What power of an ELÍZ electric boiler do I need for my house

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What power output of an ELÍZ electric boiler do I need for my house?

This is a question that practically every customer asks before deciding to buy an electric boiler. And it's the right question – choosing the wrong power output is one of the most common mistakes when renovating or installing a new heating system. A boiler with too little power cannot maintain the desired temperature in the house during freezing days, while a boiler with too much power is oversized, unnecessarily more expensive to buy, and in some cases may show worse operational efficiency. Correct power sizing is therefore the foundation of the entire project.

In this article, we'll go through all the key factors that influence the required boiler power, show you specific calculations and real-life practical examples, and help you understand whether and why ELÍZ electric boilers are a suitable choice for your particular situation.

Why is the correct boiler power output so important?

Before we get into the numbers, it's important to understand what actually happens when the boiler's power doesn't match the house's real needs. From experience, we know that customers underestimate both extremes.

An underpowered boiler – the house cannot be heated properly at outside temperatures of around -10 to -15 °C, which is common in Slovakia during January frosts. In the worse case, the boiler runs continuously at full power, which leads to faster wear and higher energy costs, because the system never reaches a comfortable temperature and has to constantly compensate for heat loss.

An oversized boiler – at first glance this sounds like a safe choice, but it's not. A boiler that is too powerful heats up quickly, reaches the desired temperature, switches off, the house cools down, it switches on again... This phenomenon is called cycling, and with electric boilers (unlike gas boilers) it's not a problem to the same extent, because electric boilers have no ignition and the start-stop cycle is much less harmful. Nevertheless, an unnecessarily oversized system simply costs more money to buy.

Heat loss of the house – the basis of everything

The boiler's power output must equal the heat loss of the house at maximum winter load (the so-called design outdoor temperature). In other words, the boiler must be able to supply exactly as much heat to the house as the house loses on the coldest day. Nothing more, nothing less.

The heat loss of a house depends on several factors:

Usable floor area and volume of the heated space – bigger house = greater losses
Quality of thermal insulation of the perimeter walls – from old, non-compliant structures to passive house standard
Quality of the roof and floor – ceiling, roof structure, floor above an unheated basement
Number, size and type of windows and doors – windows are the weakest link in the building envelope
Climatic region – the design outdoor temperature in Bratislava is -11 °C, while in the Orava or Kysuce regions it can be as low as -18 °C
Heated interior temperature – typically calculated with 20 °C for living spaces
Building orientation – north-facing facades lose more heat

A professional heat loss calculation according to the STN EN 12831 standard is carried out by a designer or energy auditor. However, for typical family houses there are proven approximate values of specific heat output (in W/m²) that allow you to make a rough estimate without a project.

Approximate power values according to building type

In practice, for a quick estimate of the required heating system output, the specific power per square meter of floor area is most commonly used. These values reflect common construction standards and climatic conditions in Slovakia:

Building type / insulation	Specific output (W/m²)	Example: 100 m²	Example: 150 m²
Passive house (≤ 15 kWh/m²/year)	15 – 25 W/m²	1.5 – 2.5 kW	2.3 – 3.8 kW
Low-energy house (new build after 2016)	30 – 45 W/m²	3 – 4.5 kW	4.5 – 6.8 kW
Standard new build (2000–2015)	45 – 65 W/m²	4.5 – 6.5 kW	6.8 – 9.8 kW
Older house after complete envelope renovation	50 – 70 W/m²	5 – 7 kW	7.5 – 10.5 kW
Older house partially insulated (windows + facade)	70 – 90 W/m²	7 – 9 kW	10.5 – 13.5 kW
Older non-insulated house (panel building, brick without insulation)	90 – 130+ W/m²	9 – 13+ kW	13.5 – 19.5+ kW

Note: The values apply to the climatic conditions of central Slovakia with a design outdoor temperature of -13 to -15 °C. For significantly colder areas (e.g., Orava, the Tatras), increase the estimate by 10–15%.

Slovakia's climatic region and its impact on power output

Slovakia has relatively diverse climatic conditions. While in Bratislava and the surrounding Danube lowlands winter minimums reach around -10 to -12 °C, in mountainous areas the design temperatures are up to 6–8 °C lower. This directly affects the boiler power requirements.

