What power of SLOKOV electric boiler do I need for my house

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Why calculating the boiler's power is a key step before any purchase

One of the most common mistakes I see among customers buying an electric boiler is underestimating or overestimating the required power. Both extremes are problematic – a weak boiler will not be able to heat the house, and thus will run at full power continuously, shortening its lifespan and still not being sufficient. On the other hand, an overdimensioned boiler unnecessarily increases installation costs (stronger circuit breaker, larger cable, higher power draw from the grid) and will operate in short cycles, which is also detrimental to its performance.

Electric boilers SLOKOV are available in several power levels – from 10 kW up to 26 kW – and each of them has its place. This article will help you understand how to correctly estimate which power is right for your specific house, without the need to immediately call a heating system designer.

Basic principle: what the boiler's power actually means

The boiler's power is given in kilowatts (kW) and indicates how much thermal energy the boiler can supply to the heating system in one hour. The higher the power, the more heat – but also the higher the electricity consumption. Unlike gas or heat pump boilers, electric boilers convert electrical energy into heat with an efficiency close to 100%, which simplifies the calculation: 10 kW electrical input = 10 kW thermal output.

In practice, this means that if a boiler runs at full power for one hour, it will consume 10 kWh of electrical energy (for a 10 kW boiler). Therefore, correct power dimensioning is directly linked not only to heating comfort, but also to monthly energy costs.

Main factors affecting the required boiler power

The power your house needs is not a single number valid for everyone. It depends on a combination of several variables that need to be considered together:

Heated (heated) volume or area of the house – a larger house requires more power, that is obvious.
Thermal insulation properties of the building envelope – a new low-energy building and an old, non-insulated house of the same size can differ in required power by up to three times.
Climatic region – the calculated outdoor temperature in Slovakia ranges from –11 °C (warm southern regions) to –18 °C (mountainous and northern areas). A 7 °C difference has a significant impact on heat losses.
Building orientation and glazing – large glazed areas on the north increase heat losses, while on the south they can help through solar gains.
Ceiling height – the higher the ceilings, the greater the volume of air that needs to be heated.
Type of heating system – floor heating, radiators, or fan coils have different water temperature requirements and also affect the boiler's operating point.
Preparation of hot domestic water (HDW) – if the boiler also heats water for the household, it needs additional power reserves.

Approximate method: W/m² – quick rough estimate

For a practical first estimate, the specific heat loss of the building is used, expressed in watts per square meter of heated area (W/m²). This value depends on how well the building is insulated:

Old, non-insulated house (before 1980, no insulation): 80–120 W/m²
House partially renovated (added polystyrene, new windows): 50–70 W/m²
House built according to standards from 2000–2010: 40–55 W/m²
Low-energy new build after 2015: 25–40 W/m²
Passive house: 10–20 W/m²

Calculation method: Required power (kW) = Heated area (m²) × Specific loss (W/m²) ÷ 1000

Example 1 – family house 150 m², built in 1985, no insulation:
150 × 100 = 15 000 W = 15 kW. A suitable boiler for such a house would be, for example, SLOKOV SL ELEKTRO 15, or with a slight reserve, SLOKOV SL ELEKTRO 18.

Example 2 – new build 180 m², standard thermal insulation (year 2018):
180 × 40 = 7 200 W = 7.2 kW. In this case, a 10 kW boiler – SLOKOV SL ELEKTRO 10 – would theoretically be sufficient, as a slight excess of power is actually beneficial, not a drawback, in the case of an electric boiler.

Example No. 3 – larger older house 280 m², partially renovated:
280 × 65 = 18 200 W ≈ 18 kW. For such a case, the SLOKOV SL ELEKTRO 18 is suitable. However, if the house also has a storage water heater for domestic hot water (DHW) connected via the boiler, you should consider the SLOKOV SL ELEKTRO 23.

How to account for hot water preparation

The calculation of the building's heat loss gives you the required power only for heating. If you want to use the electric boiler to heat domestic hot water (DHW) as well – which is the case in most households – it is necessary to add additional power for water heating to the heating power.

