In this article:

• Passive and Active Systems
• Solar Collectors
• Solar Water Storage Tanks

The first, most rudimentary solar water heaters were simply black metal containers, such as barrels or drums, that were placed in sunlight. Water filled them, absorbed heat from the sun, and then traveled through a pipe to the home's hot-water system. 

Modern solar water heaters use the same basic principles but have incorporated a sophisticated system of pumps, storage containers, and collection devices to maximize the amount of the sun’s energy transferred to the water, the hot water generated, and the energy retained. Nearly all cycle hot water into an existing conventional tank-style hot water heater that has a secondary source of heat, and thus reduce the energy load needed to maintain hot water. The conventional water heater backs up the solar heater on cloudy days and times of increased demand.

Because solar water heaters are serious energy savers, buying one qualifies you for a federal tax credit of up to 30% of the installation and material cost, with no upper limit. This incentive does not apply to solar pool heaters.

Many designs are manufactured, distinguished by the type of collector and circulation system they employ, as discussed below.

Passive and Active Solar Systems

All solar water heaters fall within two main categories: passive and active. As discussed in the article, Solar Water Heaters: Passive or Active?, active systems utilize circulating pumps whereas passive systems move water via water's natural tendency to rise when it is warmed.

Active systems may have either direct or indirect circulation. With a direct system, a pump circulates your home's water through the collectors. Indirect systems don't circulate your household water through the collectors but, instead, pump a non-freezing heat-transfer fluid through the collectors to be heated. This anti-freeze fluid then travels through a heat exchanger inside a storage tank, where it transfers its heat to your water. The indirect system is more complex, but better in climates where freezing temperatures would freeze-up a direct system.

Passive systems, though less expensive than active ones, are not as efficient at transferring heat because they don't circulate the the heated water as quickly. On the plus side, they are less expensive, more reliable, and more durable.

Solar Collectors

Regardless of whether a home's system is active or passive, it will have one of three types of solar collectors: flat-plate, integral collector-storage system, or evacuated-tube solar collectors.

Flat-plate collectors are like glazed, weatherproof, insulated boxes that contain a dark absorber plate beneath at least one glass or plastic cover. The water flows through this absorber plate, where it is heated by the sun. The insulation helps keep the heat inside the box.

Integral collector-storage ("batch") systems (ICS) also employ a glazed, insulated box but utilize one or more black tubes or tanks instead of absorber plates to heat the water. This preheated water is sent to the conventional backup water heater, where it is stored. Outdoor pipes are vulnerable to freezing, so this type of setup isn't appropriate for severely cold climates.

More commonly used in commercial settings, the evacuated-tube solar collectors employ parallel rows of transparent glass tubes, metal absorbers, and coated fins. These high-tech collectors minimize heat loss through radiation.

Solar Water Storage Tanks

Solar water heater storage tanks are well insulated and have inlets and outlets connected from and to the collector. One-tank systems utilize a back-up heater/solar storage tank combination. Two-tank systems have a solar water heater that preheats the water before it travels to a conventional water heater.