Solar Water Heater Efficiency


Efficiency ratings for solar thermal collectors refer to either the thermal energy output or the “amount of energy offset” by a system (the extent to which the solar water heating system reduces conventional gas or electric energy use). Below are the three main indicators of solar hot water efficiency.

Solar Energy Factor (SEF)

The SEF is the energy delivered by the system divided by the conventional energy input (energy used by the backup heater plus any energy used to power pumps or controllers in the solar water heating system). The values range from 1 to 11, with most residential systems having a water heater efficiency of 2 or 3. The higher the value, the more energy efficient the system. That means the solar thermal system contributes more to water heating so less conventional energy is required.


Photo Credit: SOLARNOR

Solar Fraction (SF)

The SF is the portion of the total hot water heating load provided by solar energy. The higher the SF, the greater the solar contribution to water heating. The value of the SF ranges from 0 to 1, with residential water heating systems typically having a solar factor of 0.5–0.75. The SF is a standard developed by the Solar Rating and Certification Corporation (SRCC), and is outlined in the OG-300 standard.


This rating comes from the Florida Solar Energy Center, which provides a rating for most models of solar collectors. Btu stands for British thermal units – the measurement of the thermal energy needed to heat water. Btu/(ft²day) indicates how much thermal energy per square foot of collector is generated in a day. A higher Btu/(ft²day) rating means that fewer square feet of collector space can heat the same amount of water.
You can use this rating to compare the energy supplied per dollar invested of a collector. To do so, divide the Btu/(ft²day) by the total system cost. Multiply the result by the number of collectors used.
Note that these ratings consider solar water heating efficiency in terms of thermal energy output. Such ratings do not judge the reliability or longevity of a collector.

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