Actually, there is no fixed answer for this very common question since the sun is not fixed and changes its position (means angle to our ground plane) constantly. Naturally, we have to adjust the tilt angle as per our requirement in view of sun’s known moving path/direction. If we are not using a tracker for our Panels then we shall look at our consumption pattern and optimize the fixed angle accordingly. If our load is more in summer then a lower angle will be better say between (4° to 15°), if our load is more in winter then a higher angle (say from 38° to 55°) might be better and if we need a round year optimization then we may be looking to fix our panels at around 25° to 35°.
The other important factor in selecting the angle is the availability of mounting space and its cost economics.