For best results the panel should be in 90 degrees with the solar radiations to get the best out of this flux. think of it as if u have a hose and spraying water through a 3 feet far window, more water will cross the window if the window was straight, if it is incline the opening willdirectly get smaller and thus less water will get in.
same thing for solar, if angle is not 90 degrees less radiations get in, if the angle is less than 30 degrees radiations do not penetrate.
cos θ = (sin δ sin φ cos β) – (sin δ cos φ sin β cos γ) + (cos δ cos φ cos β cos ω) + (cos δ sin β sin γ sin ω ) + (cos δ sin φ sin β cos γ cos ω)
θ: Incidence angle
δ: Declination
γ: Surface azimuth
β: Tilt angle
φ: Latitude
ω: Hour angle