This is an interesting question, and I must say that not much rigorous work and research has been done in finding answers to this.
When our team at Solar Mango did a review of literature to find out what others are saying about the generation loss from dust on solar panels, we found that opinion was divided! To our surprise, some knowledgeable folks were even claiming that accumulation of dust on solar panels should not really be a concern and that it is unlikely to reduce the output significantly. Of course, whether these folks were speaking for the world in general is in question because some of them essentially implied that periodical rains wash away most of the accumulated dirt, thus making the solar panels quite clean without us needing to clean them. But then, all regions in the world do not get regular rainfall month after month!
What we have seen at Solar Mango seems to suggest however that in many dusty regions around the world that get rainfall only a few months a year, dust could cause significant decrease in generation.
Based on some of the analyses Solar Mango made of real life solar power plant installations both at rooftops and solar farms, it can be said that solar panels that have not been cleaned at all for a year could lose as much as 15% of the output compared to regularly (once in 2 weeks) cleaned solar panels.
Now interestingly, some of our studies showed that the decrease in output could be as high as 40% during cold seasons (compared to cleaned panels), which would be only about 15% decrease during summers and hot periods.
Overall, it can be said that it makes helluva sense to clean your solar panels, once every two weeks.
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