With a storage water heater (boiler 100–200 liters), in a typical family house (3–5 people), you need to calculate the power required for rapid heating of the tank. The speed of heating depends on the power:

150 L tank, heating from 10 °C to 55 °C: Required energy ≈ 7.9 kWh. At a boiler power of 10 kW (when not heating) = heating takes about 50 minutes.
If the boiler must heat both the house and the water at the same time, the total power in the peak is added together.

Therefore, for larger houses with DHW heating via the boiler, it is advisable to have a reserve of 3–5 kW above the calculated power for heating alone. This is one of the common reasons why a customer with a house theoretically requiring 15 kW ends up with the 18 or 23 kW variant.

Climatic zones of Slovakia and design temperature

Slovakia is not a climatically homogeneous country. In the southern Danubian Lowland, the design outdoor temperature (i.e., the temperature used in heating design as the worst possible) reaches –11 °C. In mountainous areas of northern and central Slovakia, this temperature can be as low as –18 °C. For typical cities such as Bratislava, Trnava, Nitra, it is –12 to –13 °C, for Žilina, Banská Bystrica –14 to –16 °C.

Why does this matter? Because the boiler is sized to cover the maximum heat loss under the worst possible weather conditions. The difference between the design indoor temperature (20 °C) and the outdoor temperature (e.g., –15 °C) creates a temperature gradient of 35 °C. If you were to calculate for –11 °C, the gradient is only 31 °C – meaning the heat loss is about 11 % lower.

Practical consequence: A house in Žilina requires about 10–15 % more powerful boiler for the same building than the same house in Bratislava. Therefore, location is not a detail, but a relevant factor in choosing the power.

Overview of SLOKOV SL ELEKTRO models and their typical use

The SLOKOV SL ELEKTRO range covers a power range from 10 to 26 kW. Let's look at which situations each model is typically suitable for:

SLOKOV SL ELEKTRO 10 – for smaller houses and apartments

SLOKOV SL ELEKTRO 10 with a power of 10 kW is suitable for new builds and low-energy houses up to 200 m², or for older smaller houses up to about 80–100 m² with average insulation. In practice, we use it for higher-standard apartments in apartment buildings (where spaces adjacent to other apartments do not transfer heat loss to the exterior), or as a secondary source in combination with another main heating system.

SLOKOV SL ELEKTRO 15 – the most common choice for a standard family house

SLOKOV SL ELEKTRO 15 with a power of 15 kW is the most popular variant in practice for standard Slovak family houses. It covers houses with a heated area of 120–180 m² with standard insulation or smaller older houses with insulation. It is the power "sweet spot" – strong enough for most common situations, but still relatively low in electrical demand for network connection.

SLOKOV SL ELEKTRO 18 – for larger or older houses

SLOKOV SL ELEKTRO 18 with a power of 18 kW is a suitable choice for houses of 150–250 m² with average insulation, or for houses with TÚV heating via a boiler, where some power reserve is needed. Typically, these are older houses with replaced windows and insulated walls, but with original ceilings and floors without insulation.

SLOKOV SL ELEKTRO 23 – for large houses or demanding conditions

SLOKOV SL ELEKTRO 23 with a power of 23 kW is suitable for larger family homes over 200 m², older non-insulated houses, or wherever the boiler also covers a significant TÚV volume. This also includes the case where a customer has a cottage or recreational building with occasional heating and needs to quickly heat the building after a longer break.

SLOKOV SL ELEKTRO 26 – for the most powerful applications

SLOKOV SL ELEKTRO 26 with a power of 26 kW is intended for large family homes over 250 m², old non-insulated buildings, or industrial and commercial spaces with higher heat demand. In the residential segment, we install it exceptionally – mostly in houses with historically poor building insulation and no or no possibility of insulation.

Floor heating vs. radiators – does it affect the choice of boiler power?

The boiler power does not directly depend on the type of heating element – the thermal loss of the building is the same regardless of whether you have floor heating or radiators. However, there is an indirect connection:

Radiators operate with higher water temperature (typically 60–80 °C in the supply), the heating system reacts faster, but the boiler must deliver water at a higher temperature. For an electric boiler, this is not an issue – it heats water to any required temperature equally efficiently.

