What eloBLOCK power do I need for my house – kW calculation based on area
How to calculate the required eloBLOCK power output for your home – a practical guide to choosing kW
When a customer asks "which eloBLOCK do I need", they usually expect a simple answer – something like a table where they enter the area and get the power output. The reality is a bit more complicated, but not impossible to grasp. After years of experience with similar orders, I know that choosing the correct power output of an electric boiler depends on at least five factors at once, and underestimating even one of them means either cold radiators in freezing weather, or an unnecessarily oversized boiler that costs you more than it should.
In this article, I'll show you how to do the whole calculation systematically – from a basic estimate based on area to a detailed calculation of heat losses. We'll go through concrete practical examples, compare individual eloBLOCK models, and I'll also tell you what most articles leave out: when it's better to go one level higher and when a lower power output is actually sufficient.
Why "100 W per square meter" is not a sufficient rule
Perhaps the most widespread myth in the world of heating goes like this: take the floor area in square meters, multiply by a hundred, and you get the required power output in watts. So for a house with a floor area of 120 m², you would need 12 kW. This rule is practical as an initial orientation, but in real life it can be off by as much as 40 – 50%.
Why? Because it doesn't take into account:
- the thickness and type of insulation of the perimeter walls
- the age and condition of the house (a timber-frame house from 2022 vs. a brick house from 1978)
- ceiling height (3-meter ceilings = a third more air to heat)
- the quality of windows (double glazing vs. triple glazing, their area and orientation)
- the location of the house (a valley with temperature inversion vs. a southwest-facing slope)
- Slovakia's climatic zone (the Danube Lowland vs. the Orava Basin)
- how the spaces are used (permanently inhabited vs. a weekend cottage)
Therefore, I always recommend at least a three-step process: first a rough estimate based on area, then a correction according to the thermal 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 based on floor area
Although the blanket rule is not perfect, it's good to start with it as a reference point. Professional practice has established these approximate specific heat output values according to building type:
| Building type | Approximate specific power | Example: 100 m² |
|---|---|---|
| Low-energy new build (after 2015) | 40 – 60 W/m² | 4 – 6 kW |
| Standard new build (2000 – 2015) | 60 – 80 W/m² | 6 – 8 kW |
| Timber-frame house without certification | 50 – 70 W/m² | 5 – 7 kW |
| Masonry building from the 80s/90s | 80 – 110 W/m² | 8 – 11 kW |
| Older building without insulation (before 1990) | 100 – 140 W/m² | 10 – 14 kW |
| Cottage / recreational property (intermittent heating) | 120 – 160 W/m² | 12 – 16 kW |
As you can see, the range is enormous – for the same 100 m², we range from 4 to 16 kW. Therefore, from the table, select only the type that best fits your house, and consider the result as a starting point for the following steps – not as the final figure.
Step 2 – Correction according to the thermal quality of the building
Thermal quality is a summary term for how well a 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 heat transfer coefficient U of around 1.5 – 2.0 W/(m²·K). The same wall with 10 cm of EPS insulation has U ≈ 0.3 – 0.4 W/(m²·K). The difference is fivefold. If your house's walls are not insulated, increase the result from the rough estimate by 25 – 40%.
Windows
Double-glazed windows (older, metal frame) have 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 your window area exceeds 20% of the floor area (typical for modern houses with large glazed areas or panoramic walls) and you have old windows, add another 10 – 15%.
Roof and floor
An uninsulated roof (especially in old family houses with an unheated attic) can account for as much as 20 – 30% of total heat losses. An uninsulated floor on the ground adds another 5 – 15%. These losses are less dramatic, but not negligible in old houses.
Step 3 – Correction according to Slovakia's climatic zone
Slovakia has relatively significant climatic differences. The design outdoor temperature (the coldest design temperature for sizing) ranges from -11 °C in the warmest areas of the Danube Lowland to -18 °C in the Orava Basin and some mountain areas. Each additional degree Celsius of design temperature means 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 your rough estimate gave you 10 kW and you live in Orava, you actually need 10 × 1.23 = 12.3 kW. That's a difference of a whole eloBLOCK power level.
Sample calculation – three concrete examples from practice
Example 1: New build in Bratislava, 130 m²
A terraced family house from 2019, low-energy standard, 15 cm EPS insulation, triple-glazed windows, minimized thermal bridging. Floor area 130 m², ceilings 2.6 m. Climatic zone: Bratislava, design temperature -11 °C.
