I assume that you refer th thermal collectors. Their efficiency may vary from 0 to 95 % depending on their type, on the irradiance, on the temperature difference between the heat transferring fluid and the external ambience, and even on the wind velocity.
The thermal collectors may be characteriused by two numbers:
Also look at the Swiss testing laboratory :http://www.spf.ch/Collectors.84.0.html?&L=6
and on https://en.wikipedia.org/wiki/Solar_thermal_collector
Dear Mahesh Kumar
The important factor that affecting flat plate solar collector efficiency, for a given place, is the difference between inlet and outlet water temperatures. The increase in temperature difference negatively affects efficiency. This is due to increased in entropy generation.
Therefore, the temperature of the outside water should be selected depending on the task.
As the outlet water temperature approached to the task temperature, the entropy generation decreases and the collector thermal efficiency increases.
Please see the attached book.
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