The Solar Hot Water: A Primer gives a very good overview of the various types of solar water heating systems, and provides a useful set of ground rules for sizing these systems.
If you are building your own system, then I believe that you should consider the information provided below, and potentially adjust the collector and tank size accordingly.
Commercial solar collectors cost in the neighborhood of $30 per square foot, and storage tanks designed for solar water heating systems are expensive and large tanks are very expensive. So, commercial systems have high cost components and the total cost of the system goes up markedly as the size of the system is increased. The sizing ground rules in the primer article recognize this and try to strike a reasonable compromise between a system that will meet a good fraction of your hot water needs, but not break the budget. On the other hand, if you build a system such as this one, the collector cost is only about $5 per sqft. In addition, the tank cost is low and increases very little with tank size.
If you are building your own system, the cost of sizing the system up to provide a greater fraction of your hot water needs is small. You may want to add collector area and increase the storage tank size. This will provide a greater solar fraction because on each sunny day, the system is heating more water than you will actually use and storing the heated water in the larger tank so that you can continue to use solar heated water over a cloudy day or two or three. With such a system, the collector tilt can be increased to favor winter collection when shorter days and more cloudy weather reduce the output of conventionally sized systems. The end result is a very high annual solar fraction for a modest increase in initial investment and build work.
Gary April 26, 2009