I am keen to figure this out as well and not just for solar radiation but also for other specified inputs like thermal energy in a volume to name just one - to confirm you are hitting what you are aiming at. I have suspicions that what you ask for is not always what you get depending on mesh and other issues. Maybe its an enhancement request.
After a little head scratching I've solved my issue and its relativly simple ( isn't it always ! )
Here's how I did it
Setup a measurement body (see below) aligned to the incoming solar radiation
Give the body a Black Body Radiative surface.
I then setup the following ;
Add a SG Net Radiant Flux (Solar) to the face of the Measurement body
Surface Plot - Net Radiant Flux (solar)
Surface Parameters - - Net Radiant Flux (solar)
I'm now getting the values I was expecting .
Initially I was getting confusing values as I was using Heat Flux which includes all forms heat flux not just Solar.
I've seen far too many people just 'turn the handle' with CFD and trust the results , thats why I have an Excel Solar calculator and SMath worksheet that I use before I even fire up the CFD.
I think its crucial that I'm able to check what the software is giving me , otherwise how else can you rely on the values.