Solar water heaters -- sometimes called solar domestic hot water systems -- can be a cost-effective way to generate hot water for your home. They can be used in any climate, and the fuel they use -- sunshine -- is free.
How They Work
Solar water heating systems include storage tanks and solar collectors. There are two types of solar water heating systems: active, which have circulating pumps and controls, and passive, which don't.
Active Solar Water Heating Systems
There are two types of active solar water heating systems:
Direct circulation systems
Pumps circulate household water through the collectors and into the home. They work well in climates where it rarely freezes.
Indirect circulation systems
Pumps circulate a non-freezing, heat-transfer fluid through the flat panel solar collectors and a heat exchanger. This heats the water that then flows into the home. They are popular in climates prone to freezing temperatures.
Passive Solar Water Heating Systems
Passive solar water heating systems are typically less expensive than active systems, but they're usually not as efficient. However, passive systems can be more reliable and may last longer. There are two basic types of passive systems:
Integral collector-storage passive systems
These consist of a storage tank covered with a transparent material to allow the sun to heat the water.  Water from the tank then flows into the plumbing system. These work best in areas where temperatures rarely fall below freezing. They also work well in households with significant daytime and evening hot-water needs.
Thermosyphon systems
Water is heated in a collector on the roof and then flows through the plumbing system when a hot water faucet is opened. The majority of these systems have a 40 gallon capacity.  
Storage Tanks and Solar Collectors
Most solar water heaters require a well-insulated storage tank. Solar storage tanks have an additional outlet and inlet connected to and from the collector. In two-tank systems, the solar collector water heater preheats water before it enters the conventional water heater. In one-tank systems, the back-up heater is combined with the solar storage in one tank.
Three types of solar collectors are used for residential applications:
Flat-plate collector
Glazed flat-plate collectors are insulated, weatherproofed boxes that contain a dark absorber plate under one or more glass or plastic (polymer) covers. Unglazed flat-plate collectors -- typically used for solar pool heating -- have a dark absorber plate, made of metal or polymer, without a cover or enclosure.
Integral collector-storage systems
Also known as ICS or batch systems, they feature one or more black tanks or tubes in an insulated, glazed box. Cold water first passes through the solar collector, which preheats the water. The water then continues on to the conventional backup non pressure solar water heater, providing a reliable source of hot water. They should be installed only in mild-freeze climates because the outdoor pipes could freeze in severe, cold weather.
Evacuated-tube solar collectors
They feature parallel rows of transparent glass tubes. Each tube contains a glass outer tube and metal absorber tube attached to a fin. The fin's coating absorbs solar energy but inhibits radiative heat loss. These collectors are used more frequently for U.S. commercial applications.
Solar water heating systems almost always require a backup system for cloudy days and times of increased demand. Conventional storage water heaters usually provide backup and may already be part of the solar system package. A backup system may also be part of the solar collector, such as rooftop tanks with thermosyphon systems. Since an integral-collector storage system already stores hot water in addition to collecting solar heat, it may be packaged with a tankless or demand-type water heater for backup.
Selecting a Solar Water Heater
Before you purchase and install a solar water heating system, you want to do the following:
Estimate the cost and energy efficiency of a solar water heating system
Evaluate your site's solar resource
Determine the correct system size
Investigate local codes, covenants, and regulations.
Also understand the various components needed for solar water heating systems as solar air project, including the following:
Heat exchangers for solar water heating systems
Heat-transfer fluids for solar water heating systems
Installing and Maintaining the System
The proper installation of solar water heaters depends on many factors. These factors include solar resource, climate, local building code requirements, and safety issues; therefore, it's best to have a qualified solar thermal systems contractor install your system.
After installation, properly maintaining your system will keep it running smoothly. Passive systems don't require much maintenance. For active systems, discuss the maintenance requirements with your system provider, and consult the system's owner's manual. Plumbing and other conventional water heating components require the same maintenance as conventional systems. Glazing may need to be cleaned in dry climates where rainwater doesn't provide a natural rinse.
Regular maintenance on simple systems can be as infrequent as every 3–5 years, preferably by a pre heat solar water heater contractor. Systems with electrical components usually require a replacement part or two after 10 years. Learn more about solar water heating system maintenance and repair.
When screening potential contractors for installation and/or maintenance, ask the following questions:
Does your company have experience installing and maintaining solar water heating systems with solar accessory?
Choose a company that has experience installing the type of system you want and servicing the applications you select.
How many years of experience does your company have with solar heating installation and maintenance?
The more experience the better. Request a list of past customers who can provide references.
Is your company licensed or certified?
Having a valid plumber's and/or solar contractor's license is required in some states. Contact your city and county for more information. Confirm licensing with your state's contractor licensing board. The licensing board can also tell you about any complaints against state-licensed contractors.
Improving Energy Efficiency
After your water heater is properly installed and maintained, try some additional energy-saving strategies to help lower your water heating bills, especially if you require a back-up system. Some energy-saving devices and systems are more cost-effective to install with the water heater.
solar water heater, device that uses solar heat energy to produce hot water. A typical solar water heater consists of a solar collector mounted on the roof of a building and connected to a water-storage tank. Depending on the system, unheated water either can be circulated from the tank through the collector to be heated directly or can be heated by a high-capacity heat-exchange fluid that was warmed in the collector and transfers its heat through tubes in the water in the tank. While heat transfer from the solar collector to the unheated water can be facilitated passively without mechanical means, “active” solar hot water systems use electricity to circulate the heat-exchange fluid and to operate mechanical pumps and controllers.
Although the practice of using the sun for heating water for domestic use can be traced back to several ancient cultures, it was not until 1891 that the first patented high pressure solar water heater system was sold commercially. Invented by Clarence Kemp in Baltimore, Maryland, the system was called the “Climax” and was popular in California and other warm American states. Given the comparatively high cost and inconvenience of using conventional fuels to heat water, many households were eager to invest in these solar hot water heaters. However, the Climax system was limited in that the heating element doubled as the storage tank, thus restricting the amount of hot water available. In 1909 William J. Bailey patented a system that separated the water-storage tank from the solar heating element, forming the basis of the design of solar hot water heaters used today.