Wich formula is used for the global_horizontal_rad in the CreateEPW comp?

Dear ,

I have a curiosity about the creation of a EPW-File.

By creating an EPW file through the _CreateEPW component, even if I don’t enter any information regarding the direct solar radiation and any “sun elevation” to calculate the “sun zenith angle”, how the _ImportEPW can generate the GHI - Global Horizontal Irradiance? with which formula is it calculated? because is not readable inside the component

Thanks and best regards