The average US house hold uses 10% of its power for hot water.  We could heat our water using electricity from the solar cells, and we certainly will have this capability as a back-up, but solar water collectors have become very efficient and in terms of energy payback on the cost, they are 4 to 5 times more efficient. (More info: Water Heaters vs Solar Cell pay back times)

A solar collector consists of a network of pipes through which water (or in colder climates, antifreeze) is heated. Collectors come in various sizes, with 4 by 8 feet (1.2 x 2.4 m) the most common.

On a typical summer day (sunny and warm), the fluid in the collectors reaches 140°F to 180°F (60°C-80°C). On a clear winter day (sunny and cold), it can reach 120°F to 150°F (50°C-65°C). When it´s cloudy and warm, collectors can reach 70°F to 90°F (20°C-30°C), and when it´s cloudy and cold, 50°F to 60°F (10°C-15°C). As long as the temperature in the collector is greater than that of your incoming cold water (usually about 50°F; 10°C), your solar hot water system is saving you energy.

Several types of solar collectors are on the market. Flat-plate are thin (3-4 in.; 7-10 cm), black, and covered with glass to hold in the sun´s energy. In evacuated tube collectors, (photo above) a glass tube surrounds each individual pipe in a vacuum. This nearly eliminates the influence of ambient air temperature. Evacuated tubes perform better than flat-plate collectors in cloudy weather, and can achieve higher temperatures compared to other collector types, but are typically more expensive.

Five main configurations of solar water heating systems are used today. We will look at the pros and cons of each later.
• Batch
• Thermosyphon (shown to right)
• Open-loop direct
• Pressurized glycol
• Closed-loop drainback