Here’s visual evidence of the effect of a small amount of shade on a PV array. This is why your installers won’t recommend putting panels where there’s a chimney or a tree. In our case, ONLY for half an hour or so in the morning at this time of year, next door’s roof shades a corner of our array. As you can see from the picture, we have 14 panels, and the shadow is covering a total area of about 1.3 panels. Now have a look at the graph below, which is the inverter out put for the SAME day as I took the photograph. The peak output during that day (27th March 2012 – sunny all day, not a cloud, no haze) was 2.56Kw. I took the photograph just after 9am, and as you can see, at that time, the output was barely 250-300W. You can see the vertical leap from there to around 2.2kW when the shadow finally moved off the corner of the panels. So even though the shadow was covering only about 10% of the total area of the array, the power output was cut by about 80%.
If I blow up the neighbour’s house (I’ll wait until he’s out of course!) then the extra power I’ll get off the roof on a day like this is indicated by the “missing” area on that graph – if you imagine a smooth curve from the zero line at around 7am when the sun first hits it joining up with the curve going on from the top of the vertical leap, the curved triangle you get is the amount of power the roof’s shadow cost.
So if you have a chimney, tree, or similar, whose shadow will move across the panels during the course of the day, don’t put panels there…