Horizontal surface with downwads facing normals

I’m using the incident radiation component.

I am getting results.

The problem is that the surfaces that are horizontal have normals facing down hence the value being calculated is huge compared to the rest of the geometry. When I flip them and calculate them On their own (just the horizontal) It works , and I get good results, but when I flip them and combine them with the rest of the surfaces (that are not horizontal and not flipped) and insert that as geometry . The horizontal surfaces don’t generate any results, but the rest of the geometry does.

I just wanted to know what could be the possible reason.

Thanks