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

Figure 6.14. This figure illustrates the quantitative questions that must be asked of any proposed biofuel. What are the additional energy inputs required for farming and processing? What is the delivered energy? What is the net energy output? Often the additional inputs and losses wipe out most of the energy delivered by the plants.
Figure 6.15. Power produced by the Sanyo HIP-210NKHE1 module as a function of light intensity (at 25°C, assuming an output voltage of 40 V). Source: datasheet, www.sanyo-solar.eu.