rather than re-invent the wheel...
Do a Google search on "solar Panel" +cleaning
You will find a number of companies providing cleaning services or automatic cleaning devices
e.g.
<a href="http://www.solarpanelcleaningsystems.com/"Heliotex (who also make anti-theft devices for these high-value installations Heliotex suggest washing every 2 weeks, with a simple rinse every 2-3 days.
a waterless electromagnetic cleaning system (pdf)