I was on a rooftop working on an air conditioner and noticed that my customer's solar panels were filthy. Is this an issue? How dirty can they be and still provide acceptable performance?
Define 'acceptable'. How does 85% sound? Because that would be a very dirty solar panel. Sunlight is powerful and has a way of getting through translucent films of dirt.
Should they be cleaned periodically?
Define 'should' and 'periodically.' Here's what I tell my customers: 'I don't tell my customers to clean their panels because I don't think I should be telling people to take that risk going on their roof. The rain does a good enough job most years. If you have a one story house and it is easy to use a garden house to rinse off the panels in between watering your plants, that's a great strategy. If you do take the risk of getting on your roof to scrub them, don't do it too often: The more often you do it the less of a boost you get each time. Doing it once every couple years might be all that's worth it.'
Is this a service i can offer?
Yes but it won't make you rich. It will be by far most cost effective for large commercial arrays, and perhaps larger residential systems. The money saved by average residential customers would not be enough to pay you for driving to the house and setting up your ladder.
And I'll just note again that the more often you clean, the more the returns diminish. The boost in production will be less if they are still pretty clean from the last time you were there. So if it's worth it to someone to pay you, say, $250 to show up once a year, it may not be worth it to them to pay you $500 to show up twice.
Before I finished this post, I looked up your location in your profile: You're in a pretty dusty place with not much rain. So if there are places in the country where it
is worth it and someone
can make decent money offering a service, you might be in one of them. So worth a try perhaps. I would find some solar systems where someone is willing to cooperate with you in doing a pretty controlled survey of the production monitoring before and after cleaning, to develop some data and find if there's a good financial case to be made.