What eloBLOCK power do I need for my house – kW calculation based on area

We are still preparing products.

But you can look at other categories.

How to calculate the required performance of eloBLOCK for your house – a practical guide to selecting kW

When a customer comes with the question "which eloBLOCK do I need", they usually expect a simple answer – something like a table where they enter the area and get the power. Reality is a bit more complicated, but not unclear. After years of experience with similar orders, I know that the correct choice of electric boiler power depends on at least five factors simultaneously, and neglecting even one of them means either cold radiators in the cold or an unnecessarily oversized boiler that costs you more than it should.

In this article, I will show you how to do the whole calculation systematically – from the basic estimate through the area to the detailed calculation of heat losses. We will go through specific examples from practice, compare individual eloBLOCK models, and I will also tell you what most articles keep silent: when it is better to go one step higher and when, on the contrary, a lower power is sufficient.

Why the "100 W per square meter" rule is not sufficient

I guess the most widespread myth in the world of heating goes like this: take the area in square meters, multiply by a hundred, and you get the required power in watts. So for a house with a floor area of 120 m², you would need 12 kW. This rule is practical as a first orientation, but in real life, it can be 40–50% away from the truth.

Why? Because it does not take into account:

thickness and type of insulation of the perimeter walls
age and condition of the house (a wooden house from 2022 vs. a brick house from 1978)
ceiling height (3-meter ceilings = one-third more air to heat)
quality of windows (double glazing vs. triple glazing, their area and orientation)
location of the house (a valley with inversion vs. a south-western slope)
climatic region of Slovakia (Podunajská nížina vs. Oravská kotlina)
use of the spaces (permanently inhabited vs. weekend cottage)

Therefore, I always recommend at least a three-step procedure: first a rough estimate via the area, then correction according to the thermal technical quality of the building, and finally verification according to the climatic zone. The result of these three steps is much more reliable than any single-factor table.

Step 1 – Rough estimate via floor area

Although the general rule is not perfect, it is good to start with it as a reference point. Professional practice has established these approximate values of specific thermal power according to the type of building:

Type of building	Approximate specific power	Example: 100 m²
Low-energy new building (after 2015)	40 – 60 W/m²	4 – 6 kW
Standard new building (2000 – 2015)	60 – 80 W/m²	6 – 8 kW
Un-certified wooden house	50 – 70 W/m²	5 – 7 kW
Masonry building from the 80s/90s	80 – 110 W/m²	8 – 11 kW
Old building without insulation (before 1990)	100 – 140 W/m²	10 – 14 kW
Cottage / recreational building (intermittent heating)	120 – 160 W/m²	12 – 16 kW

You can see that the range is huge – for the same 100 m², we move from 4 to 16 kW. Therefore, take only the type from the table that your house best fits, and consider the result as the starting point for further steps – not as the final number.

Step 2 – Correction according to the thermal technical quality of the building

Thermal technical quality is a general term for how well the building retains heat. The simplest way to estimate it without an energy audit is to look at three things: the thickness and material of the walls, the type of windows, and the condition of the roof or floor.

Walls

A 25 cm thick brick wall without insulation has a thermal transmittance coefficient U of about 1.5 – 2.0 W/(m²·K). The same wall with 10 cm EPS insulation has U ≈ 0.3 – 0.4 W/(m²·K). The difference is fivefold. If your house has no insulated walls, increase the result from the rough estimate by 25–40%.

Windows

Old double glazing (older, metal frame) has U ≈ 2.5 – 3.5 W/(m²·K). Modern triple glazing with a plastic frame achieves U ≈ 0.6 – 0.8 W/(m²·K). If you have a window area larger than 20% of the floor area (typical for modern houses with large glazing or panoramic walls) and old windows, add another 10–15%.

Roof and floor

An uninsulated roof (especially in old family houses with an unheated attic) can account for up to 20–30% of total heat losses. A floor on the ground without thermal insulation adds another 5–15%. These losses are less dramatic, but in old houses, they are not negligible.

