Solar hot water heaters can provide households with a large portion of their hot water needs while cutting back on home energy costs. The amount of hot water that solar energy will provide depends on the type and size of the system, the climate, and the quality of the site in terms of solar access. A back-up heating system for water will be necessary during times when solar radiation is insufficient to meet hot water demands. Solar water heating is a simple, efficient, and affordable technology that provides hot water and saves on energy bills.
Solar water heaters lower the cost of electric bills. A typical consumer can save about 30%-50% on his or her electric bill, while lessening the use of oil and the impact on the environment.

Water (or non-toxic antifreeze solution) travels through dark colored pipes that are located in direct sunlight. The sun heats the water in the pipes. Once heated, the water is stored in an insulated tank until needed. In instances where there is no sun for a prolonged period of time, an auxiliary back up water heating system is used. Often the back up water heater is the storage tank itself and therefore does not cost extra. A southern orientation is optimal, but not required.
The systems may be used to heat water for a wide variety of uses, including home, business, and industrial uses. Heating swimming pools, under-floor heating, or energy input for space heating are more specific examples.

Residents can typically cut their water heating needs by 40-80 percent and save between $120 and $240 annually.
In many climates, a solar heating system can provide up to 85% of domestic hot water energy. In many northern European countries, combined hot water and space heating systems (solar combisystems) are used to provide 15 to 25% of home heating energy.

Why Solar Water Heating? Besides saving money, solar water heating increases the amount of hot water available, requires little maintenance, and reduces the amount of pollution created by burning fossil fuels.

The first step in considering solar water heating is to adopt water and energy efficiency measures. For example, low-flow shower heads, aerators, and frontloading washing machines can drastically reduce the amount of hot water used.

Types and Systems:
Passive: A system not driven by mechanical means, but by thermo-siphoning only. Hot water rises - cold water falls, thus creating a circular motion.
Active: A system driven by mechanical means, usually a pump.
Direct: A system where the water and heated medium is the same.
Indirect: A system where the heated medium and water are separated by a heat- exchanger.

Passive Direct System: A passive system also known as a monobloc (thermosiphon) system, a compact system consists of a tank for the heated water, a solar collector, and connecting pipes all pre-mounted in one frame. The water flows upward (thermo-siphon) when heated in the panel. When this water enters the tank (positioned higher than the solar panel), it expels some cold water from inside so that the heat transfer takes place without the need for a pump. A typical system for a four-person home in a sunny region consists of a tank of 150 to 300 liters (36.9 to 79.2 gallons) and three to four square meters of solar collector panels.

Active System: Conventional Active systems use a powered circulation pump whenever the hot water tank is positioned below the solar panels. Most systems in northern Europe are of this type. The storage tank is placed inside the building, and thus requires a controller that measures when the water is hotter in the panels than in the tank. The system also requires a pump for transferring the fluid between the parts.

Indirect System: A non-toxic antifreeze solution is heated in the solar panel and is pumped through a heat exchanger. The heat exchanger exchanges the heat from the medium to the water - water that that feeds into the hot-water tank.

The medium in the collector can reach very high temperatures in good sunshine, or if the pump fails. Designs should allow for relief of pressure and excess heat through a heat dump.

A well-made, properly-maintained system should last 15 to 20 years or more, which means your decision to invest will have long-lasting results.

A solar system places all the costs upfront. From there the sun serves as a renewable fuel source that costs the homeowner virtually nothing to use. A family of four needs two 4' x 10' collector panels connected to a 40-80 gallon storage tank.

Types of thermal collectors:

Evacuated tube collector
Evacuated tube collectors are made of tubes, mounted in parallel, whose number can be added to or reduced as hot water delivery needs change, each of which contains an absorber tube. The tubes are covered with a special light-modulating coating. In an evacuated tube collector, sunlight passing through an outer glass tube heats the absorber tube contained within it. The absorber can either consist of copper (glass-metal) or specially-coated glass tubing (glass-glass).

Formed plastic collector
Formed plastic collectors (such as polypropylene, EPDM or PET plastics) consist of tubes or formed panels through which water is circulated and heated by the sun's radiation. These are often used for extending the swimming season in swimming pools. In some countries, heating an open-air swimming pool with non-renewable energy sources is not allowed, and then these inexpensive systems offer a good solution. This panel is not suitable for year-round uses like providing hot water for home use, primarily due to its lack of insulation which reduces its effectiveness greatly when the ambient air temperature is lower than the temperature of the fluid being heated.

Flat plate collector
A flat plate collector consists of a thin absorber sheet backed by a grid or coil of fluid tubing and placed in an insulated casing with a glass or polycarbonate cover.

Fluid is circulated, using either mains or solar electricity, through the tubing to remove the heat from the absorber and to transport it to an insulated water tank, sometimes directly or via a heat exchanger.

As an alternative to metal collectors, new polymer flat plate collectors are now being produced in Europe. These may be wholly polymer, or they may be metal plates behind which are freeze-tolerant water channels made of silicone rubber instead of metal. Polymers, being flexible and therefore freeze-tolerant, are able to contain plain water instead of antifreeze, so that in some cases they are able to plumb directly into existing water tanks instead of needing the tank to be replaced with one using heat exchangers. By dispensing with a heat exchanger, these flat plate panel temperatures need not be quite so high for the circulation system be switched on, so such direct circulation panels, whether polymer or otherwise, can be somewhat more efficient, particularly at low light levels.

As with evacuated tubes, most flat plate collectors have a life expectancy of over 25 years.