What power of electric boiler do I need for my house?
What power electric boiler do I need for my house?
This is probably the first question that every person considering an electric boiler asks. And at the same time, it is a question for which there is no single universal answer – it depends on a whole range of factors that must be evaluated together. If you have ever ordered an electric boiler based only on the number of square meters your house has, and then found out that the boiler was not enough to heat the house or, on the contrary, was cycling unnecessarily every five minutes, you are not alone. Such an experience is quite common in practice.
In this article, we will go through the calculation of the required thermal power thoroughly and in sufficient depth – from the basic formula through the heat losses of the building envelope to specific practical scenarios. By the end, you should be able to determine whether an 8 kW boiler is sufficient for you or whether you need to look for 15 kW or even 24 kW.
Why choosing the right power is so important
A boiler with too low power simply cannot maintain the desired indoor temperature in the cold winter. That is obvious to everyone. Fewer people, however, realize that an oversized boiler is also a problem – it creates short operating cycles (so-called short cycling), in which the boiler turns on, quickly reaches the desired temperature, and turns off. This phenomenon accelerates component wear, increases operating costs, and reduces heating comfort.
A properly dimensioned boiler should run in long, even cycles – especially on the coldest days of the year, it should operate almost continuously at full or near-full power. On mild cold days, it is sufficient for it to operate at partial power or alternate with breaks.
Basic calculation: heat loss of the building
The required boiler power is equal to the heat loss of the building under design outdoor conditions. In other words – the boiler must supply exactly as much heat as the house loses on the coldest day of the year at the outdoor temperature for which it is designed.
The heat loss of the building (in kW) is calculated in a simplified form as follows:
Q = V × ΔT × k / 1000
- Q – heat loss [kW]
- V – volume of the heated space [m³]
- ΔT – difference between indoor and outdoor design temperatures [K]
- k – heat loss coefficient of the building [W/m³·K], depends on the construction and insulation
The value of k is a key parameter that reflects how well the house is insulated. In practice, it ranges as follows:
| Type of building | k [W/m³·K] |
|---|---|
| Old, non-insulated building (before 1980) | 1.0 – 1.4 |
| Partially insulated building (e.g., windows, façade) | 0.6 – 0.9 |
| New building according to standards 2000–2015 | 0.4 – 0.6 |
| Low-energy building | 0.2 – 0.4 |
| Passive house | 0.1 – 0.2 |
Let’s apply this to practice. Take a family house with a living area of 120 m² and an average ceiling height of 2.6 m – the volume is therefore 120 × 2.6 = 312 m³. The house was built in 1985 and has since undergone window replacement and partial façade insulation. We estimate the coefficient k at 0.7 W/m³·K. It is located in an area with a winter design temperature of -12 °C, and the desired indoor temperature is 21 °C, so ΔT = 33 K.
Q = 312 × 33 × 0.7 / 1000 = 7.2 kW
This means that an electric boiler with a power of around 8 kW would be sufficient for this house. For example, the Attack Electric Easy 8 would be a logical choice for such a case.
Approximate calculation based on area: a quick reference, not the final answer
In practice, I often encounter customers who come with just one piece of information: “I have a 150-square-meter house.” That is understandable – not everyone has access to project documentation or an energy certificate. Therefore, I provide approximate values of specific heat demand (W/m²) that serve as a rough estimate:
- Old, non-insulated house (before 1980): 80–120 W/m²
- Partially renovated house: 60–80 W/m²
- New building (standard 2000+): 40–60 W/m²
- Low-energy house: 20–40 W/m²
- Passive house: 10–20 W/m²
For the same house with an area of 120 m² and a value of 60 W/m² (partially renovated), we get: 120 × 60 = 7,200 W = 7.2 kW – which matches the previous calculation. Sometimes such a match confirms that we are on the right track.
Factors that skew the calculation – what not to forget
Even the best approximate calculation cannot account for all real conditions. In practice, I have often encountered cases where the heat loss was calculated correctly, but the boiler ended up being undersized or oversized by 2–3 kW because someone forgot to consider one of the following factors.
Climatic zone and design outdoor temperature
Slovakia has several climatic zones. In the southwest of the country (Záhorská nížina, the area around Bratislava), the design outdoor temperature is around -12 °C. In the more mountainous areas of central and northern Slovakia, it can drop to as low as -17 °C or even lower in extreme locations. A 5 °C difference in design temperature can mean an increase in the required boiler capacity by 15–20%. If you live, for example, in Orava or Liptov and have a house with a larger floor area, the difference compared to southern Slovakia is very real.
House orientation and shading
A house located on the north side of a hill with a north-facing façade will objectively have higher heat losses than an identical house on a sunny southern slope. Shading from other buildings, trees, or terrain reduces solar gains and increases the time during which the boiler must actively heat the space.
Ceiling height and layout complexity
Attic apartments, open stairwells, or loft layouts with ceiling heights of 3–4 m have significantly more air volume to heat. A standard formula assuming a height of 2.5–2.6 m will be underestimated in such cases.
