Comment by DamonHD
1 day ago
If the panels don't have a slope of at least 5 or 10 degrees dirt can easily accumulate on them and may not wash off effecively with rain. I have a couple near flat on my porch roof for example, that suffer from dirt accumulation more than others in my off-grid and grid-tied systems that have inclinations ranging from ~23 degrees to vertical.
5-10 degrees of slope often isn't enough to keep panels clean on their own, depending on the climate and dust environment, some form of mechanical cleaning is still needed. This video is from a multi megawatt scale ground mount PV system in Rajasthan, India.
https://www.reddit.com/r/Damnthatsinteresting/comments/18p1q...
https://www.naturaecoenergy.com/robotic-solar-panel-cleaning...
How frequently really depends on the local climate, but at least a few times a year even in a place that doesn't have dust storms.