For a rough calculation, a simple rule applies: each degree Celsius decrease in the design outdoor temperature increases the required boiler power by approximately 5–8% (depending on the specific building). So if you have a house in Žilina, where the design temperature is -15 °C instead of -12 °C in Bratislava, and your rough calculation comes out at 6 kW, you actually need around 6.9 – 7.2 kW.

Practical calculation example: three different houses

Let's show this with specific examples that we see in practice. Every house is different, and these differences fundamentally change the resulting boiler power.

Example 1 – New build, low-energy standard, 130 m²

A family from Žilina built a two-story family house with a floor area of 130 m². The house is built according to the low-energy standard – thick facade insulation of 20 cm, triple-glazed windows, security doors with a thermal break, and heat-recovery ventilation. Design outdoor temperature for the Žilina area: -15 °C.

Rough calculation: 130 m² × 40 W/m² = 5,200 W = 5.2 kW

Conclusion: For this house, a boiler with an output of 5–6 kW is sufficient. ELÍZ offers solutions in this category, and many models feature continuously adjustable power, so the boiler can reduce or increase output as needed.

Example 2 – Older family house after renovation, 160 m²

A customer from Prievidza bought a brick house from 1978 and had it renovated: a new facade with 12 cm EPS insulation, double-glazed plastic windows (not triple-glazed), and a new insulated roof. The basement is unheated. The floor area of the heated spaces is 160 m². Design temperature: -13 °C.

Rough calculation: 160 m² × 75 W/m² = 12,000 W = 12 kW

It should be noted that although the house underwent partial renovation, the old brick wall without internal insulation and double-glazed windows still mean relatively higher heat losses. A boiler with an output of 10–12 kW is suitable for this house. Some ELÍZ models have multi-stage power control, which is advantageous for a larger house.

Example 3 – Old uninsulated village house, 90 m² living area

A retiree from the mountainous Orava region has an older house built from cinder block, without any insulation. He only heats 90 m² of the ground floor and does not heat the attic. Design outdoor temperature for Orava: -18 °C. The house has original two-panel radiators.

Rough calculation: 90 m² × 120 W/m² = 10,800 W = 10.8 kW

An interesting case – a smaller house, but extreme climatic conditions and a poor building envelope. This customer needs a boiler of at least 10–11 kW. At the same time, we would recommend considering at least basic insulation, because with such heat loss, the operating costs of electric heating will be very high, regardless of which boiler is purchased.

ELÍZ electric boiler – power options and modulation

ELÍZ electric boilers are available in several power variants, which is an advantage for the customer – you don't have to buy a boiler with a large power margin if you don't need it. An important feature of modern electric boilers is continuous or step-based power regulation.

For example, the ELÍZ EQH 802 is a compact electric boiler that can cover the needs of smaller houses and apartments. Multi-stage power control means the boiler doesn't always run at full power – it only switches on as many heating elements as are currently needed. This achieves more efficient operation and smaller fluctuations in heating water temperature.

When choosing a specific model, it's necessary to consider not only the total output but also:

Number of power stages – the more stages, the smoother the regulation and the smaller the current load spikes on the electrical installation
Maximum current load – larger boilers (8 kW and above) require a three-phase 400 V power supply, while smaller ones can run on single-phase 230 V
Maximum working pressure – 3 bar is sufficient for most family houses, although some systems may require more
Type of regulation – integrated thermostat, external thermostat, weather-compensated control, programmable timer

You can find more about specific parameters and features of the boilers in the article How to choose an ELÍZ electric boiler: what to focus on before buying and in the article ELÍZ electric boiler vs. other electric boilers: parameter comparison.

Single-phase vs. three-phase power supply – a critical factor

This is a topic that is very often forgotten when choosing the boiler power, yet it can complicate the entire project. The electrical connection of most Slovak family houses is three-phase (3×400 V), but there are also houses or apartments where only single-phase (230 V) is available.

Why does this matter when choosing the power output?

On a single-phase connection with a standard 25 A fuse, you can connect an electrical device with a maximum output of around 5 – 5.5 kW
On a three-phase connection with 3×25 A fuses, an output of up to 17 kW is theoretically available
Most ELÍZ electric boilers with an output above 4–6 kW require a three-phase connection

If your house only has a single-phase connection and you need an output higher than 5 kW, you have two options: either have a three-phase connection installed (which is an additional investment), or consider combining a lower-power electric boiler with supplementary heating (e.g., storage heating, electric convectors in colder rooms).

You can find more about installation requirements, including the electrical installation, in the separate article Installing an ELÍZ electric boiler: procedure and installation requirements.

