On ground mounts in the north of England were are using approx 1000 litres per 600 panels. We get lots of lichen and bird lime so the water usage is really heavy, and we have to pre treat prior to using rotary brush. Hope that helps, but I guess it will vary so much depending on where you are (conditions).

The frequency and level of lichen on many of the installations we look after is quite high, so it's quite intensive in terms of time, water use and lichen remover. Getting supply inset really helps but often too far away to fill the tank. Those are the challenges I guess.