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In a word, Yes! You really do need to keep them clean.
MYTH Solar panels are "Self Cleaning".
The thing is, inanimate objects typically don't clean themselves. You want to know what cleans itself? A cat. Solar panels? Not so much.
Okay, to be fair, there is a certain amount of truth to the self cleaning claim. That is if you consider a rainstorm to be the same thing as a thorough-but-gentle wash, dry, and polish.
How well does rain wash things anyway? Not that well. Rain has dirt in it! Just think about how your car looks after it rains. How clean did it really get? Rain does not leave your car spotless, and depending on how much dirt has accumulated on the vehicle, and the amount of rainfall; sometimes it makes things worse.
The exact same thing happens to your solar panels in a rainstorm. A rainstorm just doesn't compare to a good, thourough cleaning.
Solar panels with a light layer of dust can lose around 5% of thier power output from what's known as the soiling effect. A panel with a lot of grime and bird droppings will lose aroung 20% or more of it's potential output.
Even after it rains, there can still be a layer of dusty water spots that WILL restrict UV rays to some degree. Not only that, if the layer of dirt is stubborn enough, and you have a lot of bird poop; the rain just won't remove it all conpletely.
After solar panels are first installed, they will work at 100%. However, it doesn't take long for dust to start accumulating on them. The more dirt, grime, and bird poop that is allowed to accumulate, the more power output will be lost and cost you more on your power bill.
If you never clean them, you will lose power output over time and can expect the output to never get back to 100%. This will fluctuate though. After a strong rainstorm, you will likely get a positive spike in power output. But, it will likely be lower than 100%, and it won't last as dust starts accumulating again.
So here's the question... How often should you have your panels cleaned?
Simple, when they get dirty enough to start costing you a significant amount of money on your power bill from a lack of solar output.
A good way to make sure your panels stay as close to 100% output as possible is to simply have regularly scheduled cleanings. This way, you can put your solar panel maintenance out of mind knowing that it will be taken care of for you on a regular basis.
The frequency of your need will depend largely on where you live. Here in the Phoenix area it usually doesn't rain all that much throughout the year. However, we do get a lot of dust storms and rain in the summer time throughout the monsoon season.
A common occurance is a heavy layer of dust will be left on your panels from a windstorm, followed by a light shower. This will leave a think layer of grime on your panels. This is also the time of year that homeowners consume the most energy and need the panels to work at their best.
In Phoenix, it pays to have them cleaned frequently during the Monsoon season, then peroidically throughout the year.