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

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.