The vast majority of solar-thermal collector installations place the collectors at a slope.
One rule of thumb that has been in the solar-thermal industry for decades is that using a collector slope angle (measured upward from horizontal), and equal to local latitude, optimizes the annual solar-energy incident on the collectors, when the collectors face due south. Since domestic water heating is a year-round load, most collectors used in solar DHW systems are installed with a slope that’s within a few degrees of local latitude.