We have cleaned solar panels from time to time, depends what type of glass you are cleaning... Plexiglass, or regular glass. LOW LOW pressure on fiber glass or plexiglass!
On regular glass solar panels we first apply a soap, and with a soft brush scrub it in, then wash it off with DI water, and squeegee it down.
Beware of plexiglass, I wouldn't even use a pressure washer just soap, brush, squeegee...