What heat pump capacity do I need for my home

We are still preparing products.

But you can look at other categories.

What heat pump capacity do I need for my house?

This is one of the questions we deal with with our customers literally every day. Although at first glance it may seem that choosing a "medium-sized" heat pump will be fine, the reality is much more complex. An undersized heat pump will struggle in winter, you won't keep the house warm and your electricity costs will skyrocket. An oversized one, on the other hand, will run for short periods, turn off, turn on again – and will wear out much faster than it should. The correct capacity of the heat pump is the foundation of the entire project.

In this article, we will go through everything from the basics: what the capacity of a heat pump actually means, how to calculate the heat loss of a house, what are typical values for different types of buildings, where people most often make mistakes and how to make an informed decision based on real numbers – without having to be energy auditors.

Why capacity is not just "square meters multiplied by a number"

One of the most widespread misconceptions is that it is enough to take the number of square meters of the house and multiply it by some factor – for example, 50 W/m² for a new building or 100 W/m² for an older house. The result is quick, but not reliable. Why?

Because two buildings with the same floor area can have up to three times different heat loss. It depends on where the house is located (elevation, region), how many windows it has and what kind, how it is oriented to the cardinal directions, the thickness and type of thermal insulation of the walls, ceiling, floor, whether it has a basement or a garage, how many people live in the house and what their ventilation habits are. The correct calculation of the heat pump capacity must be based on the building's heat loss, not on the area.

The heat loss of a building is the amount of heat (in kilowatts) that the house will lose in one hour at so-called design outdoor temperature – which is in most parts of Slovakia -15 °C, in more mountainous areas even -18 °C or -20 °C. It is precisely at this temperature that the heat pump must cover the maximum load.

How to calculate the heat loss of a building – the basis for dimensioning

Formally, the heat loss is calculated according to the standard STN EN 12831. The calculation includes losses by heat transfer through the structures (walls, ceiling, floor, windows, doors) and ventilation losses. The result is a number in kW, which tells you how much thermal power needs to be supplied to keep the house at the desired indoor temperature (usually +20 °C) even in the harshest frost.

An energy auditor or designer will make an accurate calculation. If you want, however, to have a rough idea, you can use a simplified approach:

New construction in standard A0/A1 (passive house): 20–35 W/m² of floor area
New construction in low-energy standard (A2–A3): 35–50 W/m²
House built after 2000, insulated: 50–70 W/m²
House from the 80s–90s after partial insulation: 70–90 W/m²
Older house before 1990, not insulated: 90–130 W/m² and more

For a concrete example: a family house with a floor area of 160 m² built in 2005, walls insulated with 10 cm polystyrene, plastic windows, located in central Slovakia. A typical specific heat loss will be around 60–70 W/m², which gives a total heat loss of 9.6–11.2 kW. For such a house, we would consider a heat pump with a capacity of 10–12 kW.

Calculation temperature and bivalent point – key terms

Air-to-water heat pumps have one unpleasant characteristic: the colder it is outside, the less power they can deliver and the more electricity they consume. This relationship is important for sizing.

Bivalent point is the outside temperature at which the heat pump alone is not sufficient to cover the building's heat loss and must be assisted by an additional source – most often an electric heating element (integrated into the unit) or an external boiler. With a ground-to-water heat pump the situation is different – the ground collector or borehole has a relatively stable temperature throughout the entire winter, so the performance drops much less dramatically.

In practice this means: if we have a house with a heat loss of 10 kW at -15 °C and we choose an air-to-water heat pump with a nominal power of 10 kW (measured typically at an outside temperature of +7 °C and water temperature of 35 °C), then at the actual frost of -15 °C this machine may deliver only 6–7 kW. The rest must be covered by the electric heating element.

This is normal, acceptable and economically justifiable solution. The electric heating element runs only on the coldest days (in Slovakia this is typically 200–400 hours per year), so its contribution to the overall consumption is not dramatic. But you need to take it into account during installation and dimensioning of the electricity supply.

Monovalent vs. bivalent operation – what to choose?

If you want the heat pump to cover 100 % of the thermal load even at -15 °C without any electric heating element, we are talking about monovalent operation. This requires a significantly larger and more expensive heat pump that runs most of the year at only a fraction of its capacity – which is energetically inefficient (frequent cycling, reduced COP).

Much more common and economically better approach is bivalent operation – the heat pump is sized to cover 70–85 % of the annual heat demand, and in the coldest days it is assisted by a heating element or an additional boiler. Such a solution is more cost-effective both at purchase and in operation.

