In a study conducted in Spain, researchers measured an average performance reduction of only 4.4 percent after a year’s worth of dust buildup. Most solar panel owners could probably live with that. But the study also found that in “long periods without rain, daily energy losses can be higher than 20%.”1
So, in a dry climate where dust buildup is the main issue, the problem remains pretty simple: a lack of rain results in dust buildup that might get progressively worse the longer the dry spell lasts.
Meanwhile, normal rainfall can rinse away dust and keep solar panels producing at around 95 percent of their maximum capacity. That conclusion might lead you to assume that solar panels in climates with frequent rainfall should require less maintenance. Not so fast!
Most of the time there’s more to the equation than just dust. Pollen, bird droppings, leaves, ash, and a hazy film that can form when smog or smoke hang heavy in the atmosphere can all combine into a layer of surface soiling, which may or may not wash away in the rain. Here’s a report from Renewable and Sustainable Energy Reviews2 that digs into the many complex variables involved.
The research concludes that allowing rain to do the work of cleaning solar panels is a fine decision if, and only if, you live in the right kind of place, such as:
Think about it: If rain was really an effective method of cleaning, then why is the car wash everyone’s first stop when a rainstorm ends? With the wrong combination of pollutants, a little rain might not clean anything. It could just become the base ingredient that combines a collection of contaminants into a kind of slurry.
Case in point: The account of one family living in a temperate rainforest where the climate completely rules out dust as a problem. Three years after installing their solar panels, they noticed a 10 percent performance drop. When they called local installers to ask about cleaning services, they got a canned answer about how the frequency of rain in their area should prevent any significant buildup from forming. But when a window washer who they eventually hired went up to clean their panels, he found a sticky brown haze that he said wasn’t likely to rinse off in the rain. Then once the panels were cleaned, the homeowner claimed that productivity jumped back up by 20 percent, exceeding their original production level by 10 percent!3