- Rough estimate: 130 m² × 50 W/m² = 6,500 W = 6.5 kW
- Insulation correction: 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 extend the house, or want a reserve for colder winters, 6 kW is at the limit. When the temperature drops below -11 °C (which happens a few days a year in Bratislava), a 6 kW boiler might not be enough to maintain 20 °C in all rooms. The 9 kW model provides a comfortable reserve and doesn't force the boiler to run at full capacity constantly.
Example 2: Masonry building from 1985, Žilina, 150 m²
A detached family house, 30 cm brick, no facade insulation (renovation planned but not yet done), double-glazed windows (replacement planned), ceiling insulation added in 2010, uninsulated floor on the ground. Floor area 150 m², ceilings 2.7 m. Climatic zone: Žilina, design temperature -15 °C.
- Rough estimate: 150 m² × 100 W/m² = 15,000 W = 15 kW
- Correction: uninsulated 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 state needs 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 real options: either insulate the house and replace the windows first (which would reduce the power requirement below 12 kW, one of the major benefits of renovation), or consider a combined solution, or possibly a different type of boiler with higher output. In this case, insulation is a much more economically favorable investment than buying two boilers.
Example 3: Timber-frame house in Liptov, 90 m², recreational use
A log cabin, walls 15 cm wood + 10 cm mineral wool on the outside, triple-glazed windows, large window area (southwest-facing façade, panoramic glazing). Floor area 90 m², ceilings 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 a higher backup 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's usually wiser to go one level higher. When you let the cottage cool down to 10 – 12 °C and then want to quickly heat it back up to 20 °C, you need a power reserve precisely for this quick reheating. A smaller boiler could manage it, but at the cost of a very long warm-up time.
What is "power reserve" and why you need it
Sizing a boiler exactly 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:
- Domestic hot water (DHW) heating: If the boiler has a DHW tank or works with an external boiler, it needs power for water heating as well. This is added to the heating power, not divided.
- Cold starts: In the morning after a night, or after an extended setback, the boiler must heat cold water in the system. Without a reserve, room temperatures are reached much later.
- Extreme frost: The design temperature is a statistical value. In practice, winters colder than the norm occur. Having a 15% reserve means that even at -20 °C in Orava, the system won't shut down and the house won't cool.
- Degradation of insulation over time: Older buildings lose their thermal properties. A façade after 20 years is not the same as when it was new.
- Planned house extensions: Building a terrace, conservatory, or attic increases the heated area.
So if your calculation comes out to 11 kW, don't reach for the 9 kW model. The correct choice is the Vaillant eloBLOCK VE 12/14 EU, which covers the calculated power with an adequate reserve and enables stable operation even under peak load conditions.
Power modulation – why eloBLOCK isn't an either-or choice
An important aspect that many people overlook: eloBLOCK is not a boiler with one fixed power output. Models in the VE 9/14, VE 12/14, VE 14/14 and VE 18/14 range are equipped with switchable power stages via electric heating elements. The boiler can operate at lower power during transitional seasons (autumn, spring) and deploy full power only on truly cold days. This stepwise power dimming is more energy efficient than constantly switching a full-power unit on and off.
Specifically: the 18 kW model can operate at, for example, 6, 9, 12, 15 or 18 kW, depending on the setting and the requirements of the weather-compensating controller. This means that even though you correctly sized an 18 kW boiler for winter, during autumn it runs at a much lower power and doesn't waste energy. This flexibility is one of the key advantages of eloBLOCK over simpler electric boilers – you can learn more about this topic in the article How to choose a Vaillant eloBLOCK electric boiler – what to watch out for.
Power vs. domestic hot water preparation – don't forget about DHW
If your system also includes domestic hot water preparation via an external or internal tank, you must add the power needed for water heating to the heating power. This is one of the most frequently overlooked points I encounter in practice.
The rule is simple: tank volume 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
So in addition to 9 kW for heating, you need about 3 – 4 kW extra for parallel water heating = 12 – 13 kW total. This is precisely why, for houses with a DHW tank, it's usually recommended to go one model higher than what the heating calculation alone would suggest. In practice, the tank is heated with priority (the boiler temporarily switches to water heating), so both power demands don't occur simultaneously, but it's still good to have a sufficient reserve. More details on combining with a PV system and energy storage can be found in the article Vaillant eloBLOCK and photovoltaics – is combining it with your own electricity worth it.
