Following on from discussion about planning reports last week, here’s a chart I put together showing roughly how much PV you can fit on a flat roof. It’s based on the formulas described by Volker Quaschning, the German Godfather of Sol (Thank you! I’ll be here all week. Try the crab).
The shading angle is the angle from the bottom of the panel behind to the top of the panel in front. As a rule of thumb, you can use the height of the sun at noon on the winter solstice – for London, this is about 15°. Utilisation factor is the ratio of panel area to roof area.
So for London, with panels inclined to 35°, the utilisation factor is 34%, or around 1-in-3 (follow the 35° tilt line to where it meets the 15° shading angle, then go straight down and read off the utilisation factor).
Interestingly, all things being equal, in Rome this would be 1-in-2. So not only do you get more sun, but you can cram your panels in tighter too. Ah, Italy.
Note this doesn’t allow for parapets or the requirement for free space near the edge of flat roofs to reduce wind load (which the International Solar Energy Society – German Section – put at 1.5m at the sides at 1.2m at front and back; see this kick ass book).
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