2 W/m2 ×4000 m2/person = 8000 W per person,

if wind turbines were packed across the whole country, and assuming
2 W/m2 is the correct power per unit area. Converting to our favourite
power units, that’s 200 kWh/d per person.

Let’s be realistic. What fraction of the country can we really imagine
covering with windmills? Maybe 10%? Then we conclude: if we covered
the windiest 10% of the country with windmills (delivering 2 W/m2), we
would be able to generate 20 kWh/d per person, which is half of the power
used by driving an average fossil-fuel car 50 km per day.

Britain’s onshore wind energy resource may be “huge,” but it’s evi-
dently not as huge as our huge consumption. We’ll come to offshore wind

I should emphasize how generous an assumption I’m making. Let’s
compare this estimate of British wind potential with current installed wind
power worldwide. The windmills that would be required to provide the
UK with 20 kWh/d per person amount to 50 times the entire wind hardware
of Denmark; 7 times all the wind farms of Germany; and double the
entire fleet of all wind turbines in the world.

Please don’t misunderstand me. Am I saying that we shouldn’t bother
building wind farms? Not at all. I’m simply trying to convey a helpful
fact, namely that if we want wind power to truly make a difference, the
wind farms must cover a very large area.

This conclusion – that the maximum contribution of onshore wind, al-
beit “huge,” is much less than our consumption – is important, so let’s
check the key figure, the assumed power per unit area of wind farm
(2 W/m2), against a real UK wind farm.

The Whitelee wind farm being built near Glasgow in Scotland has 140
turbines with a combined peak capacity of 322 MW in an area of 55 km2.
That’s 6 W/m2, peak. The average power produced is smaller because the
turbines don’t run at peak output all the time. The ratio of the average
power to the peak power is called the “load factor” or “capacity factor,”
and it varies from site to site, and with the choice of hardware plopped
on the site; a typical factor for a good site with modern turbines is 30%.
If we assume Whitelee has a load factor of 33% then the average power
production per unit land area is 2 W/m2 – exactly the same as the power
density we assumed above.

Figure 4.3. Chapter 4’s conclusion: the maximum plausible production from on-shore windmills in the United Kingdom is 20 kWh per day per person.
Table 4.4. Facts worth remembering: wind farms.
Table 4.5. Facts worth remembering: population density. See page 338 for more population densities.