Type of operation	Advantages	Disadvantages	Suitable for
Monovalent	No additional source, clean solution	Larger and more expensive HP, worse efficiency in summer	Passive houses, low-energy buildings
Bivalent alternating	Optimal HP size, lower price	Need for an additional source	Most family houses
Bivalent parallel	HP + boiler run simultaneously, maximum comfort	More complex control, higher investment costs	Renovations with an existing boiler

Practical examples of power calculation for different types of houses

Example 1: New construction 120 m², low-energy standard, Trenčín

The house has a floor area of 120 m², two-story, walls with a heat transfer coefficient U = 0.18 W/m²K, triple-glazed windows U = 0.7 W/m²K, air recovery with 80 % efficiency. The calculation outside temperature for Trenčín: -15 °C. Specific heat loss: approx. 30 W/m². Total heat loss: 120 × 30 = 3 600 W = 3.6 kW.

For such a house a heat pump with a power of 4–6 kW is sufficient. A smaller unit will be more efficient, cheaper to operate and will cycle less. We often see that customers for such houses unnecessarily buy 10–12 kW units because "just to be sure" – and the result is counterproductive.

Example 2: House from 1985, 180 m², partially insulated, Banská Bystrica

The house has gas-silicate masonry, 5 cm polystyrene on the facade (old insulation), wooden windows replaced by plastic ones. Ceilings are well insulated, but the floor is not. Specific heat loss: approx. 80–85 W/m². Total heat loss: 180 × 82 = 14 760 W ≈ 15 kW. The calculation outside temperature for B. Bystrica: -15 °C.

Here we would recommend a heat pump with a power of 10–12 kW in a bivalent configuration, where an electric heating element (usually 6–9 kW) helps on the coldest days. Alternatively, if the customer wants complete coverage without a heating element, then a 14–16 kW unit – but this is less economically advantageous.

Example 3: Passive house 200 m², Bratislava

Modern new construction with excellent insulation, air recovery and triple glazing. Specific heat loss under 20 W/m². Total loss: 200 × 18 = 3 600 W = 3.6 kW. Bratislava has a calculation temperature of -11 °C, so the actual numbers will be even more favorable.

Surprise: such a large passive house needs the same small heat pump as a small new construction. It is important to also consider the need for hot water and possibly cooling in summer – this can influence the choice of equipment more than heating alone.

Impact of the heating system on sizing

The performance of a heat pump is closely related to the temperature gradient required by your heating system. This is another point where many customers run into unpleasant surprises after installation.

Heat pumps are optimized for low heating water temperatures – ideally 35–45 °C. This corresponds to floor heating. The higher the required water temperature, the lower the COP (coefficient of performance), and the less efficient the device operates.

Floor heating (35–45 °C): Ideal for a heat pump, COP 3.5–5 and more
Low-temperature radiators (45–55 °C): Still acceptable, COP 2.8–3.5
Standard radiators (55–70 °C): Possible, but COP drops to 2–2.5; economics worsen
High-temperature radiators (70–80 °C): Unsuitable for standard heat pumps; special high-temperature models are required

If you have an older house with classic cast iron radiators sized for 70/55 °C and want to install a heat pump, you have two options: replace the radiators with larger ones (to suffice with lower temperatures), or opt for a high-temperature heat pump. Both options cost money – and this must be considered in the performance and overall project budget. More about this decision can be read in the article How to choose a heat pump – what to focus on before purchase.

Hot water (DHW) – another factor that changes performance

Most heat pumps for single-family homes serve not only for heating but also for heating domestic hot water. And this is not a negligible load. An average four-person family consumes 150–200 liters of hot water per day (at 45–55 °C), which represents a thermal power requirement in the range of 2–4 kW equivalent during the day.

To make hot water circulation efficient and comfortable, it is worth considering accessories. For example, SET of hot water circulation for TČ series ITEC-T, ATHENA-T, LEGEND, CALIBRA and ATLAS solves circulation directly as an integrated set – without the need for an external circulation pump and complex wiring. Such a solution saves time and costs during installation and ensures that hot water is immediately available.

From the perspective of sizing performance: if the heat pump also heats DHW, you must account for the fact that during the tank heating period (usually in the morning or during night tariff), the heat pump operates at full power for DHW and less capacity remains for heating. Modern devices manage this intelligently, but when designing the system, it is good to know this.

