Following articles will help you to calculate angle of incidence of the Sun for tilted surface.

https://www.pveducation.org/pvcdrom/properties-of-sunlight/solar-radiation-on-a-tilted-surface

http://www.itacanet.org/the-sun-as-a-source-of-energy/part-3-calculating-solar-angles/

This slide will help you.

http://slideplayer.com/slide/8996633/

for i=1:365

teta(i)=(360/365)*(284+n(i));

delta(i)=23.45*(sind(teta(i)));

omega_s(i)=acosd((-tand(phi))*tand(delta(i)));

H_o(i)=(((24*3600*G_sc)/pi)*(1+(0.033*cosd(360*n(i)/365)))...

*((cosd(phi)*cosd(delta(i))*sind(omega_s(i))+((pi*omega_s(i)/180)*sind(phi)*sind(delta(i))))));

end

This could help you

http://www.itacanet.org/the-sun-as-a-source-of-energy/part-3-calculating-solar-angles/

how can I calculate Optimum tilt angle of a surface (βo)

Get equation for Incident angle using,

• Plate inclination angle
• Solar zenith angle
• Plate azimuth angle
• Solar azimuth angle

Answer is this attachment.

Ashika Siriwardhana the resulting equation in your PDF is like the second equation in the main post of Verhaegen Yoni .

cos(solar zenith)*cos(surface slope) + sin(solar zenith)*sin(surface slope)*cos(solar azimuth - surface aspect)

They both are correct.

If DNI is direct normal irradiance, and DTI is direct tilted irradiance, then:

DTI = DNI * [ cos(solar zenith)*cos(surface slope) + sin(solar zenith)*sin(surface slope)*cos(solar azimuth - surface aspect) ]

Formula 2) is the one I've always been using...

Following this book: solar engineering of thermal processes. Ch.1_ Page of 14

Verhaegen Yoni,

You can use the following equation to calculate the angle of incidence for a tilted surface.

AOI = cos-1 [cos⁡(aM ).cos⁡(aS ).cos⁡(AM-AS )+sin⁡(aM ).sin⁡(aS )]

where

aM = 90° - θM = altitude of the module

aS = altitude of the sun

AM = azimuth of the module

AS = azimuth of the sun