How big should my solar thermal system be, and which features should I look for?

Some of the answers to questions about system size and features depend on how you plan to use the solar system. Here, you'll find general information on sizing systems for water heating, swimming pools, and space heating. To locate more specific information, please see Getting help.

Sizing a for heating domestic water

Just as conventional water heaters come in different sizes, so do solar water heaters. Sizing your solar water heater involves determining the total collector area and volume you need to meet 90% to 100% of your household's hot water needs during the summer. Solar- experts use worksheets and computer programs to help determine system requirements and collector sizing.

Contractors usually follow a guideline of about 20 square feet (2 square meters) of collector area for each of the first two family members. For every additional person, add 8 square feet (0.7 square meters) if you live in the Sun Belt area of the United States, or 12 to 14 square feet (1.1 to 1.3 square meters) if you live in the northern United States.

For active systems, the size of the solar storage tank increases with the size of the collector—typically 1.5 gallons per square foot of collector. A small, 66-gallon system is usually big enough for one to three people; a medium-size, 80-gallon system works well for a three- or four-person household; and a large, 120-gallon system is appropriate for four to six people.

In recent years, homebuilders have begun including solar water heating systems as standard features in some subdivisions. The systems are the same size for every home, and they all work equally well. However, large families with greater hot-water demand find that a smaller percentage of their hot water is provided by the solar energy system.

Sizing a solar thermal system for heating swimming pools

Heating your swimming pool with solar energy requires a collector that is 50% to 100% of the surface area of your pool. Your geographic location and other factors determine the exact size.

For example, a 15-by-30 foot swimming pool in Florida typically requires a collector that equals 100% of the pool's square footage, which translates to 450 square feet of unglazed flat-plate collectors. This is because many Florida swimming pool owners use their pools year round. In contrast, in northern California, most pools are used only 6 to 8 months per year, so systems are typically sized at 60% to 70% of the pool's surface area.

In general, adding more square footage lengthens the swimming season and allows owners to use the pool in colder weather. A pool cover or blanket reduces heat loss and helps maintain warm temperatures for a longer period.

Sizing a solar thermal system for space heating

In contrast to solar water heating, solar space heating usually requires a larger, more complicated system. Space-heating systems have to store heat for use when solar energy is least available and your house is coldest—at night and during the winter months. Solar space-heating systems are usually combined with water heating, and they are sized to accommodate both uses.

The amount of area needed to heat your home depends on many factors. These include the available solar energy, collector efficiency, local climate, and heating requirements. Heating requirements are based on insulation levels, the house's airtightness, and the of the residents. Generally, the area of solar collector is about equal to 10% to 30% of the floor area of the house.

heat your water with the sun