The Schwebebahn in Wuppertal is part powered by solar panels. 558 W over 74.53 square metres means 7.4 W per square metre when the photograph was taken. I looked at a solar insolation map for Germany
http://www.solarserver.com/knowledge/lexicon/s/solar-radiation.html This seems to give Wuppertal as 950 kWh per year per square metre which would give an average of 0.108 kW or 108 kW per square metre. If that's the case, we were well below average. But then I first heard of the place in the phrase "Es regnet immer in Wuppertal"!