Overview of eloBLOCK models and who they are suitable for
To conclude this calculation section, let's create a clear overview so you know 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, passive-standard houses |
| VE 9/14 EU | 9 kW | Low-energy houses of 100–140 m² in warmer areas, insulated houses with new insulation |
| VE 12/14 EU | 12 kW | Family houses of 120–160 m² with medium energy standard, new builds in higher climatic zones |
| VE 14/14 EU | 14 kW | Larger family houses of 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 actually allow
This is an aspect that is completely overlooked when choosing power output, yet it is critical: not every house has an electrical connection that can handle 18 kW. Electric boilers are either single-phase or three-phase. Important limits:
- Single-phase connection (230 V): Maximum 3.7 – 7.4 kW with 16 – 32 A protection. So on a single-phase connection, you cannot operate a 14 or 18 kW boiler.
- Three-phase connection (3 × 230 V / 400 V): Here we're in the range of 3 × 16 A to 3 × 32 A, which corresponds to 11 – 22 kW. eloBLOCK models from 9 kW upward require three-phase power supply.
- Current protection and type of fusing: the boiler must be protected by correctly sized circuit breakers and an RCD. Incorrect protection is a safety risk, but also a cause of outages.
If your calculated required power comes out to 12 kW or more, check your house's electrical connection before buying the boiler. In many older houses, the connection needs to be upgraded to three-phase – this is a matter for an electrician and the distribution company, not just the boiler itself. This entire topic is discussed in detail in the article Converting to three-phase power supply for eloBLOCK – when it's necessary and how to do it.
Frequently Asked Questions (FAQ)
I have 100 m² and an old uninsulated house – is a 9 kW eloBLOCK enough?
Almost certainly not. For an old, uninsulated house with a 100 m² floor area, we're in the range of 100 – 130 W/m², i.e. 10 – 13 kW before the climatic correction factor. In the Záhorie region, 9 kW might just barely be enough, but certainly not in Liptov. I recommend considering at least the VE 12/14 EU, ideally combining the boiler purchase with a plan to insulate the house, which will dramatically reduce long-term electric heating costs.
Do I have 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 cycles (turning on and off too often), which increases wear and reduces efficiency. eloBLOCK does have power regulation, but even so, an unnecessarily oversized boiler (for example, 18 kW for a 70 m² new build) is not an economically sensible solution.
How does underfloor heating change the calculation compared to radiator heating?
The actual power required to heat the house does not fundamentally change – the heat losses of the house are the same regardless of the heat distribution method. The difference lies in the heating water temperatures: underfloor heating operates with low temperatures (30 – 45 °C), which is favorable for electric boilers in terms of system stability, but not in terms of the power output itself. More on this topic can be found in the article Connecting the Vaillant eloBLOCK to an underfloor heating system – what you need to know.
Is the calculation different if I live alone vs. a large family?
For space heating itself, no – the heat losses of the building are a physical property of the structure, not of the people in it. The difference arises with domestic hot water preparation: a larger family uses more DHW, which increases the demand on the boiler's total power output if water heating is handled by the same boiler. For a family of 4+ people, I recommend a tank of at least 200 l and adding 4 – 5 kW to the heating power.
Is it worth considering planned photovoltaics when choosing power output?
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 you get part of the electrical energy for free. So for choosing boiler power, PV is neutral. More on combining eloBLOCK + PV is discussed in a separate article Vaillant eloBLOCK and photovoltaics – is combining it with your own electricity worth it.
What if my calculation comes out exactly at the border between two models – for example, 8.8 kW?
In such a case, always choose the higher model. With a borderline result of 8.8 kW, a 9 kW boiler would be working at the edge of its maximum on cold days – this is unfavorable in terms of lifespan. The VE 12/14 EU model with power regulation will work more reliably, more quietly, and with a longer lifespan in this case, while automatically reducing power to the required level during transitional seasons.
Conclusion – three things not to forget
Choosing the correct eloBLOCK power output isn't rocket science, but it does require 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 take into account the energy standard of the house. A new build and an old, uninsulated house can have the same area but twice as different a power requirement. This is the biggest source of errors when choosing.
Second: The climatic zone can increase your requirement by 10 – 20%. If you live in a colder area of Slovakia and buy a boiler sized according to Bratislava temperatures, you'll be freezing in January.
Third: Don't underestimate the electrical connection. A boiler over 6 kW requires a three-phase connection, and this is a condition that needs to be verified before ordering – not after installation. Further technical details on installation, protection, and wiring can be found in the article Installing the Vaillant eloBLOCK – procedure, electrical installation and protection requirements.
The entire eloBLOCK range is designed to cover most common situations in family houses in Slovakia – from 6 kW for low-energy new builds to 18 kW for larger or poorly insulated houses in colder areas. The key is a correct calculation before purchase, not trying out different models after installation.
Do you have a question on this topic?
Can't decide, or dealing with a specific situation in your household? Write to us - we're happy to help.