On Saturday my new PV system modules had seagull poo on them (nothing else produces that much). I climbed on the shed roof and cleaned it off. By Monday they had done even more. It was due to rain and so I left it. For once heavy rain was useful -its all gone. This system is almost horizontal so needs some cleaning. The old system is at 27 degrees to the horizontal and tends to self clean, although I have got moss growing on the edge of the frames now. Its difficult to know how much difference poo makes to power output but must make some, although its not the major problem that I heard someone say it was on the radio (the implication being PV wouldn’t work because of bird poo).