For a solar water heating system, use the solar energy factor (SEF) and solar fraction (SF) to determine its energy efficiency.
The solar energy factor is defined as the energy delivered by the system divided by the electrical or gas energy put into the system. The higher the number, the more energy efficient. Solar energy factors range from 1.0 to 11. Systems with solar energy factors of 2 or 3 are the most common.
Another solar water heater performance metric is the solar fraction. The solar fraction is the portion of the total conventional hot water heating load (delivered energy and tank standby losses). The higher the solar fraction, the greater the solar contribution to water heating, which reduces the energy required by the backup water heater. The solar fraction varies from 0 to 1.0. Typical solar factors are 0.5–0.75.
Don't choose a solar water heating system based solely on its energy efficiency. When selecting a solar water heater, it's also important to consider the following:
Sizing your solar water heating system basically involves determining the total collector area and the storage volume you'll need to meet 90%–100% of your household's hot water needs during the summer.
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Before purchasing a solar water heating system, you can estimate its annual operating cost and compare it with other more and/or less efficient systems. This will help you determine the energy savings and payback period of investing in a more energy-efficient system, which will probably have a higher purchase price.
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