Inverter vs. on/off – impact on sizing

Modern heat pumps are almost exclusively inverter-based – that is, the compressor adjusts its speed according to current demand. Older and cheaper models had a compressor that was either on or off (on/off). This difference has a significant impact on how precisely you must size the performance.

With an inverter heat pump with a power range of, for example, 3–12 kW, you can cover a house with a heat loss of 6 kW and 10 kW – the device adapts itself. With an on/off device, you must hit the power much more precisely, otherwise it will run short and cycle, which shortens the compressor's lifespan.

An inverter compressor also means that the device can run at minimal power (e.g., 2–3 kW) even when it is 5 °C outside and the house only needs a little heating – which is energy efficient and prolongs the lifespan. This is why almost every manufacturer today offers only inverter solutions.

What happens if the heat pump is oversized?

Many customers choose the logic of "bigger = better" or "better bigger for safety". Practice, however, shows the opposite. An oversized heat pump has these problems:

Short cycles (short cycling): The device quickly reaches the desired temperature, turns off, cools down, turns on again – and so on. Each start is a load on the compressor.
Worsened COP: The heat pump has the best efficiency during long, smooth operation – not during cycling.
Increased noise: Frequent starts are noisier than continuous operation.
Shortened lifespan: The compressor is stressed the most during start-up; more starts = faster wear.
Higher device cost: You pay for power that you do not use.

Optimal sizing is therefore not "as big as possible", but "exactly right" – with a slight inverter range upwards for certainty on the coldest days.

What happens if the heat pump is undersized?

The opposite extreme is equally problematic. An undersized device:

Cannot maintain the desired temperature in the coldest weather – the house will be cold
Runs continuously at maximum power even in milder frost, which increases wear
The electric heater must run longer and at higher outside temperatures, which dramatically increases electricity consumption
The customer is dissatisfied, service centers handle complaints, sometimes the device is replaced with a larger one – with additional costs

From practice: a very typical scenario is a house from the 90s, 200 m², not insulated – the owner bought a cheap 8 kW heat pump. The first winter was survived with the heater running almost continuously, electricity bills were similar to those with an electric boiler. In the end, we replaced the device with a 14 kW model and added a stratified tank. A preventive correct calculation would have been much cheaper.

Regional differences – where you live in Slovakia

Slovakia is quite climatically diverse. The calculated outdoor temperature (so-called design temperature) varies according to the region and elevation:

Bratislava, Dunajská streda, Komárno: -11 °C to -13 °C
Trnava, Nitra, Trenčín, Žilina, B. Bystrica, Košice: -13 °C to -15 °C
Hilly areas (Orava, Liptov, Gemer, Spiš above 600 m a.s.l.): -15 °C to -18 °C
High mountain areas above 800 m a.s.l.: -18 °C to -22 °C

A house in Bratislava with a heat loss of 10 kW at -13 °C will have a heat pump working more efficiently and for more hours per year without the help of an electric boiler, compared to the same house with the same heat loss in Liptovský Mikuláš at -18 °C. This must also be considered when selecting the size of the storage tank and setting the bivalent point.

Accessories and expansion modules – impact on the overall system

Proper sizing is not only about the heat pump itself. The entire system must be designed so that individual components work together. During installation and system expansion, correct accessories and modules are also important.

For example, when installing dual units (DUO configurations), proper hydraulic connection is essential. Connection piping for the CALIBRA DUO and ATLAS DUO heat pumps ensures tight and properly dimensioned hydraulic connection – a seemingly small detail that can cause problems with flow and system performance if the wrong diameter or unsuitable type of piping is chosen.

If you plan to expand the system or need to integrate a mixed circuit with another controller, Relay kit for pump signal with mixed circuit can be helpful – this module converts the 0–10 V control signal from the heat pump to on/off for an external pump, which is practical when integrating into older systems. More on this and similar topics can be found in the article Expansion module for heat pumps – when and why you need it.

In cold environments, protection against freezing of the supply piping is also important. For outdoor installations of the ITEC range, Self-regulating heating cable – 2 m is available, which automatically keeps the pipe temperature above freezing without unnecessary electricity consumption – self-regulating technology ensures that the cable produces heat only where and when it is needed.

Domestic hot water tank – another dimension of sizing

The volume of the DHW tank and possibly the buffer tank for heating is directly related to the performance of the heat pump. A larger tank allows the heat pump to run longer and more efficiently in batches – the fewer starts, the better for the compressor.

Recommended buffer tank volumes are at least 20–30 liters per each kW of installed capacity. For a 10 kW heat pump, this is 200–300 liters. A DHW tank for a four-person family should have a volume of 200–300 liters, for a larger family or if you have solar collectors, even 400 liters.