Floor heating operates with low water temperature (25–40 °C in the supply), the system reacts more slowly (thermal inertia of the concrete slab), but it is ideal for low-temperature sources. For an electric boiler, this means lower heat losses in the pipes and less strain on the system.

Practical effect: In a new build with floor heating, you can consider a slightly lower peak power, as the system operates continuously and does not require rapid heating spikes. With radiators, on the other hand, after a night setback, the system needs a short power boost – in this case, it pays to have a small power reserve.

Impact of a thermal storage tank on boiler sizing

If you plan to use night or low-rate electricity (so-called dual-rate tariff, HDO), a thermal storage tank (buffer tank) comes into play. In this case, the boiler power does not need to cover the immediate thermal loss of the building, but must be able to charge the tank during tariff D and the tank then covers consumption during the more expensive tariff V.

This approach can change the selection strategy – sometimes it is more advantageous to have a more powerful boiler with a smaller tank, other times a medium power with a large tank is sufficient. More about this approach can be found in the article Electric boiler SLOKOV and night tariff – how to set up heating, where this issue is discussed in detail.

Step by step: how to calculate the required power yourself

The whole process looks like this:

Determine the total heated area of all floors (m²).
Determine the thermal insulation category of the house and assign a specific loss value (W/m²) according to the table above.
Calculate: Area × specific loss = heating power in W, convert to kW.
Add a climatic correction: for colder areas in northern Slovakia, add 10–15 %.
If you plan to heat TÚV via the boiler, add 3–5 kW.
Round up the result to the nearest available SLOKOV power level.

Why a slight overcapacity is better than undercapacity

This is a question customers often ask: "Is a smaller boiler enough? I will save on the purchase." The answer is usually no, and for several reasons:

Security reserve for extreme cold: Thermal losses of the building are calculated based on the design outdoor temperature, which is a statistical value. In practice, there may be days when the temperature drops below this design point. A boiler without a reserve will not be able to maintain the desired indoor temperature.

Deterioration of insulation over time: The thermal insulation of houses deteriorates over time – windows lose tightness, insulation in the ceiling settles. A house in 10 years may have 5–10 % worse thermal protection than today.

Renovations and extensions: Do you plan to build a winter garden, an insulated garage, or convert the basement in a few years? A power reserve will save you from having to replace the boiler.

Longer boiler lifespan: A boiler that never runs at 100 % capacity has a longer trouble-free life. Electric heating elements wear out due to cyclic heating and cooling – a less intense mode is more beneficial for them.

On the other hand, significant overdimensioning (e.g., a 26 kW boiler for a house requiring 8 kW) is not reasonable. The boiler would operate only at a small power level, which is not a problem in terms of consumption (an electric boiler consumes only what is needed), but unnecessarily increases the initial investment and the requirements for the electrical connection.

Electrical requirements of individual power levels

This is an aspect that customers often do not think about in advance: a more powerful boiler requires a stronger electrical connection. SLOKOV electric boilers are three-phase devices (400 V, 3+N+PE), which is standard for garden and family houses, but the capacity of the fuse and the cable cross-section vary:

10 kW: Current approx. 14.5 A per phase – circuit breaker typically 3×16 A, cable min. 4 mm² Cu
15 kW: Current approx. 21.7 A per phase – circuit breaker typically 3×25 A, cable min. 6 mm² Cu
18 kW: Current approx. 26 A per phase – circuit breaker typically 3×32 A, cable min. 6–10 mm² Cu
23 kW: Current approx. 33.3 A per phase – circuit breaker typically 3×40 A, cable min. 10 mm² Cu
26 kW: Current approx. 37.7 A per phase – circuit breaker typically 3×40–50 A, cable min. 10–16 mm² Cu

Before selecting the boiler power, always check the capacity of your home electrical connection. If you have a main circuit breaker of 3×25 A, you will not be able to connect a 23 kW boiler without increasing the capacity of the connection – and this can be an administratively and financially demanding step (depends on the distribution company and the condition of the grid).

More about electrical installation and connection requirements can be found in the article Installation of SLOKOV electric boiler – procedure and requirements.

