Comment by walrus01

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.