Energy.gov has the details on solar hot water:
CHOOSING A SOLAR WATER HEATING SYSTEM
There are two types of solar water heating systems: active and passive.
Direct circulation systems: Pumps circulate household water through the collectors and into the home.Pros: Automatic controllers installed on the system sense when sunlight is being collected. Cons: Best for areas where temperatures rarely fall below freezing. Only works for single application domestic use. Integral collector-storage passive systems: Made of a storage tank, solar collection tank, and the pipes that run between them, these systems require cold water to be pumped into the solar collector in batches.Pros: They work well in households with significant daytime and evening hot-water needs. Cons: Best in areas where temperatures rarely fall below freezing but overall can be less effective. Indirect circulation systems: Pumps circulate a non-freezing, heat-transfer fluid through the collectors and a heat exchanger. This heats the water that then flows into the home.Pros: Still functions in freezing temperatures. Suitable for multiple solar heating applications like a solar swimming pool as well as domestic use. Cons: Can be more expensive than direct circulation systems. Thermosyphon systems: Water flows through the system when warm water rises as cooler water sinks. The collector must be installed below the storage tank so that warm water will rise into the tank.Pros: Reliable system. Cons: Contractors must pay careful attention to the roof design because of the heavy storage tank. Can be more expensive than integral collector-storage systems.
There's a lot more information at the link. Check it out and then give us a call for all your solar hot water needs.