Thought that the following article from CleanTechnica on cleaning solar panels was worth passing along.
The writer's position is that for the most part solar panels simply don't need cleaning --rain is good enough. If you're intent on doing it, however, then a water hose or a squeegee should be fine. Obviously, caution is always needed, especially given that most installations are located on roofs. You'll also want to stay away from the inverter.
That said, given that an estimated 80% of solar installations in Australia are hemorrhaging power, monitoring is critical to determine faults and appropriate cleaning can be part of the approach of keeping systems generating the most they can -- especially when the power generated is part of your business' financial health and every kilowatt is precious. So, again, it's worth a read as long as you keep the larger business considerations for solar in mind.