i take into consideration, travel, time to clean, and overheads, never per panel. 
I have my hourly rate plus travel.
i always ask the customer, age of connie, when last cleaned etc, if im not sure myself, and cant visit, i get them to mail some pics.
 Also Steve, this wasnt just the  roof, it was full external, i always push for a full clean, i cant remember the last time i just did the roof.