The right collector type
Solar collectors can be installed on a ground rack, if you have the necessary space, on a large shed or a south, west, or east facing roof on your house. Because all materials do not provide the same amount of heat per square foot, you should choose polypropylene. The lesser conductive materials will require a larger area and more collectors.
If your prime area for the collectors is the roof on your house, ultimate care should be taken to choose the right collector type. The lower the incline (1/12 to 6/12) the more heat you will get in June and July; the greater incline (7/12 to 12/12) will perform better in May and September. Anything beyhond 12/12 will require 15 to 30% more collector area.
It should be lightweight, and easy to install (even if you choose to have it professionally installed). It's design should reduce wind load to a minimum and allow moisture to escape from underneath. It should require as little as 3 roof perforations per collector, and allow contraction and expansion under extreme weather conditions. It should be possible to replace the collectors without removing any of the screws that hold the collectors in place (under no circumstance other than re-shingling, should they be removed, not even for replacing damaged collectors). They should be easily repairable without removal. Their design should allow them to drain properly when shut down, and they should not be removed in the Fall. They should not increase the pressure on your pump by more than 4 PSI.
If esthetics are of concern, choose a design with no gaps between the collectors, no visible metal hardware and no tie-down straps. For all these reasons, you should choose Heliocol.