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A minor point, it's at its peak of 1kWm-2 when the sunlight is perpendicular to the Earth, that is, when the sun is directly overhead, because that's when it has to pass through the least atmosphere. Sunlight reflecting off the building where the sun is not directly overhead will necessarily be less intense.

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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.

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.

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