For solar water heating in your home, there are basically two types of solar collectors: conventional flat-plate collectors and evacuated tube collectors. Evacuated tubes have a number of advantages: the work on overcast days, in colder weather, and the tubes can be installed and replaced individually without special tools. The evacuated tubes act like giant thermos, allowing 93% of the sun's radiation in, but only 3 to 5% out.
Thermal conduction and convection losses are almost nothing (under 2%) because of the vacuum gap. This design allows the system to work in very low temperatures (down to about -40°F).
The only disadvantage is the higher price of these systems. For a residential installation you are looking at about $1500-$2500.
When using solar water heaters, you need back-up heating method. Tankless water heater work best with solar installations.
The Solar Rating Certification Corporation rates solar collectors for sale in the U.S.