Heat pumps - which power to choose?

Proper selection of the heat pump power allows not only to meet the heat demand of our home, but also to use this heat source economically in the interior. A heat pump is an ecological and economical solution when it comes to heating a house or building. Nevertheless, poorly matched in terms of power, it may expose us to higher electricity bills than we expected.

That is why it is so important to choose a device with performance parameters suited to our needs. The following factors determine the size of our heat pump:

– heat demand of the building

– house area

– number of household members

– temperature characteristics of the area

– heat demand of the building


The area of a single-family house will help us calculate the total heat demand of our house. Currently built semi-detached houses, terraced houses or detached houses have an area of 80 m2-160 m2. Therefore, for our calculation scheme, we can take the area of the house, e.g. 120 m2.


Heat pumps generally have two functions. They are used as a heat source for the building itself and for heating domestic hot water (DHW). In the latter case, it is important how many people live in our house, because it affects the water consumption.

Based on the number of household members, we determine the power needed to heat the water in our house. In general, the value is 250W per person. Thus, a household of 4 people needs approx. 1 KW to heat DHW.


It mainly concerns air heat pumps and their efficiency in given weather conditions. The efficiency of these devices varies with the temperature outside and the temperature of the internal heat source.

Manufacturers provide the performance parameters of their devices for the climatic conditions of a given area. The energy label shows indicative power parameters of the device, carried out on the basis of temperature analyzes of a specific region. Thus, the estimated power of the device for our climate region can be read from the energy label.


We select the heat pump power for the building with the following parameters:

– house area – in this case 120 m2

– heat demand of the building – 40 W / m2

– 15 cm thick mineral wool insulation

– heat source – water underfloor heating with a nominal temperature of 35OC

– number of household members – 4 people

We can initially calculate the heating power (Pg) of our heat pump. The house area (120 m2) is multiplied by the power demand indicator (40 W/m2).

120 x 40= 4800 W= 4,8 KW.

To this should be added the need for heating power needed to heat the domestic hot water. 4 people live in the house. So 250 W x 4 = 1 KW of thermal energy needed to supply the inhabitants with domestic hot water.

By adding the heating power of 4.8 KW + 1 KW of DHW heating power, we get 5.8 KW, which is the total power needed to supply our house with thermal energy. In this case, the ideal solution will be a heat pump with a capacity of at least 6 KW.


When calculating the heat pump’s heating power, we also take into account the so-called bialence coefficient [Kb]. This parameter determines to what extent our heat pump is to cover the heat demand in our home. If it is to be one source of heat in our house, then kb = 1, as in the above case. When our heat pump is to work with other heat sources, this coefficient will change accordingly. For example, if the device is to meet our thermal needs in 70% and there will be an auxiliary heat source connected to it (e.g. an electric heater), in this case kb = 0.7.

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