One method for comparing the heating efficiency of MSHPs is the “heating seasonal performance factor” (HSPF). This value is the ratio of the system’s heat output (in Btu) to the amount of electricity consumed (in Wh) to move this capacity of heat. The HSPF is essentially a seasonal coefficient of performance (COP) for a unit. To convert to COP, simply divide the HSPF by 3.412, which is the number of Btu/hour in a watt.

While HSPF can be an effective measure for a particular MSHP model’s efficiency, it is also important to evaluate the unit’s heat capacity tables to best understand how the unit will perform for your site’s conditions. Some units may have a higher HSPF than a competing model, but may have a far different capacity at lower temperatures. If the MSHP is being used as the sole source of heat in a particular portion of the house, the total heating capacity of the unit is as important as how efficiently it can provide this heat.

For example, the heat capacity table illustrates the performance of a particular single-zone MSHP model. As the outdoor temperature decreases, so does the heat capacity of the unit. If the design temperature is 5°F and the anticipated room temperature is 70°F, the unit can provide 15,400 Btu/hour of heat. Comparing this value to the heating demand for the room aids in the selection process.