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.