A recent article in Mother Earth News explains how solar water heaters are rated and tested. The rating of a collector depends on the amount of heat in Btu that it will produce based on a certain amount of radiation that shines on the collector. A flat plate collector is more efficient when the inlet/ambient temperature difference is between 10 degrees F and about 70 degrees F. An evacuated tube solar collector becomes more efficient at higher temperatures.
The writers at Mother Earth News stress that different types of solar water heaters function differently under a variety of conditions and climates, so picking the best one for a project takes a wholistic approach.
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