Step 3 – Correction according to the climatic zone of Slovakia

Slovakia has relatively large climatic differences. The calculated external temperature (the coldest design temperature for dimensioning) ranges from -11 °C in the warmest areas of the Podunajská nížina to -18 °C in the Oravská kotlina and some mountainous areas. Each additional degree Celsius of the calculated temperature means that the boiler must cover a larger temperature difference between the interior (+20 °C) and the exterior.

The practical impact of this factor is significant. If you roughly estimated 10 kW and you live in Orava, you actually need 10 × 1.23 = 12.3 kW. This is the difference of one whole performance step of eloBLOCK.

Sample calculation – three specific real-life examples

Example 1: New build in Bratislava, 130 m²

Row house built in 2019, low-energy standard, 15 cm EPS insulation, triple-glazed windows, thermal bridge minimized. Floor area 130 m², ceiling height 2.6 m. Climatic zone: Bratislava, design temperature -11 °C.

Rough estimate: 130 m² × 50 W/m² = 6 500 W = 6.5 kW
Correction for insulation: good insulation → no upward correction
Climatic factor: 1.00 (reference zone)
Result: 6.5 kW → suitable Vaillant eloBLOCK VE 6/14 EU (6 kW) or with reserve eloBLOCK VE 9/14 EU (9 kW)

In this case, I recommend the 9 kW model for one specific reason: if you plan to expand the house or want to have a reserve for colder winters, 6 kW is on the edge. During temperatures below -11 °C (which occurs several days a year in BA), a 6 kW boiler might not be sufficient to maintain 20 °C in all rooms. The 9 kW model provides a comfortable reserve and avoids the pressure of 100% boiler utilization.

Example 2: Brick house from 1985, Žilina, 150 m²

Detached house, 30 cm brick, no facade insulation (planned renovation, not yet completed), double-glazed windows (planned replacement), ceiling insulated in 2010, floor on ground without insulation. Floor area 150 m², ceiling height 2.7 m. Climatic zone: Žilina, design temperature -15 °C.

Rough estimate: 150 m² × 100 W/m² = 15 000 W = 15 kW
Correction: non-insulated walls +30 %, old windows +10 %: 15 × 1.40 = 21 kW
Climatic factor: 35/31 ≈ 1.13 → 21 × 1.13 ≈ 23.7 kW
Result: the house in its current condition requires about 22 – 24 kW

This is a case where one eloBLOCK is not enough. The maximum power of the available model is 18 kW (Vaillant eloBLOCK VE 18/14 EU). The customer has two realistic options: either insulate the house and replace the windows first (which would reduce the required power to under 12 kW, one of the key advantages of renovation), or consider a combined solution or another type of boiler with higher power. In this case, insulation is a much more economically advantageous investment than buying two boilers.

Example 3: Wooden house on Liptov, 90 m², recreational use

Log cabin, 15 cm wood + 10 cm mineral wool from outside, triple-glazed windows, large window area (south-west facade, panoramic glazing). Floor area 90 m², ceiling height 2.8 m. Climatic zone: Liptov, design temperature -17 °C. Used mainly on weekends and holidays, intermittent heating.

Rough estimate (recreational, intermittent): 90 m² × 140 W/m² = 12 600 W = 12.6 kW
Correction: good insulation, but intermittent operation requires higher reserve power
Climatic factor: 37/31 ≈ 1.19 → 12.6 × 1.19 ≈ 15 kW
Result: 14 – 16 kW → suitable Vaillant eloBLOCK VE 14/14 EU (14 kW) or eloBLOCK VE 18/14 EU (18 kW)

For intermittent heating, it is usually wiser to go one step higher. When you let the cabin cool down to 10 – 12 °C and then want to quickly heat it up to 20 °C, you need a power reserve exactly for this quick heating. A smaller boiler could handle it, but at the cost of a very long warm-up time.

