How to calculate Diffuse Horizontal Irradiance?

I have been preparing solar data from the Australian Bureau of Meteorology - they provide the GHI and DNI parts of the equation below, but not the DHI which I need to simulate a solar panel in SAM.

Global Horizontal (GHI) = Direct Normal (DNI) X cos(θ) + Diffuse Horizontal (DHI)

Does anyone understand where the data comes from for the cosine response to incident angle section of the equation? I think that if I can figure that part out I can use that to solve for the DHI part of the equation. Thanks so much for the help.

Hi Jack,

SAM's Detailed PV model accepts a weather file with any combination of two of DHI, DNI, or GHI, so you should be able to use the weather data without calculating the DHI. See my reply to the following post for instructions:

sam.nrel.gov/forum/forum-general/3184

For a fixed array with no tracking, theta is equal the array tilt angle input.

Best regards,
Paul.

Thanks so much for the help!