How often do you need to clean them?
Posted before I say your question. The study from the early 80's I mentioned looked at the maintenance costs, specifically cleaning.
With the exception of cleaning after severe dust storms, never. The penalty was a 4 to 5% reduction in electrical output.
You can do your own trade on value of your cleaning time vs. say a 500 W loss of output on a 10kW array.
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