38The average raw power of sunshine per square metre of south-facing roof in
Britain is roughly 110 W/m2, and of flat ground, roughly 100 W/m2. Source:
NASA “Surface meteorology and Solar Energy” [5hrxls]. Surprised that
there’s so little difference between a tilted roof facing south and a horizontal
roof? I was. The difference really is just 10% [6z9epq].
39... that would be about 10 m2 of panels per person. I estimated the area of
south-facing roof per person by taking the area of land covered by buildings
per person (48 m2 in England – table I.6), multiplying by ¼ to get the south-
facing fraction, and bumping the area up by 40% to allow for roof tilt. This
gives 16 m2 per person. Panels usually come in inconvenient rectangles so
some fraction of roof will be left showing; hence 10 m2 of panels.
–The average power delivered by photovoltaic panels...
There’s a myth going around that states that solar panels produce almost as
much power in cloudy conditions as in sunshine. This is simply not true. On
a bright but cloudy day, solar photovoltaic panels and plants do continue to
convert some energy, but much less: photovoltaic production falls roughly
ten-fold when the sun goes behind clouds (because the intensity of the in-
coming sunlight falls ten-fold). As figure 6.15 shows, the power delivered
by photovoltaic panels is almost exactly proportional to the intensity of the
sunlight – at least, if the panels are at 25°C. To complicate things, the power
delivered depends on temperature too – hotter panels have reduced power
(typically 0.38% loss in power per °C) – but if you check data from real pan-
els, e.g. at www.solarwarrior.com, you can confirm the main point: output
on a cloudy day is far less than on a sunny day. This issue is obfuscated by
some solar-panel promoters who discuss how the “efficiency” varies with
sunlight. “The panels are more efficient in cloudy conditions,” they say; this