Energy Plus: Surface Outside Face Incident Solar Radiation Rate per Area of tilted surface

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I am using EnergyPlus to verify a lower order simulation model. When I compare the surface outside face incident solar radiation rate per area from Energy+ to my model I see differences, shown in Fig 1.

How does EnergyPlus modify the direct and diffuse radiation to calculate the incident solar radiation on the buildings surfaces?

Any guidance on how EnergyPlus calculates the Surface Outside Face Incident Solar Radiation Rate per Area would be greatly appreciated!