You are finding that in the real world, things are much more complicated than in the laboratory. There are many factors that affect the output of a solar cell in real conditions, almost all of them for the worse. This is why any attempt to evaluate the output of a solar array 'accurately' can only be an academic exercise. Even if you could quantify all the variables for one array in one location at a given time - wait ten minutes and things will have changed. So don't spend too much time trying to work out to 3 or more significant figures what you solar array is going to do when you actually install it. Just accept it won't give quite the output you expect.
Mr maine... do you think the researchers whom have the ideal diagram in real condition maybe using the optimization tools to design their results or they succeed to have ideal results .
Traditionaly, solar radiation divides on try parts: Direct normal, Diffuse and Reflected. Only Direct part describes by Cos-law.
This is the main problem on recalculations from Horisontal surface to Tilt panel surface. The meteo data base set consist (mainly) on Horisontal surface data, with lack of Direct normal radiation data.
First, what is an ideal diagram? The IV plot of ANY two solar panels even of the same type and same manufacturer under conditions as similar as possible won't be identical. Optimisation tools - what are they optimising and how do you know if you can achieve in a real array the operating condition the tools have calculated? Thirdly - what do you mean by ideal results? Do you mean results from a real array that exactly match the results of some calculation? (They won't). I am not clear what you exactly want to do here.