Floor heating vs. radiators
This is not a factor that directly changes the building's heat loss, but it affects how quickly and at what temperature the boiler must supply heat. Floor heating operates with low water temperatures (about 30–45 °C), which is advantageous for electric boilers in terms of regulation. Radiators usually require higher temperatures (55–75 °C). If you have a combined system, the boiler must be able to cover both circuits.
Preparation of hot water (HW)
This is a factor that customers often forget most frequently. If you want to use an electric boiler to prepare hot water in an external tank, you must account for the fact that the boiler may consume the entire available power during the heating of the HW. For a family of 3–4 people with a 150–200-liter tank, this is a real load even for a 15 kW boiler. For this purpose, there is a special TÚV module set (preparation of TÚV in an external tank), which expands the capabilities of the electric boiler by controlling the tank. You can read more about this topic in the article TÚV module for electric boiler: how does the preparation of hot water in an external tank work?
Heat loss from adjacent rooms and internal gains
In the case of apartments in apartment buildings, adjacent heated apartments can significantly reduce heat losses – an apartment building has different thermal properties than a detached house. Internal gains (people, appliances, lighting) can reduce the actual heat requirement by 10–15% in a larger household.
Practical scenarios from practice: how many kW do I need?
Let's go through a few typical situations that I encounter in practice. Each case has different results and highlights why the generic formula "1 kW per 10 m²" is so unreliable.
Scenario 1: Apartment in a panel building, 65 m², after renovation
A panel apartment after a comprehensive renovation (new windows, insulation of the apartment building) in southern Slovakia. Ceiling height 2.55 m, volume approx. 166 m³. Adjacent apartments are heated. Heat loss in a panel building after renovation is typically 40–50 W/m². Result: 65 × 45 = 2 925 W ≈ 3 kW. For such an apartment, a smaller boiler or direct heating electric convectors may be sufficient. If a boiler is still to be installed here for TÚV and flexibility, Attack Electric Easy 8 would be significantly oversized in this case, but still functional and reliable.
Scenario 2: New construction of a family house, 150 m², low-energy standard
A house built in 2019, thick insulation of walls (EPS 200 mm), triple-glazed windows, air recovery, heat loss approx. 30 W/m². Result: 150 × 30 = 4 500 W = 4.5 kW. Surprisingly low number, isn't it? Many customers want a 15 kW boiler for such a house. In this case, with air recovery and good insulation, the boiler would operate at 20–30% load all year and would be brutally oversized. Attack Electric Excellent 8, 7.5 kW would be at the upper limit here and would operate at significantly reduced power on most days.
Scenario 3: Older family house, 180 m², no insulation, mountainous area
A house built in 1975, original double-glazed windows, no insulation of the façade, non-heated attic. We are in an area with a design temperature of -15 °C. Estimated heat loss is 100–110 W/m². Result: 180 × 105 = 18 900 W ≈ 19 kW. In this case, a boiler with a power of 20–24 kW would be appropriate. Attack Electric Excellent 24, 22 kW would provide the necessary reserve in this case even in the event of sudden cooling.
Scenario 4: Renovated family house, 130 m², central Slovakia
A house built in 1990, replaced windows, insulated façade with 100 mm EPS. Heat loss approx. 65 W/m², design temperature -13 °C. Result: 130 × 65 = 8 450 W ≈ 8.5–9 kW. If the family plans to connect an HW tank as well, I would recommend going for Attack Electric Excellent 15 kW, which provides significantly better comfort during simultaneous HW heating and heating, and also has more room for power regulation.
Why "1 kW per 10 m²" is a dangerous shortcut
The rule of "1 kW per 10 m²" – that is, 10 W/m² – comes from a time when buildings were constructed completely differently and their energy demands were much higher. Today, this rule would only apply to very old and poorly insulated buildings with high ceilings. For a low-energy house, it would mean three times the required power.
I also encounter the opposite extreme: customers who have read about passive houses and want a 4 kW boiler for a 160 m² house, which in reality has 60 mm facade insulation from 2004 and double-glazed windows. In this case, the real need is closer to 10–12 kW.
Therefore, I always recommend at least roughly recalculating the heat losses and considering at least three parameters: the age and condition of the house's insulation, the climatic region, and whether the boiler will also be used for DHW.
Boiler power and available electrical connection
One aspect that is not only considered when calculating heat losses but also in technical reality is the capacity of the electrical connection. An electric boiler with a power of 15 kW at three-phase 400 V draws a current of approximately 21.7 A per phase. A 24 kW boiler draws approximately 34.6 A. This is not an insignificant load.
If your house has a 3×25 A fuse (a common value for single-family homes), a 24 kW boiler will be at the limit, and when you add a stove, a dryer, and other appliances – you are realistically asking for problems. Before choosing the boiler power, consult with an electrician and possibly request the distribution company to increase the fuse or the main circuit breaker. For more technical details on the connection requirements, see the article Installation of Attack Electric boiler: procedure, requirements and connection.
