How does SAM calculate the irradiation on a tilted and oriented plan for the PV systems?

Hi!
I wanted to know how does SAM exactly calculate the irradiation on a plan with given tilt and azimuth. I've tried to understand with the 'Help' option but I only found the way that SAM calculates the beam irradiation initially to use as data.
Hopefully someone could help me to understand this.

Thank you in advance,
Raquel

Dear Raquel,
Please see Chapter 6 of the SAM Photovoltaic Reference Manual available from the Performance Model Documentation page for a detailed description of the calculations. This is the final draft of the manual and should be published as an NREL publication soon.
Best regards,

Paul.

Thank you very much!

I was wondering if you could explain one expression in Chapter 5. Just before the first expression (5.1), you say that the AOI is a function of sun azimuth angle, sun zenith angle, etc. But the sun azimuth angle is not a part of that equation, why?
Another doubt that I have, and maybe it a little stupid question, if the zenith angle is (PI/2-solar altitude) why do you use cos(Z) and not sen(solar altitude)?

Best regards,
Raquel

Dear Raquel,
That is an error in the equation. Thank you for pointing it out. The correct equation should be:

As for the second part of your question, you could use either for the calculation. 
Best regards,

Paul.

I have another question. In equation 6.2 why don't you use the diffuse irradiance in the horizontal?
Thank you in advance!

Hello,

Equation 6.2 in Section 6.1 is for the beam irradiance on a horizontal surface, so it uses the beam normal irradiance from the weather file rather than the diffuse irradiance.

The incident diffuse irradiance equations are described in Section 6.2.

Best regards,
Paul.