The panels were dusty and cleaning them did add some lustre. The output measuring seems accurate enough, the customer is of a scientific background and seems to understand the whole setup. The panels are monitored and results are available on line, in real time - and linked to this are two neighboring farms with their own panel setup. One of these setups was cleaned at a similar time and the resulting output increase was 30% - speculation is that those panels were cited at a shallow angle on a barn roof, overlooking a courtyard where vehicles regularly drive through, many of which are tractors, carrying hay, straw etc and Molasses Animal feed is present at the location. Due to this combination the panels had a sticky residue with a substantial dust covering and caused a marked decrease in output. Presumably then a domestic setup on a house roof and near a road would see a considerable drop in efficiency due to it's location and immediate environmental factors whereas the setup I cleaned is largely self cleaning ?