Just for speculation and with limited factors like accessibility I'd be at $1.50 per panel. I'd be flushing the panel with tap water, while another guy brushed the dirt loose with a truck wash brush or boars hair brush, then a 3rd guy follows quickly with a pure water rinse. Similar to Scott's suggestion but no soap as it would require more rinsing slowing the process down and you're gonna need speed to make a profit on this one.