Since the area of all south-facing roofs is 10 m2 per person, there certainly
isn’t space on our roofs for these photovoltaic panels as well as the solar
thermal panels of the last section. So we have to choose whether to have the
photovoltaic contribution or the solar hot water contribution. But I’ll just
plop both these on the production stack anyway. Incidentally, the present
cost of installing such photovoltaic panels is about four times the cost of
installing solar thermal panels, but they deliver only half as much energy,
albeit high-grade energy (electricity). So I’d advise a family thinking of
going solar to investigate the solar thermal option first. The smartest solu-
tion, at least in sunny countries, is to make combined systems that deliver
both electricity and hot water from a single installation. This is the approach
pioneered by Heliodynamics, who reduce the overall cost of their
systems by surrounding small high-grade gallium arsenide photovoltaic
units with arrays of slowly-moving flat mirrors; the mirrors focus the sun-
light onto the photovoltaic units, which deliver both electricity and hot
water; the hot water is generated by pumping water past the back of the
photovoltaic units.

The conclusion so far: covering your south-facing roof at home with
photovoltaics may provide enough juice to cover quite a big chunk of your
personal average electricity consumption; but roofs are not big enough to
make a huge dent in our total energy consumption. To do more with PV,
we need to step down to terra firma. The solar warriors in figure 6.6 show
the way.

Figure 6.5. Solar photovoltaics: data from a 25-m2 array in Cambridgeshire in 2006. The peak power delivered by this array is about 4 kW. The average, year-round, is 12 kWh per day. That’s 20 W per square metre of panel.
Figure 6.6. Two solar warriors enjoying their photovoltaic system, which powers their electric cars and home. The array of 120 panels (300 W each, 2.2 m2 each) has an area of 268 m2, a peak output (allowing for losses in DC–to–AC conversion) of 30.5 kW, and an average output – in California, near Santa Cruz – of 5 kW (19 W/m2). Photo kindly provided by Kenneth Adelman.