The collector is the part of a solar water heater system that collects the warmth from the sun and transfers it to the water. An example of the simplest collector is the old fashioned 'hot box,' which is a recycled hot water tank that has been stripped of its shell, painted black, and placed in a well-insulated box with a glass lid. Though incredibly basic, this is actually an effective system that is easy to build and install for very little investment.

The next step up is a hot box collector that uses a black absorber plate to collect the solar radiation. These collectors usually have a dark absorber plate placed under one or more glass or clear plastic covers. The plate transfers the sun's heat to copper tubing (painted black as well, usually with an electrostatic paint). This is superior to the hot box in delivering larger volumes of heated water. Absorber plate collectors have a much lower profile and weigh less than a rudimentary hot box, making them an attractive low-cost solution for existing homes (and especially economical for heating swimming pools).

The top-of-the-line technology for solar hot water heating is an evacuated-tube collector. This type of system features rows of transparent glass tubes that run parallel to one another. Some styles employ glass tubes within tubes or have a metal absorber tube attached to a fin. With the latter type, the fin's coating absorbs solar energy but inhibits heat loss. A vacuum is created inside the tubes, which eliminates conductive and convective heat loss, which seriously diminishes the efficiency of the more basic collector systems. Evacuated-tube or vacuum-style collectors can achieve surprisingly high water temperature, sometimes in excess of 300 degrees F.