Under or oversized heat pumps constantly run and counteract any savings by cycling on and off repeatedly. Determining the size of a heat pump for your home requires multiple measurements. Its size directly relates to how much energy or heat the heat pump can produce. If you do not install a large enough heat pump, it will be ineffective in bringing the room to the desired temperature. If the heat pump is too large, it will turn off and on frequently, which causes the machine to wear out faster. Several online calculators can help you determine the right size heat pump for your home.

Measure the room size and the ceiling height in meters, to calculate the room volume. The room size is the total floor area of the room with a final measurement in meters squared, or m^2. The room volume is calculated as follows, with a final measurement in meters cubed, or m^3. Room size times ceiling height equals room volume.

Determine the room's heating factor based on the type of insulation in the ceiling and walls. If the room has one external wall with no insulation, use a heating factor of 15. If the ceiling has insulation, use a factor or 20. if the ceiling and walls have insulation, use a factor of 22. For a room with two external walls and no insulation, use a heating factor if 13, for just ceiling insulation use 17 and for both ceiling and wall insulation use a factor of 22.

Divide the room volume by the chosen heating factor. This provides the kilowatt size of the necessary heat pump to heat the room. Compare the kilowatt output of each size heat pump and ensure heating capacity of the unit at 2-degrees Celsius is at minimum, the number calculated.