The tilt of the solar panel will depend on the season. As the season's change, the sun gets higher and lower in the sky, and you want as much of the panel to be hit directly with the sunlight. You may also want to consider that during the morning and evening, the sun will be lower in the sky which may also affect the tilt you want, to capture the most direct sunlight at all times during the day. I have found an interesting website here that helps to explain the angle required, which even depends on your latitude location.