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Solar Panel Voc and Isc

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I have had solar installed on my roof for 8 years. For the first 6 years the power degradation was about 0.8% per year which approximates the panel specification. For the last 2 years the power degradation has been about 6-15% per year (depending on the panel). I have determined that the microinverters are OK because their efficiency has always been 95-100% (even during the bad 2 years). So the problem must be the panels.

At the request of the panel manufacturer, the installer is going to measure Voc (open circuit voltage) and Isc (short circuit current). Of course these measurements depend on the amount of sunlight hitting the panels so the values will vary.

I already know that the Voc will be normal because the voltage at the microinverter (not open circuit) has dropped about 0.8% per year from day 1  until present (including the 2 bad years) which is normal. But we have not measured Isc yet. I have all the V and I data from the microinverters. But I expect that Isc will be slightly lower than what the microinverter measures (not short circuit).

What information about panel health can be determined from Isc that is different from the I measured by the microinverter (not short circuit)?

Other than placing the panel under a known light source, how can the panel be tested on the roof in sunlight?

The attachments show (1) voltage at the microinverter, (2) current at the microinverter and (3) peak 15-minute period power generated in each month.

You say 6-15% depending on the panel, so you have the individual panel data?
The worst being 6% drop and the best being 15% drop right?

Seems like an odd coincidence that ALL panels are suddenly dropping off, if it was just one it would make sense. I assume you've checked everything is clean and no possible new sources of shading exist right.

Yes, I have data for each individual panel. The microcontroller company furnished data reports that contained data for every 15-minute period of the day for Power Produced (w-hr), DC Volts and DC Amps. The data for the curves posted were from 15-minute period of the month that had the maximum power produced (all between 12-1pm). This removed concerns of clouds, haze, obstructions, etc.

DC Volts and DC Amps are the voltage and current from the panels into the microinverters. Multiplying these gives us the power into the inverters. The Power Produced is the output of the microinverters. Dividing power in by power out equals microinverter efficiency which is unchanged from the beginning. The microinverter efficiencies haven't degraded at all. That's why I believe the problem must be with the panels.

Correct, the best panel dropped 6% per year and the worst panel dropped 15% per over the last 2 years (12% and 30% total for 2 years).

The panels were last cleaned in July 2023 and data from Aug 2023 is consistent with the other monthly and yearly drops. I found previously that even very dirty panels dropped the production by only about 5%. There are no obstructions.

I agree that it is odd that all panels started degrading about the same time. It's almost as if we have had a permanent solar eclipse for the past 2 years! The date codes for all the panels is the same month and year. Maybe there were defects in a batch of panels?

Any data on mains / line voltage? Maybe the microinverters are throttling down due to excessive mains voltage on sunny days.

I find it extremely odd that all panels start to degrade at the same time. Only thing I can think of is that the protective layer at the back of the panels may have degraded and now the panels start to corrode from the rear. But this should be happening on all panels.

AC Voltage is about 240 VAC and 60 Hz which seems normal. Since the microinverter efficiency is close to 100% they don't seem to be throttling down.

One possibility is moisture entering the panels due to poor seals. But my best guess is failing connectors. Either corrosion or loose connections. Bad production tooling or contaminated wire might cause this. Since all the panels were built in the same month, there could have been a short-term systemic problem at the factory. But that's only a guess.

I still have no idea what measuring the Isc will do without knowing the intensity of the incoming sunlight. That is the next step as directed by the panel manufacturer.


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