generation of electricity is creating heat where normally it would not.

  8. Terrapod says:

    Cdr Zero is on the correct trail. As an old EE my first thought was that something in that particular cell is using some of the electricity to generate heat. That it is neatly bounded by the grid conductors says it is just that cell. Silicon cells are doped with some traces of rare metals to make them semi-conductive. If a tiny bit of moisture did get inside, it could deteriorate the cell making is slightly resistive, just barely enough to produce a smidge of heat.

    Another thought is that maybe all it takes is a tiny bit of air getting in to change the thermal gradient in the cell and the sunshine heated it up a bit faster than the rest.

    Nothing you can really do other than keep an eye on it to see if the condition propagates to other cells nearby.

    That you have had those running for 15 or more years is really amazing. Must be old phone company units which were overbuilt and over designed for very rugged service (pre-Chinese crap)

  9. Terrapod says:

    Oh, and one more thought, look behind that cell, is anything warm blooded nesting or sleeping under it? 😉

  10. Zendo Deb says:

    Panels have a 20 to 25 year life. The glass may have been hit by a bird or a hale, or

  11. Zendo Deb says:


    Nothing lasts forever….

    As for the “look” of the panel, it could be delamination of the glass from the underlying substrate.

  12. Mike says:

    Looking again at the photos, I did a google search for “visable signs of potential induced degradation in solar cells” and found the following. Number 3 seems to fit what is going on with the solar cell.

    Once again I will add that I’m I’m just guessing.

  13. John says:

    CZ and Terrapod may be zeroed in? Bet the panel seal integrity is fine. That melt pattern implicates that single demon cell so nicely. It seems to be displaying resistive heating and may no longer be adding any voltage (~0.5v) contribution to the panel.

    It may also adversely degrade the panel beyond its 1/72nd proportion thereof (like shade does), but maybe not by itself enough to be an issue yet?

    Might be informative and educational to try what Ben suggested and test that panel compared to the other three for short circuit current and open circuit voltage, assuming it isn’t a huge hassle to isolate them from each other.

    Assuming the four panels are wired in parallel, a drop of output current from any one just shrinks effective bank amp capacity some. If the voltage of one drops much, that might be more of an issue.

  14. Malatrope says:

    CZ is right. Possibly the reversing diode for that cell has failed. The overall ability of the panel to generate power won’t change in any significant way.

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