Oak trees kill solar panels.
Ok, lets not be dramatic..in the right conditions an Oak tree can dramatically reduce your solar-panels ability, if it is close enough to the panels.
Oak trees emit a dark residue that is very tough to remove from the slick surface of a solar panel when you try to wash it. Just water and a glass-safe brush may not do it, it may even require a razor blade! Oak trees give off a dark oily substance that settles on the panels, then the night air and moisture blend with the oil to thicken it the final blow is the hot daytime sun to crystallize and harden the strange tree-glue masking your panels off from the sun they desperately need to make power.
Most window cleaning companies do not have specialized brushes to remove this goo, the brush would be far stronger than the type we use on the glass and the kind we use on glass is usually fine for most dust when mixed with water and soap.
Whats the answer? Hard work is usually the answer, just scrub harder and for longer but that does not always work, so custom tools that hold fresh razor-blades at the perfect angle are generally the answer, mix this with good soap and slice the thick layer of goo off of the panels. Washing the panels fairly often is also a good answer. You know who to call. ;]