From: WC Maddox
The 1.4 Kw/m2 value is at the top of the atmosphere. At the surface on a clear day it is closer to 1.0 Kw/m2. This is with the Sun overhead. In Chicago in Jan & Feb, the Sun even at noon would not be overhead. The energy would be spread over a larger area so the 0.86 mW/mm2 (value before concentration) could be correct.
End Message