What is "power reserve" and why you need it

Dimensioning a boiler precisely to the calculated power is a deliberate mistake made by many designers and salespeople. A power reserve of 10–20 % above the calculated power is standard and justified for several reasons:

Heating of domestic hot water (DHW): If the boiler has a DHW tank or works with an external boiler, it also needs power for heating water. This is added to the heating power, not divided.
Cold starts: In the morning after a night or a prolonged period of inactivity, the boiler must heat the cold water in the system. Without a reserve, room temperatures will be reached much later.
Extreme frosts: The calculated temperature is a statistical value. In practice, winters can be colder than the norm. Having a 15 % reserve means that even at -20 °C in Orava, the system will not shut down and the house will not freeze.
Insulation degradation over time: Older buildings lose their thermal properties. A façade after 20 years is not the same as when it was first handed over.
Planned house extension: Building a terrace, a winter garden, or an attic increases the heated area.

So if your calculation comes out to 11 kW, don't reach for a 9 kW model. The right choice is Vaillant eloBLOCK VE 12/14 EU, which covers the calculated power with an appropriate reserve and ensures stable operation even under load conditions.

Power modulation – why eloBLOCK is not an either-or

An important aspect that many people overlook: eloBLOCK is not a boiler with a single fixed power. Models in the VE 9/14, VE 12/14, VE 14/14, and VE 18/14 series are equipped with switchable power levels via electrical resistors. The boiler can operate at a lower power during the transitional seasons (autumn, spring) and only use full power on genuinely cold days. This stepwise reduction in power is more energy-efficient than constantly switching on and off at full capacity.

Specifically: the 18 kW model can operate, for example, at 6, 9, 12, 15, or 18 kW depending on the setting and the demand from the thermostat. This means that even if you have correctly oversized a 18 kW boiler for winter, it will run at a much lower power in the autumn and will not waste energy. This flexibility is one of the key advantages of eloBLOCK over simpler electric boilers – learn more about this topic in the article How to choose an electric boiler Vaillant eloBLOCK – what to watch out for.

Power vs. hot water preparation – don't forget about DHW

If your system also includes the preparation of domestic hot water via an external or internal tank, you must add the power required for heating water to the heating power. This is one of the most frequently overlooked points I encounter in practice.

The rule is simple: volume of the tank in liters × temperature rise × 1.163 (Wh/l·K) = energy in Wh. If you want to heat a 150-liter tank from 15 °C to 55 °C in two hours, you need:

150 l × 40 K × 1.163 = 6,978 Wh ÷ 2 hours = 3,489 W ≈ 3.5 kW extra power

Therefore, to 9 kW for heating, you need an additional approximately 3–4 kW for simultaneous water heating = total of 12–13 kW. This is exactly why it is usually recommended to choose one model higher than the heating calculation alone for houses with a DHW tank. In practice, the tank is heated with priority (the boiler temporarily switches to water heating), so both powers do not occur simultaneously, but it is still good to have sufficient reserve. More about the combination with a PV system and an energy storage tank is discussed in the article Vaillant eloBLOCK and photovoltaics – is the combination with self-generated electricity worth it.

Overview of eloBLOCK models and who they are suitable for

To conclude this calculation section, let's make a clear overview so you can see which model fits which situation:

Model	Power	Typical situation
VE 6/14 EU	6 kW	Low-energy new builds up to 80–90 m², apartments, studios, houses with passive standard
VE 9/14 EU	9 kW	Low-energy houses 100–140 m² in warmer areas, insulated houses with new insulation
VE 12/14 EU	12 kW	Family houses 120–160 m² with medium energy standard, new builds in higher climate zones
VE 14/14 EU	14 kW	Larger family houses 150–200 m², houses in mountainous areas, older houses after partial insulation
VE 18/14 EU	18 kW	Large houses 200+ m², houses with DHW, poorly insulated houses in cold areas, intermittent heating

Electrical installation limitations – what power your grid will allow

This is an aspect that is completely overlooked when choosing the power rating, and yet it is critical: not every house has an electrical connection that can handle 18 kW. Electric boilers are single-phase or three-phase. Important limits:

Single-phase connection (230 V): Maximum 3.7 – 7.4 kW with fusing at 16 – 32 A. Therefore, you cannot operate a 14 or 18 kW boiler on a single-phase connection.
Three-phase connection (3 × 230 V / 400 V): Here we are in the range of 3 × 16 A to 3 × 32 A, which corresponds to 11 – 22 kW. eloBLOCK models from 9 kW require three-phase power supply.
Circuit breaker and type of protection: the boiler must be protected by a correctly dimensioned circuit breaker and residual current device (RCD). Incorrect protection is a safety risk and also a cause of power outages.

