When adding the 4 panels this August, the old panels looked to be covered with pollen.  We had to remove the lower row to reverse their mounting. While on the ground we hand washed the panels and saw how dirty they really were.  After washing, the lower row had 20% higher production than the unwashed upper row.  See photo below.  I got a long solar panel brush to wash the uppers. After washing the uppers, they all produce about the same.