Solar panel cleaner - so you need to haul a supply of window cleaner around, a method to dispense cleaner, a method to wipe off cleaner, maybe a hook/winch system to move up & down the panel when wheels are wet & slippery, maybe the hook on a roller to move laterally across panels, or maybe a hook on two corners of the panel(s) that the robot can traverse across - think the overhead camera system at football games, camera can be moved up & down the field or back & forth across the field, and in elevation too by extending the cables - you just need back & forth, the robot can do that, and up & down the panel by letting the robot take up the slack from one or both sides to move itself up & down. I'm picturing sail boat winches & cable loops hanging down some like a mag tape reader like you'd see in computer rooms. May need to gather that up if there are a lot of panels vertically.
The durability of the panel faces could determine the weight of the robot, so maybe the wheels would have to be width of the panel outer rails to stay off the face - the operator could extend the legs to the proper width while setting up on the first panel.
So maybe a wide spray pattern dispenser, a rotary brush, and a windshield wiper blade? Spray as it goes down the panel, spinning brush for some cleaning action, and the wiper blade following to squeegee it off. Start at the top of a panel, work down to the bottom, raise the brush/squeegee up, mover over a width, start spraying, then lower & brush/squeegee, and repeat. Or maybe have the brush be the applicator.
Could be a fun project figuring out the cable control to move up & down and across.