Sizing Your Solar Water Heater
Just as you would choose a 30-, 40-, or 50-gallon (or larger) conventional water heater, you need to determine the right size solar water heater to install. Sizing a solar water heater involves figuring out how much hot water you use, and how much solar hot water you want and can afford. Then you'll need to size your collector and storage tank appropriately:
- Storage tanks. Solar storage tanks are usually 50-, 60-, 80-,
or 120-gallon capacity. A small (50 to 60 gallon) system is sufficient
for one to three people, a medium (80-gallon) system is adequate for
three to four people, and a large (120-gallon) system is appropriate
for four to six people.
- Collectors. As a general rule of thumb for flat-plate
collectors, you'll want to allow about 40 square feet of collector
area to meet the needs of two typical water users (20 square feet
each) and 8 square feet for each additional person if you live in the
Sun Belt. Allow 12 to 14 additional square feet (1.1 to 1.3 square
meters) per person if you live in the northern United States.
- Ratio between tank size and collector size. A ratio of about 1.5 gallons of storage capacity to 1 square foot of flat-plate collector area is recommended. This helps prevent the system from overheating when the demand for hot water is low. In very warm, sunny climates, experts suggest that the ratio should be increased to as much as 2 gallons of storage to 1 square foot of collector area. For example, a family of four that needs 66 square feet of collector area in a northern climate would need a 99-gallon storage tank (in a southern climate, they'd need a 132-gallon tank). Because you might not be able to find a 99-gallon tank, you may want to get a 120-gallon tank.
As an alternative to meeting some or all of your hot water needs with a solar water heater, you might consider purchasing a solar-assisted system. A solar-assisted system has a 20-to 26-square-foot collector that would be retrofitted to an existing 40-or 52-gallon water heater. Due to the reduced storage tank capacity and smaller collector size, these systems will only provide 50 to 60 percent of the hot water used.
Before you buy your system, you or your system supplier/installer will want to make more precise calculations to evaluate your particular site. Additional calculations generally take into consideration the amount of sunlight you have available and the orientation of your collector, both of which can affect the amount of solar hot water your system can produce (and therefore the size system you need).