@Pål_Herman_Sund
The source or radiation (energy) is calculated from the sun and the sun angles, for the time specified, based on the location of each cell in the input surface. Along with the sky/sun map (how the sun seemingly moves across the sky) and the visibility (is the location shaded by a mountain for part of the day) the tool calculates the the total radiation per unit area.
Total radiation is composed of,
- Direct Radiation (the radiation which comes directly from the sun)
- Diffused Radiation (the radiation which is diffused by the sky, layers of atmosphere and other surroundings)
This is highlighted in the conceptual documentation for the tools.
Let me know if I am missing something in your question.
Regards,
-Ryan