Heat pumps transfer energy by forcing a refrigerant through differing pressures. The refrigerant absorbs latent heat of vaporization when it evaporates and releases it elsewhere when it liquefies. Each refrigerant has its own heat transfer rate, a value that describes how much heat it absorbs per unit weight. Specifications typically state this value using the standard scientific unit of kilojoules per kilogram (kj/kg). Simple conversions apply this transfer rate to construction and manufacturing measurements.

Multiply your heat transfer requirement, measured in British Thermal Units, by 1.055 to convert it to kilojoules. If you must move, for instance, 250,000 BTUs in a given amount of time: 250,000 x 1.055 = 263,750 kj.

Divide this amount of heat by the refrigerant's heat transfer rate. If the refrigerant moves, for instance, 170 kj/kg, then: 263,750 / 170 = 1,551 kg.

Multiply this weight by 2.2 to convert it to pounds: 1,551 x 2.2 = 3,412 lb.

Divide this weight by the number of cycles the system will go through during the time period. If if will cycle the refrigerant, for instance, 20 times: 3,412 / 20 = approximately 170 pounds. The system therefore needs 170 pounds of refrigerant.
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References
 Free Study: Heat Pumps and Refrigeration
 "Engineering Thermodynamics;" P.K. Nag; 2008
 University of Wisconsin: Measuring and Quantifying Energy
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