Hi Abderrahman Mellalou ,

just multiply the known incident radiation with the cosinus of the tilting angle with reference to the direction of incidence.

But if you need to go one step further, e.g. calculating the output of a solar panel: In many cases, the efficiency of the device depends on the angle of incidence, too (could be virtually zero due to total reflection, for example).

Hello Abderrahman Mellalou you can use Mainel Meinel model

Chapter Simulador de perfiles fotovoltaicos utilizando el modelo de ...

I consider that these scientific contributions will allow you to have your own criteria regarding the calculation you need to make:

· Analytical Calculation of Radiation Characteristics of Metasurface-Based Solar Antennas "SOLANTs" With Uniform and Non-Uniform Cells.

· Estimation of Solar Radiation for Tomato Water Requirement Calculation in Chinese-Style Solar Greenhouses Based on Least Mean Squares Filter.

· Calculation of the irradiance of solar radiation in a greenhouse with a complex structure using a diagram for sky view factor.

· Foundation cloud image based solar radiation intensity calculation method, involves calculating irradiation intensity of to-be-predicted time horizontal plane surface according to cloud layer shielding rate clear sky irradiation model.

· Solar radiation strength calculation apparatus of air conditioning system, has solar radiation strength calculation unit that calculates solar radiation strength with which solar radiation of seat is shielded in solar radiation direction.

· Haze condition-based surface solar shortwave radiation calculation method, involves utilizing pixel is as correction pixel to calculate solar shortwave radiation of correction pixel as radiation value by using parameterization process.

· Image terrain radiation correcting method, involves calculating reflectivity of each pixel of image to-be-corrected by using preset calculation formula or relative reflectance calculating formula according to solar zenith angle.

The detailed procedure are clearly treated in the text book by Duffie and Beckman "Solar Engineering of thermal processes". You can also use the TRNSYS software to do evaluate the hourly radiation with ease.

There is method to estimate solar irradiation on tilted surface. The method is called ratio of beam (Rb) which is defined as ratio between irradiation on tilted surface to the horizontal surface. The value of Rb is mostly related to the position of the sun to the surface. You can refer to the book :

engineering process and system" by Kalogirou chapter 2.3.8 . The same equation also stated in other book of "Solar engineering if thermal process" by Duffie Chapter 1.8.