BTU: British Thermal Unit... 1 BTU is the necessary energy to rise 1 pound of water by 1° F in 1 hour.

Even if this rule is very simple and precise, it's not always easy to find out the exact and real capacity of the different pool heat pumps on the market. The reason is that the power output is directly related to the ambient air temperature and moisture level in the air at the moment the performance test is done. This explains why the same  unit will produce more BTU in Miami than if it's installed in Chicoutimi.

A standard...

To help the manufacturers, the American Society of heating, refrigerating and air-conditioning engineers, (ASHREA ) created a set of rules to standardise the performance tests conditions. Here are the ASHRAE 146 conditions, for the pool heat pump testing.

Tests	Standard	Low Temperature	Spa Conditions
Air Temperature	27° C or 80.6° F	10° C or 50° F	27° C or 80.6° F
Water Temperature	26.7° C or 80° F	26.7° C or 80° F	40° C or 104° F
Relative Humidity	63 %	63 %	63 %

During the tests, many readings are taken, to be able to calculate the real BTU output and also the Coefficient of Performance (CoP) Finally the CoP is the most important result,  because it gives the real performance of the machine. For example, if we take 1 Kw-Hr from the customer electrical panel, and we give 5 Kw-hr to the pool, we get a CoP of 5. With the ASHRAE standard test, we normally get a CoP between 4.3 and 5.9 in the best cases.

Link to AHRI: (Just select HAYWARD manufacturer, copy the code and click on search) : http://www.ahridirectory.org/ahridirectory/pages/hpph/defaultSearch.aspx

The reality...

Unfortunately, the  pool heat pumps manufacturers are not obliged to use this type of standard and to have a testing certificate from an independent laboratory like ETL (Electrical testing laboratory) which are providing the Energy guide specification on the refrigerators for example. So the BTU rating on the units varies a lot from one company to the other, for the same output, because many of them will label their machine with the maximum BTUs the machine can produce in extreme conditions like 90° F with90% relative humidity for example, while others will use conditions that are in the ASHRAE table above.

How can you find the best unit ?

 After you have selected a list of machines that are the right size for your pool, the important thing is to get a machine that has a good construction with proven components and  the best  CoP, to get the maximum output for every Kw of energy spent in heating your pool. Look for certification from an independant laboratory , which is always the best.  By the way, if you find  a CoP over 6 , you must start asking questions because this type of performance isn't frequent. Only a few machines had a CoP over 6 in all the history of tested pool heat pumps.