[waffles4all]

A minor point, the direct peak intensity is not 1 kWm^-2 at AM 1. AM1.5G is the standard spectrum and it is acquired at an oblique angle of 48.2 degrees from zenith. It is typically normalized to 1 kWm^-2 due to the inherent variability encountered when actually trying to measure these things.

[teamonkey]

Er, yeah, that's kind of the point :)

For London in Autumn/Winter will have a surface normal of z~=60-70 degrees to the Sun. Much greater than 48.2 degrees, so the peak insolation must necessarily be less than 1kWm-2.