Domestic hot water preparation – does it affect the choice of power?

Yes, significantly! If you want to use an ELÍZ electric boiler for domestic hot water (DHW) preparation as well, the total required boiler output increases. There are two main solutions:

1. Boiler with an integrated DHW tank – some models have a built-in hot water tank (for example, 8–16 liters), which is suitable for apartments and smaller households without high hot water consumption. This tank is either instantaneous or a small storage type.

2. Boiler + external DHW tank – for family houses, a combined tank of 150–200 liters is commonly used, where the boiler heats the water indirectly via a heat exchanger. In this case, the boiler must have sufficient power so that heating the DHW doesn't neglect heating the house. In practice, this means adding at least 3–5 kW extra output or properly setting DHW priority heating.

So if your house needs 7 kW for heating and you plan to also heat a DHW tank through the boiler, consider a boiler with an output of at least 9–10 kW, or alternatively use a separate electric storage water heater for DHW – this is a very common and economically favorable solution in practice.

The effect of the heating system's temperature gradient on boiler selection

Not every heating system is the same. Old-type radiator systems operate at a temperature gradient of 70/55 °C (supply water temperature / return water temperature). Modern underfloor or wall heating operates at a much lower gradient – for example, 35/28 °C. The ELÍZ electric boiler is capable of working in both cases, but you should be aware that:

At a temperature gradient of 70/55 °C (old radiator system), the boiler must heat the water to higher temperatures – this is more energy-intensive but technically not a problem
With underfloor heating (35/28 °C), the required maximum water temperature is lower, and the system operates more smoothly and with higher efficiency
When switching from an old boiler to an electric ELÍZ boiler without replacing the radiators, it's important to verify that the original radiator system is sized for sufficient output at the current temperature gradient

This topic, including the specific steps when transitioning from an old gas system, is discussed in detail in the article Switching from gas heating to an ELÍZ electric boiler: what you need to know.

How do I calculate the power output myself without a designer?

A designer's calculation according to the standard is ideal, but not always available or economically justified for a small project. Here is a simple procedure for making a rough estimate yourself:

Step 1: Measure or estimate the heated floor area (m²). Only count rooms you actually want to heat.

Step 2: Categorize your house according to its insulation (see the table above) and find the corresponding specific output value (W/m²).

Step 3: Calculate: Output (W) = Area (m²) × Specific output (W/m²)

Step 4: Take the climatic region into account – for northern Slovakia and mountainous areas, add 10–15%.

Step 5: If you'll be heating DHW in a tank via the boiler, add 3–5 kW.

Step 6: Round the result up to the nearest available output in the ELÍZ range. A small power margin (10–15%) is fine and leaves room for a possible expansion of the heating system.

If you're not sure how to categorize your house, the easiest way to verify the heat loss is to look at the consumption of your current heating system. If you heated with gas and know your annual gas consumption, we can estimate the heat loss fairly accurately retrospectively.

Combination with photovoltaics – does it change the choice of power?

If you're planning to combine an ELÍZ electric boiler with a photovoltaic (PV) system, choosing the power output takes on an additional dimension. In this case, it's advantageous to have a boiler with adjustable power that can receive and use surplus energy from the PV panels in real time. Some systems handle this via an external controller (e.g., a boiler manager, Smart Meter), which instructs the boiler to increase its energy intake when the PV system produces a surplus.

For a household with a PV system, it's not unreasonable to have a boiler with slightly higher output than a purely thermal calculation would require – for example, if you have a 10 kWp PV system and a 9 kW boiler, in theory you can cover most of the heating energy from the sun during sunny months. This topic is discussed in detail in the article ELÍZ electric boiler and photovoltaics: is the combination worth it.

Common mistakes when choosing power output – from practice

Over years of practice selling and consulting on heating systems, we repeatedly see the same mistakes. Here are the most common ones:

The customer calculates the total area of the house including the garage, basement, and unheated attic. Only the actually heated area should be included in the calculation.
The customer underestimates the heat loss of an old house. An old uninsulated house can have a specific output 4–5 times higher than a new build.
The customer plans to insulate the house a year later. If this doesn't happen, the boiler is oversized for the future state and insufficient for the current state.
The customer forgets about DHW. They then find out that the boiler can't keep up with heating the house and heating hot water at the same time.
The customer buys too large a boiler because "bigger must be better". An oversized boiler doesn't reduce lifespan as significantly as with gas boilers, but it's unnecessarily more expensive to buy, and with three-phase power supply it can cause problems with fuses during startup.