Real examples from practice – specific customer cases

Customer A: Brick house from the 50s, Žilina, 135 m²

The house had no thermal insulation and original wooden windows, which the customer replaced just before the boiler installation. Walls 40 cm brick without insulation, ceiling over unheated ground floor. Climatic zone with a design temperature of –15 °C. Calculated specific loss after window replacement: approx. 85 W/m². Power: 135 × 85 = 11,475 W ≈ 11.5 kW just for heating. Since the customer also wanted to heat a 150 l TÚV tank and the house was in a cold area, SLOKOV SL ELEKTRO 18 was recommended. Practice proved it right – during the frost below –18 °C in January, the boiler ran at full power and the house was comfortably warm.

Customer B: New build, low-energy standard, Trnava, 192 m²

House built in 2021, 20 cm wall insulation, triple glazing, heat recovery. Specific loss: 30 W/m². Power: 192 × 30 = 5,760 W ≈ 5.8 kW. The customer also had a 200 l TÚV tank. SLOKOV SL ELEKTRO 10 was recommended, since even with a reserve for TÚV it comfortably came to 10 kW. The customer was pleasantly surprised by the low electricity bills – the house was heated with an average winter consumption of approx. 900 kWh/month.

Customer C: Recreational cabin, Liptov, 80 m²

Cabin with intermittent heating – used only on weekends and holidays. Wooden construction, simple windows, design temperature –16 °C. Calculated specific loss: 110 W/m². Steady-state power: 80 × 110 = 8,800 W. But when arriving on the weekend, the customer needs to heat the cabin from 7–10 °C to 20 °C in a short time – this requires significantly higher power. SLOKOV SL ELEKTRO 15 was recommended precisely for this start-up reserve.

Most common mistakes when selecting power

Calculating floor area as heated area: Garage, technical room without a heat source, unheated basement – this is not a heated area. Do not include them in the calculation.
Ignoring the climatic zone: A house in Kysuce and the same house in Dunajska Streda require a different boiler power.
Reliance on "neighbor's recommendation": Every house is different – age, insulation, orientation, airtightness.
Not accounting for TÚV: If the boiler is to heat water, this must be considered in the power.
Underestimating electrical installation requirements: A more powerful boiler may require modification of the home distribution board and cables.

Summary overview table

Model	Power	Typical heated area (normal insulation)	Typical heated area (good insulation)	Recommended circuit breaker
SL ELEKTRO 10	10 kW	70–100 m²	120–180 m²	3×16 A
SL ELEKTRO 15	15 kW	100–150 m²	180–250 m²	3×25 A
SL ELEKTRO 18	18 kW	130–180 m²	250–320 m²	3×32 A
SL ELEKTRO 23	23 kW	160–230 m²	over 300 m²	3×40 A
SL ELEKTRO 26	26 kW	over 200 m² (old house)	–	3×40–50 A

Note: Area values are approximate for a standard climatic zone (design temperature –13 °C). For colder areas, consider a 10–15 % smaller area for the given power.

When an approximate calculation is not enough – when to get an energy audit

The approximate method of W/m² is good for most common cases, but not for all. In some situations, it is worth investing in a professional calculation of heat losses according to the standard STN EN 12831:

Historical buildings with non-standard construction (castle, aerated concrete, old stone)
Buildings with unusual shape or large glazed area
Houses in very exposed locations (hills, valleys with inversion, windy areas)
Projects where public funding or grants are planned (e.g. Green Home Program) – a professional calculation is mandatory there
Combinations of boiler with heat pump or solar collectors, where precise dimensioning is critical for the economics of the whole system

The energy certificate of the building, if available, contains information on the specific heat demand for heating in kWh/(m²·year), from which the required boiler power can be derived quite accurately.

Selecting power when replacing an old boiler – it is not exactly the same

A specific case is the replacement of an existing boiler. Customers sometimes assume that it is sufficient to buy a boiler with the same power as the old one. This is not always correct:

If you replace a 24 kW gas boiler with an electric one, you cannot automatically reach for a 24 kW electric one – gas boilers are traditionally dimensioned with a significantly higher reserve (a factor of 1.3–1.5 was common), because they are cheaper to operate at higher power and customers demanded it. An electric boiler can be more precisely oversized, because every additional kilowatt costs more to operate even with electric heating.

