It must be stressed that these comparisons are only a guide and should not be considered as a rank ordering of collectors. Such considerations as component sizing and proper installation procedures are equally important in determining a system's overall efficiency. In addition, because of the variability of weather conditions, instrumentation accuracies, and other test condition constraints, the thermal performance of any two collectors should be considered approximately the same if their ratings are within 25 Btu/(ft2day) of each other. Bear in mind that these comparisons are concerned with thermal efficiency and energy output per investment dollar only; they do not imply equivalent reliability or durability.
Approximate Comparison Model
1) To compare the thermal efficiency of different collectors, look at the Btu/(ft2day) rating; the higher the number, the better the solar energy collection efficiency (which means it will require fewer square feet of collector area to do the job).
2) To determine the collector that supplies the most energy per investment dollar, divide the Btu/day rating by the total installed cost of the system, then multiply the result by the number of collectors used. Example: Collector X has a Btu/day rating of 25,000. Your dealer has given you a price of $2,000 for an installed system consisting of two collectors. Calculate as follows: 25,000 ÷ 2,000 = 12.5, 12.5 x 2 = 25 Btu/day per dollar invested.