Solar Water Heaters (Home Energy Part 14)
Solar hot water systems can be a great way to lessen your household use of energy, and add resiliency to your some with a renewable energy source. Hot water is a large energy draw in households. Solar hot water systems typically offer much higher returns in energy savings than solar photovoltaics.
Most domestic solar hot water systems are designed to provide 40-80 percent of a households annual hot water needs. Domestic solar hot water systems are usually integrated with conventional storage or tankless water heaters. (Be sure to purchase a tankless system that is designed to operate with a solar hot water system.) The conventional tank or tankless system becomes your backup hot water supply for when the sun is not shining. 
Energysavers.gov is an excellent source of additional information on solar hot water systems. http://www.energysavers.gov/your_home/water_heating/index.cfm/mytopic=12850 
3. Chiras, Dan., “The Homeowners Guide to Renewable Energy,” March 2006.