The efficiency level of a solar collector refers to the thermal output or the "energy offset" of the system (the degree to which the solar water heating system reduces the use of traditional gas or electric energy). The following are the three main indicators of solar water heating efficiency.
Solar energy factor (SEF
SEF is the energy delivered by the system divided by the conventional input energy (the energy used by the backup heater plus any energy used to drive the pump or controller in the solar water heating system). The value ranges from 1 to 11. The efficiency of most residential water heaters is 2 or 3. The higher the value, the higher the energy efficiency of the system. This means that solar thermal systems have a greater contribution to water heating, so they require less conventional energy.
Solar energy (SF
SF refers to the part of the total load of hot water heating provided by solar energy. The higher the SF, the greater the contribution of solar energy to water heating. The value of SF varies from 0 to 1, and the solar coefficient of residential water heating systems is usually 0.5-0.75. SF is a standard developed by the Solar Energy Rating and Certification Corporation (SRCC) and listed in the OG-300 standard.
This rating comes from the Florida Solar Energy Center, which provides a rating for most types of solar collectors. Btu stands for British Thermal Energy Unit-a measure of the thermal energy required to heat water. Btu/(ft²day) indicates how much heat energy per square foot collector generates in one 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 provided by the collector for every dollar invested. To do this, divide Btu/(ft²day) by the total cost of the system. Multiply the result by the number of collectors used.
Please note that these ratings take into account the efficiency of solar water heaters in terms of heat output. This rating does not judge the reliability or life of the collector.