Solar Panel Voc and Isc

[ee753]

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.

[thm_w]

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.

[ee753]

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?

[nctnico]

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.

[ee753]

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.

[bdunham7]

How do the panels appear?  Which model panels and microinverters do you have?  If you have Enphase, do you have full access to Enlighten?

[nctnico]

AC Voltage is about 240 VAC and 60 Hz which seems normal

But is this always the case? Over here mains voltage varies between 223V and 240V depending on how much the sun is shining. In some places it varies even more up to the point solar panel inverters switch off due to the mains voltage getting too high.

[floobydust]

I suggest to borrow/rent an IR thermal imaging camera and inspect the array when it's operating.
This site is dated but full of good troubleshooting: https://photovoltaikbuero.de/en/pv-know-how-blog-en/creeping-losses-in-the-output-of-photovoltaic-installations/
https://photovoltaikbuero.de/en/pv-know-how-blog-en/why-would-the-open-circuit-voltage-in-a-solar-panel-string-be-too-low/

[mtwieg]

So my understanding is that you're observing the peak output power from the panels is lower than in previous years (mainly due to a decrease in peak output current). But I don't see any independent measurements of actual irradiance. Simplest explanation is that irradiance is actually lower, rather than some change in the panels and/or electronics.

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.

Agreed, perhaps they will also use a calibrated instrument (or a known-good panel) to measure irradiance during the measurement.

[ejeffrey]

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.

The inverters being near 100% efficiency doesn't tell you anything about whether they are throttling.   Inverters will throttle by drawing less current from the panel than available, not by dissipating extra power internally.

[nctnico]

So my understanding is that you're observing the peak output power from the panels is lower than in previous years (mainly due to a decrease in peak output current). But I don't see any independent measurements of actual irradiance. Simplest explanation is that irradiance is actually lower, rather than some change in the panels and/or electronics.

Sounds like a sensible thing to check. Dave tends to pull up a website that has data on solar panel installations around the world in his solar panel related videos. I think it is a good idea to find such a website and check the output of solar panel installations nearby. See if they have a similar decline.

[ee753]

I made a quick visual inspection of the panels with magnification did not see any obvious anomalies.

LG280S1C-B2 panel and Enphase microinverters.

I have access to the consumer Enlighten app and received full data reports from Enphase. The full data reports contain ACV, AC frequency, DCV, DCA, temperature, energy produced (joules) for every 15-minute period of daylight. That data provided the data for the DCV and DCI graphs.
The AC outputs do vary over the day but seem unrelated to power production or light levels. During one of my sample days the ACV was over 246 VAC mid-morning, approximately 244 midday and about 240 VAC just before sunset.

It is hard to believe that the irradiance during the highest 15-minute period in a given month over 8 years has significantly changed. If that were true, it has been dropping more and more each of the last 2 years. This is NOT a step function.  If decreasing irradiance were the issue, all the panels would likely drop by the same percentage. But some dropped 12% over 2 years and others dropped 30%. That's a BIG difference. And I doubt the sun is dimming

If a calibrated panel is used when the installer measures DCI later this week that would make sense. But I will be very surprised if they do that. The measurements would also be useful if the used a pryanometer or pyrheliometer to measure irradiance. I doubt they will do that either. They didn’t mention anything like that. We will see this coming Thursday . . .

Dave tends to pull up a website that has data on solar panel installations around the world in his solar panel related videos. I think it is a good idea to find such a website and check the output of solar panel installations nearby. See if they have a similar decline.

Who is “Dave” and where can I find these websites? Even if I find them, I doubt they will have the detailed data so cloud conditions can be eliminated. And if is highly unlikely that Enphase will give me a neighbor’s data report.

[nctnico]

'Dave' is the one paying to keep this forum up & running 

Maybe start here for looking at data: https://pvoutput.org/outputs.jsp

[woodchips]

You are in the USA, anywhere near the smoke cloud from Canada in the last 2 years?

It did for the dinosaurs, who knows?

[ee753]

Maybe start here for looking at data: https://pvoutput.org/outputs.jsp

Unfortunately not very helpful.

You are in the USA, anywhere near the smoke cloud from Canada in the last 2 years?

Nope. I'm in SoCal so no smoke. Good guess, though.

[bdunham7]

Who is “Dave” and where can I find these websites? Even if I find them, I doubt they will have the detailed data so cloud conditions can be eliminated. And if is highly unlikely that Enphase will give me a neighbor’s data report.

I don't know if I'm close enough to be relevant, but I'm in Upland, CA.  I'm showing 100% of estimated for Lifetime Production (start date January 2013) and 115% of estimated for the current month-to-date.  If your system was set up correctly in the beginning, the production estimates should account for panel degradation and so forth.  I think there's a way to make my system public so you could see it, but I don't know how helpful that would actually be.

[Someone]

Maybe start here for looking at data: https://pvoutput.org/outputs.jsp

Unfortunately not very helpful.

Why not? it quickly shows other systems nearby and their outputs over months/years for you to compare with.

[tszaboo]

It is probably the weather. The second year I had my panels installed, I got +10%.

[bdunham7]

Here are the detailed results of my #24375 inverter from Aug 1-7 2023.  Let me know if the format works and if this information (and more of it) would be useful.

[ee753]

No. Graphical data doesn't help. Here is a portion of the data report from Enphase

[bdunham7]

The attached chart(1).zip contains a .csv file.  You can click on it and download it.

[ee753]

OK. Yes, that is what I need for the days listed.
Thanks

[ee753]

*** UPDATE ***

Another poster was kind enough to send me data from his system. He is located about 100 mi North of me and has very similar weather conditions. Again I chose the days and times with the highest output (no clouds, no haze) when analyzing his data. It turned out that his DCI dropped about 2.0% over the last 2 years (1%/year) which is normal. My DCI dropped between 12% and 30% (6-15%/year) depending on the panel. Clearly there is something wrong with my system - and it's NOT weather.

The installers measured Voc and Isc on 4 panels in strong sunlight today. Isc is specified as 9.3A. The 4 panels measured were 9.9A, 9.2A, 7.2A and 6.0A. Voc was normal for all 4. Clearly less current is coming from at least 2 panels.

We spoke with Enphase and they are convinced that the microinverters are performing properly (as am I).

There was some discussion that unplugging/replugging the connectors might remove oxidation, corrosion, contaminants, etc. Since 2 of the panels were unplugged/replugged last week I checked the DCI today. The DCI dropped 5-8% on those panels from before they were unplugger/replugged. It got worse. My thoughts are that there are certainly issues with the connectors. Maybe the panels, also.

[nctnico]

Connectors can be the problem. I created a topic about this earlier this year:

Turns out you can't mix random connectors which look like MC4 but are in fact not compatible at all:
https://www.eevblog.com/forum/renewable-energy/fyi-mixing-mc4-connector-brands/

On top of that some installers use pliers instead of the official crimping tools to crimp the connectors.

I'd open up a few connectors to inspect the crimps and see if the connectors which are on the solar panels are compatible with the connectors on the Enphase modules.

[bdunham7]

The 4 panels measured were 9.9A, 9.2A, 7.2A and 6.0A. Voc was normal for all 4. Clearly less current is coming from at least 2 panels.

Presuming that was measured with an ammeter directly across the panel output connectors, you clearly have some bad panels.  If that reduction in current were due to a bad connection, that connection would quite quickly be glowing red hot as it would be dissipating ~150W.  Also, where could there be a bad connection external to the panel if you have it shorted with the meter?