If your required power is 12 kW or more, check your house’s electrical connection before purchasing the boiler. In many older houses, the connection must be upgraded to three-phase – this is a matter for an electrician and the distribution company, not just the boiler itself. The entire issue is discussed in detail in the article Switching to three-phase power for eloBLOCK – when it is necessary and how to do it.

Most frequently asked questions (FAQ)

I have a 100 m² old house without insulation – is a 9 kW eloBLOCK enough?

Almost certainly not. For an old house without insulation and 100 m² of floor area, we are in the range of 100 – 130 W/m², i.e. 10 – 13 kW before the climatic correction factor. On the Danubian Lowland, 9 kW may just be enough, but certainly not in the Low Tatra. I recommend considering at least VE 12/14 EU, ideally combining the boiler purchase with a plan to insulate the house, which will significantly reduce long-term electricity heating costs.

Do I need to take the most powerful model if I want a reserve?

No, a reserve of 10 – 20 % above the calculated power is optimal. An oversized boiler has problems with short cycling (turning on and off too often), which increases wear and reduces efficiency. eloBLOCK has power regulation, but even so, an unnecessarily oversized boiler (e.g. 18 kW for a 70 m² new build) is not an economically sound solution.

How does the calculation change with floor heating compared to radiators?

The required power for heating the house itself does not change – the heat loss of the house is the same regardless of the method of heat distribution. The difference is in the heating water temperatures: floor heating operates at low temperatures (30 – 45 °C), which is advantageous for electric boilers in terms of system stability, but not in terms of power itself. More on this topic can be found in the article Connecting Vaillant eloBLOCK to a floor heating system – what you need to know.

Is the calculation different if I live alone versus in a large family?

For heating the rooms themselves – no. Heat loss of the building is a physical property of the structure, not of the people in it. The difference comes with hot water preparation: a large family consumes more DHW, which increases the overall boiler power requirement if the water heating is handled by the same boiler. For a family of 4+ people, I recommend at least a 200 l storage tank and an additional 4 – 5 kW to the heating power.

Is it worth considering planned photovoltaics when selecting the power rating?

Yes, but be careful with the logic. Photovoltaics does not reduce the required boiler power – the house needs the same amount of heat regardless of where the electricity comes from. A PV system reduces operating costs because part of the electrical energy is obtained for free. From the perspective of boiler power selection, PV is neutral. More on the combination of eloBLOCK + PV is covered in a separate article Vaillant eloBLOCK and photovoltaics – is the combination with self-generated electricity worth it.

What if my calculation comes out exactly on the border between two models – for example, 8.8 kW?

In such a case, always choose the higher model. A borderline result of 8.8 kW means a 9 kW boiler that will operate at the edge of its maximum capacity on cold days – this is not suitable in terms of longevity. A model VE 12/14 EU with power regulation will work more reliably, quietly, and with a longer lifespan in this case, automatically reducing the power to the required level in the off-season.

Conclusion – three things you should not forget

Selecting the correct eloBLOCK power rating is not rocket science, but it requires a bit more than just dividing the house area by a number. If you remember only three things from all of this, let them be these:

First: Always consider the energy standard of the house. A new build and an old house without insulation can have the same area, but twice the difference in power requirement. This is the biggest source of errors in selection.

Second: The climatic zone can increase your requirement by 10 – 20 %. If you live in a colder area of Slovakia and buy a boiler dimensioned according to Bratislava temperatures, you will be freezing in January.

Third: Do not underestimate the electrical connection. A boiler above 6 kW requires a three-phase connection, and this is a condition that must be verified before ordering – not after installation. Further technical details on installation, protection, and connection can be found in the article Installation of Vaillant eloBLOCK – procedure, requirements for electrical installation and protection.

The entire eloBLOCK range is designed to cover most common situations in family homes in Slovakia – from 6 kW for low-energy new builds up to 18 kW for larger or less insulated homes in colder areas. The key is the correct calculation before purchase, not trying different models after installation.

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.