Hi Brad,
Generally for smaller PV systems there is not a need to clean PV modules. You may do more harm than good. Usually just let Mother Nature clean off.
Soiling rate averages about 2% per month without any rainfall.
Spraying water on a hot array risks cracking the glass. Your spraying water early in the morning or late in the day is better when the array is cooler.
Likewise using any kind of pole to scrub the array risks cracking the glass if you inadvertently hit the module.
The big PV farms in dusty areas often use an air blower that travels down the length of the array and blows off dust.
The only places where accumulated soiling is really a problem are in places where aerosols are in the air that cling to the dust and "glues" dust onto the module glass (e.g., Abu Dhabi Masdar has this issue from all the petroleum aerosols in the air and requires more aggressive cleaning techniques).
You can measure the module output dirty and then clean to see what the power output difference is for your array (under same sun conditions). In most places soiling is going to reduce performance by only 2 to about 5% at worst.
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Robert Foster
Assistant Professor
New Mexico State University
Las Cruces NM
rfoster@nmsu.edu------------------------------