The best ones have a motorized system to keep the parabola facing the sun at all times. This requires two sensors, with a "wall" between them mounted on the parabola, to determine how to position it so both sensors "see" the same amount of light. Usually the motor is programmed to "park" at the morning position when the system shuts down for the day. They also work best on a South facing roof. I have never seen a "horizontal" parabolic collector, so I am not sure what their efficiency would be.