The left chart below displays total kWh generated per day in July 2010. Mid-summer is almost 20 kWh per day every day. The sun always shines in this part of California.
In contrast, the right chart below shows total kWh generated per day in December 2010. It's a lot less than in summer even on a good day, and there's lots of day-to-day variation due to clouds and rain. It was very rainy from Dec 13 to Dec 25.
By 2021, the 12-year-old panels had acquired a layer of sticky grime. Probably pollen adhering to the panels, then dust sticking to the pollen. Wiping a section of a panel with a paper towel after spraying with a glass cleaner left the paper towel pretty black and the panel smoother and cleaner. But cleaning 16 large panels this way was not feasible, so we hired a cleaner who used a cleaning process with hot deionized water. I was skeptical that water alone would clean them, but it did. They now feel smooth, and glass cleaner no longer cleans anything.
That said, the improvement in energy generation was very small. It's rather hard to judge because day-to-day and year-to-year fluctuations are large. By looking at the maximum daily power generation for 3 weeks before and after cleaning, and also looking at the same time span for several other years, I estimated that I should easily see a 5% increase in performance. I didn't. At best, performance increased 1 or 2 %. That makes up for 1 or 2 years losses due to panel degradation, but it doesn't in any way make up for the steady, inexorable decline. It seems a bit surprising, but the sticky grime was not really hurting performance.
The company wanted us to set up a yearly (or more often) cleaning schedule, which I declined. Considering the limited improvement and the cost of the cleaning, a cleaning more than every 5 or 6 years probably cannot be justified.
© 2021 Randy Knight