Modulating power – a big advantage of modern electric boilers
Modern Attack Electric Excellent electric boilers have stepped or continuous power regulation. This means that a boiler with a nominal power of 15 kW does not have to operate at full 15 kW all the time – for example, in transitional periods or on mild frosty days, it may only switch on 5 kW. This stepped power significantly reduces the risk of negative effects of overdimensioning and at the same time saves electricity.
That is why it sometimes pays off to choose a boiler one power step higher than the pure calculation suggests – for two reasons: first, the boiler will have a reserve for DHW preparation without limiting heating, and second, a boiler with a higher nominal power can operate at a lower power under normal conditions, thus extending its lifespan and reducing thermal shocks to components.
More about how to correctly set the power steps and control parameters of the boiler is covered in the article Control and setting of Attack Electric boiler: how to save on electricity?
How to proceed step by step when choosing the power
To conclude this technical part, let's summarize the recommended procedure that will save you from disappointment after installation:
- Determine or estimate the heat loss of the building – if you have an energy certificate, you will find the heat loss value there directly. If not, use the formula with k values according to the table above.
- Consider the climatic region – check the design outdoor temperature for your location (available in STN EN 12831 or on SHMÚ and architectural design websites).
- Decide whether the boiler will also heat DHW – if yes, add the power required for the storage tank (usually +3–5 kW reserve).
- Check the capacity of the electrical connection – consult with an electrician and the distributor will confirm whether sufficient power is available.
- Choose a boiler with a power 10–20 % higher than the calculation suggests – this reserve covers unexpected losses, chimney loss (if present), insulation aging, and possible future expansion of the heated area.
- Prefer modulating power – boilers with stepped regulation (such as the Excellent series) offer greater flexibility and more efficient operation.
If you are also interested in comparing specific models and their technical differences, I recommend the article Attack Electric Excellent vs. Easy: what they differ in and which one to choose? or the summary article How to choose an Attack Electric boiler: power, model and accessories.
Frequently asked questions (FAQ)
Is an 8 kW boiler sufficient for a house with an area of 100 m²?
It depends on the condition of the house. For a new low-energy standard house in southwestern Slovakia, 8 kW is sufficient and even on the upper limit. For an older house without insulation in a colder region with the same area, 8 kW may not be enough on the coldest days – in that case, I would recommend at least 10–12 kW. Attack Electric Excellent 8, 7.5 kW is an excellent choice precisely for well-insulated houses in this size category.
Can I round up the boiler capacity without worries?
Yes, but not blindly. Rounding up by one performance step (e.g., from the calculated 12 kW to the available 15 kW model) is fine and even recommended. However, if you installed a 24 kW boiler in a house that needs only 8 kW, problems with short cycling and increased operating costs arise. A reasonable reserve is 15–25 % above the calculated value.
How does the required capacity change if I insulate my house?
Insulating the façade and replacing windows can reduce heat loss by 30–60 %, depending on the original condition and the thickness of the insulation layers. This means that a boiler that was originally well dimensioned will be oversized after the renovation. Nothing will be damaged – it will operate at a lower capacity and cycle less. Just don't expect electricity consumption to drop proportionally to the capacity – savings depend on the actual heat loss, not on the boiler's rated capacity.
Is one powerful boiler better or two smaller ones?
For single-family homes in the 8–24 kW range, a single boiler solution is almost always more economical – lower purchase price, simpler installation, one service location. Two boilers make sense only for larger buildings with two separate circuits or in cases where redundancy is required – for example, recreational properties, where a heating failure in winter could cause the installation to freeze.
How can I determine the exact heat loss of my house without an architect?
The most reliable method is an energy audit or at least a consultation with a construction expert who can account for thermal bridges, infiltration, and all structural details. If that is not possible, use the values from the table in this article (factor k) and calculate based on the building volume – you will get a reasonable estimate with a deviation of ±20 %, which is usually sufficient for selecting the correct boiler capacity. For a rough estimate, you can also use specific values in W/m² according to the condition of your house.
Does the water temperature in the system affect the required boiler capacity?
Not directly – water temperature does not affect the heat loss of the house. However, it does affect how quickly the boiler can compensate for that loss and how efficiently heat is delivered to the room. Systems with underfloor heating have a larger water volume and longer thermal inertia, which can slow down the response to temperature fluctuations outside. For large-volume circuits (e.g., a combined system with underfloor heating and radiators), it is advisable to choose a boiler with sufficient capacity to quickly heat up even from a cold start.
Conclusion: boiler capacity is not just a number
Selecting the right capacity for an electric boiler is about understanding the entire heating system – not just one parameter. The house, its construction, your location, the method of use (heating only or also domestic hot water), the capacity of the electrical connection, and the type of distribution system – all of these together determine which boiler is right for you.
Practical tip: if you are hesitating between two capacity levels, choose the higher one – and ensure a high-quality controller with a programmable thermostat so the boiler does not unnecessarily operate at full capacity in milder weather. A well-dimensioned boiler combined with intelligent control ensures comfort and economical operation for many years.
Do you have a question about this topic?
Not sure what to choose or dealing with a specific situation in your household? Write to us – we are happy to help.