If the heat pump heats DHW directly (without a tank), heating is interrupted and in the transitional period there may be temperature fluctuations. With a properly sized tank, the system is comfortable, efficient, and quieter – fewer compressor starts, smoother operation.

Cooling in summer – another reason for proper sizing

More and more customers are asking for a cooling function from the heat pump in summer (so-called passive or active cooling). This can affect the sizing of the unit, because:

Passive cooling (free cooling) via a ground borehole/collector works without an active compressor and does not load it
Active cooling in reverse operation (air-to-water) means that you are cooling a floor heating system in summer – and for that you need a different hydraulic mode
For cooling via fan coils, you may need more capacity than for heating alone

If you plan to use cooling, tell the designer at the beginning, not as an afterthought after installation. The system must be designed differently – a stratified tank, different regulation settings, or possibly a different unit model.

Most frequently asked questions (FAQ)

Can I do the sizing myself without an energy audit?

For a rough estimate, yes – using the measured heat loss and the house area, you can get a reasonable performance range. For binding sizing, especially if you are applying for subsidies (e.g. the Zelená domácnostiam program), an energy audit or heat loss calculation according to the EN 12831 standard is necessary. We recommend it for everyone who plans an investment over 10,000 €. The cost of the audit (usually 150–400 €) is many times returned by the correct choice of equipment.

What if I plan to insulate the house only after installing the heat pump?

This is a common situation and must be handled carefully. If the house is significantly insulated (e.g. from 80 W/m² to 40 W/m²), the heat loss will drop by half – and a previously properly sized heat pump will be oversized after insulation. In such a case, we recommend either insulating first and then installing the heat pump, or choosing an inverter unit with a sufficiently wide performance range that can adapt to the reduced load.

Is it better to have one larger heat pump or two smaller units in cascade?

For larger buildings (over 20 kW), a cascade of two smaller units is a very good strategy: in mild weather, only one unit runs at optimal performance, in freezing weather the second one is turned on. A cascade also increases reliability – if one unit fails, the other still heats. For standard family homes (up to 12–15 kW), one unit is the standard and more cost-effective solution.

Does the DHW tank affect the performance of the heat pump?

Yes, indirectly. A larger tank allows the heat pump to heat water in long, efficient cycles and then turn off. A smaller tank causes the unit to heat water more frequently and in shorter cycles. For a typical four-person household, an optimal DHW tank is 200–250 liters, and if the system has a buffer tank for heating, another 150–300 liters depending on the unit performance.

What impact does elevation have on the selection of heat pump capacity?

Elevation increases the calculated outdoor temperature (it is colder), which means higher heat loss and thus a need for higher capacity. At the same time, in higher elevations, the heating season is longer. A house in Orava at an elevation of 700 m may need 30–40 % more heat pump capacity than the same house in Bratislava. This is one of the reasons why table-based calculators from the internet may not give accurate results – always consider the specific location.

Can I later replace the heat pump with a more powerful one without changing the entire installation?

It depends on the system design. If the hydraulics (pipes, tanks, circulation pumps) are designed with sufficient reserve – typically for a capacity 20–30 % higher than the original unit – the replacement is relatively simple. Therefore, we recommend considering this possibility in advance when designing the system: a larger tank, pipes with a larger internal diameter, sufficient electrical circuit breaker. More about this process can be found in the article Heat pump installation – procedure, requirements and common mistakes.

Conclusion: The correct capacity is the foundation of the entire investment

Selecting the correct capacity of the heat pump is a decision that will affect the comfort of living, the amount of electricity bills, and the lifespan of the equipment for the next 15–20 years. It is not a decision that should be made based on price or on what the neighbor had. It is a technical question that requires specific input data: heat loss of the building, type of heating system, region, planned functions (DHW, cooling), type of operation (mono/bivalent).

If you're unsure, invest 200–400 € in an energy audit or consult an HVAC designer. The result will not only be the correct sizing of the heat pump, but also an overall system design that will operate efficiently for decades. And if you want to understand the individual parts of the system in more detail – from accessories through circulation integration to regular maintenance – further practical guides can be found in other articles from our Knowledge Center: Connecting and installing heat pump accessories step by step, Integrating hot water circulation with a heat pump, or Maintenance and servicing of a heat pump – what you can do yourself and what you cannot.

Do you have a question about this topic?

Having trouble deciding or dealing with a specific situation in your household? Write to us – we'll be happy to help.