How densely could such windmills be packed? Too close and the upwind
ones will cast wind-shadows on the downwind ones. Experts say
that windmills can’t be spaced closer than 5 times their diameter without
losing significant power. At this spacing, the power that windmills can
generate per unit land area is


This number is worth remembering: a wind farm with a wind speed of
6 m/s produces a power of 2 W per m2 of land area. Notice that our answer
does not depend on the diameter of the windmill. The ds cancelled because
bigger windmills have to be spaced further apart. Bigger windmills might
be a good idea in order to catch bigger windspeeds that exist higher up (the
taller a windmill is, the bigger the wind speed it encounters), or because
of economies of scale, but those are the only reasons for preferring big

This calculation depended sensitively on our estimate of the windspeed.
Is 6 m/s plausible as a long-term typical windspeed in windy parts
of Britain? Figures 4.1 and 4.2 showed windspeeds in Cambridge and
Cairngorm. Figure B.6 shows the mean winter and summer windspeeds
in eight more locations around Britain. I fear 6 m/s was an overestimate
of the typical speed in most of Britain! If we replace 6 m/s by Bedford’s

Figure B.4. Wind farm layout.
wind farm
(speed 6 m/s)
2 W/m2
Table B.5. Facts worth remembering: wind farms.
Figure B.6. Average summer windspeed (dark bar) and average winter windspeed (light bar) in eight locations around Britain. Speeds were measured at the standard weatherman’s height of 10 metres. Averages are over the period 1971–2000.