On the other hand, if the old boiler's power was insufficient and the house was cold, replacing it with the same power will not solve the situation.

Most Frequently Asked Questions (FAQ)

Is the SLOKOV SL ELEKTRO 10 boiler sufficient for a new build of 160 m²?

If the building meets a truly low-energy standard (heat loss below 35 W/m²), a 10 kW boiler could theoretically be sufficient: 160 × 35 = 5,600 W = 5.6 kW. However, if you plan to use the boiler for domestic hot water (DHW) heating, add 3–5 kW, bringing you to 9–11 kW. In this case, a 10 kW boiler is on the edge – I recommend the SLOKOV SL ELEKTRO 15, which will give you a comfortable reserve for DHW and for colder winters. Remember that extra capacity in an electric boiler does not mean higher consumption – it consumes exactly what is needed.

Can I use the SLOKOV SL ELEKTRO 26 boiler in a house where the calculation gives 12 kW?

Technically yes, but it is not ideal. A significant overdimensioning (26 vs. 12 kW) unnecessarily increases the initial investment in the boiler, circuit breakers, and cabling. Electric heating elements do operate in shorter cycles when regulated to a low power, but that is not a big issue in itself – modern boilers have stepped power output. The main issue is economic: you pay for power you will never use, and you have overdimensioned electrical circuit breakers.

I have a house with two floors – how do I calculate the area?

Add up the heated floor area of all floors. If the ground floor is 80 m² and the upper floor is 70 m², the heated area is 150 m². Do not include unheated areas – garage, basement, technical room without a heating appliance. Also, be careful with sloped parts of the ground floor – if there is a heating appliance and the space is usable, include only the part where the ceiling height is at least 1.2 m.

Can the SLOKOV 18 kW boiler heat my house if the calculation gives 16 kW?

Yes, and this is actually a standard situation. Available power steps are 10, 15, 18, 23, 26 kW – there is no boiler with exactly 16 kW. In such a case, you always choose the next higher power, which is 18 kW. The boiler will operate at a lower power than maximum most of the time, and that is perfectly fine. An advantage is also the mentioned reserve for extreme cold or DHW.

What if I plan to insulate my house in a year or two? Should I buy a smaller boiler now?

This is a classic dilemma. I recommend buying a boiler suitable for the current state of your house and not underestimating future insulation (because plans can change). Insulation will significantly reduce heat losses, but a higher capacity boiler will not "die" – it will simply operate at a lower power. The advantage is that with better insulation, the boiler's running times will be shorter and electricity consumption will decrease. Replacing the boiler with a smaller one after insulation rarely pays off economically.

I have an old house where the calculation is close to 24 kW – is there a larger boiler than the SLOKOV SL ELEKTRO 26?

The SLOKOV SL ELEKTRO series ends at 26 kW, which covers almost all single-family homes in Slovakia. If you really need more (e.g., a larger multifunctional building), you may consider another technology (heat pump, condensing boiler) or connecting two electric boilers in cascade – technically feasible but requiring a project solution. For more information on comparisons with other solutions, I recommend the article SLOKOV SL ELEKTRO vs other electric boilers – comparison.

Conclusion: power sizing is fundamental, not a detail

Choosing the right power for your SLOKOV electric boiler is not just a formal step before ordering – it is the foundation for ensuring your house is reliably heated in winter, electricity bills are reasonable, and the boiler lasts for years without problems. The calculation is not complicated, but it requires you to know the basic parameters of your house: heated area, age and condition of thermal insulation, geographic location, and planned method of water heating.

If you are unsure about the calculation, I always recommend choosing a higher power step – a slight overcapacity is much less problematic in practice than undercapacity. And don't forget to check the capacity of your electrical connection and home distribution board before purchase, as it can be a limiting factor for more powerful models.

Further practical information on installation, wiring, and settings can be found in the related article.

Do you have a question about this topic?

Not sure how to decide or dealing with a specific situation in your household? Write to us – we are happy to help.