If you're unsure about the calculation, we recommend consulting a specialist retailer or installer. If you've decided on an ELÍZ electric boiler, you can find more about the whole selection process in the article How to choose an ELÍZ electric boiler: what to focus on before buying and in the Frequently asked questions about ELÍZ electric boilers section.

Specific products from the ELÍZ category, such as the ELÍZ EQH 802, can be found directly in the ELÍZ category on Atria.sk, where you'll also find exact technical parameters including power inputs, voltage requirements, and power stages.

Frequently Asked Questions (FAQ)

How do I find out the output of my old boiler so I know what to replace it with?

Check the label on the old boiler – every boiler has its rated output in kW listed. If the label is unreadable, try to find the boiler's documentation or look up the model online. Alternatively, ask the installer who installed the boiler. Keep in mind, however, that the old boiler's output may not have been optimal – it could have been oversized or undersized as well. It's best to make a new heat loss calculation, especially if you've since insulated the house or replaced the windows.

Can I use an ELÍZ electric boiler to heat a 200 m² older uninsulated house?

Technically yes, if you choose a sufficiently powerful model and have a three-phase connection available. For a 200 m² uninsulated older house, you would need a boiler with an output in the range of 18–26 kW, which is at or beyond the limit of a typical home connection. In such a case, we would strongly recommend first considering insulating the house, because heating costs at such a level of heat loss would be unsustainable regardless of the type of boiler. Electric heating is cost-effective mainly in well-insulated buildings, ideally combined with photovoltaics or a cheap night electricity tariff.

Is it better to buy a boiler with a slightly higher output as a reserve?

A small reserve (10–15% above the calculated output) is reasonable and standard. It gives you certainty even on extremely cold days and in case of a possible expansion of the heating system (a new room, a winter garden...). A larger margin (30% or more) is unnecessary – it increases the purchase price and, for a three-phase boiler, also the requirements for sizing the electrical installation. ELÍZ electric boilers with adjustable output can also work at a lower current output, so a small margin poses no operational problem.

Can I connect an ELÍZ boiler to a single-phase connection if I need 8 kW?

No. A single-phase connection with a standard 25 A fuse won't allow you to connect a device with an input above approximately 5.5 kW. A boiler with an output of 8 kW requires a three-phase 3×400 V power supply. If your house only has a single-phase connection, contact your distribution network to request a change (installation of a three-phase connection) – this is a common procedure, but it comes with a cost and administrative burden. An alternative is to split the heating into several smaller electrical devices (a boiler up to 5 kW + electric convectors in rooms), which, however, can be complicated for higher power needs.

What if my house is undergoing renovation – should I buy a boiler now or wait?

If the renovation includes insulating the building envelope, we recommend waiting to buy the boiler (or at least installing it) until the insulation work is complete. Heat loss after insulation can be 30–50% lower than before, so a boiler sized for the old condition will be unnecessarily large for the new one. If you need to heat during the renovation, consider a temporary solution (electric convectors, infrared heating) and choose the final boiler only after the work is finished.

What output do you recommend for a typical 3-room apartment in a panel building?

A typical 3-room apartment in a panel building has an area of around 60–75 m². Panel buildings after comprehensive renovation (insulation, window replacement) have a specific output of 30–45 W/m², which for 70 m² comes to 2.1–3.2 kW. An original, non-renovated panel building may have a specific output of 60–80 W/m², i.e., it needs 4.2–5.6 kW. For most apartments, a boiler with an output of 3–5 kW is therefore sufficient, and it can also run on a single-phase power supply. However, it's necessary to check whether the apartment's connection and electricity meter allow such an input – in old panel buildings, electrical connections tend to be sized for a lower load.

Conclusion: boiler output is not a matter of chance, it's a calculation

Choosing the right output for an ELÍZ electric boiler is a combination of technical calculation, common sense, and knowledge of your specific situation. It's not just about the area of the house – the state of insulation, climatic region, type of heating system, power supply, and whether you plan to also heat hot water through the boiler are equally important.

A rough calculation according to the specific output table will give you a good foundation. For certainty and larger projects, we recommend having a heat loss calculation done by a designer – the investment of a few dozen euros in the project will pay off in the form of a correctly sized system for the coming decades.

If you've estimated the output and are looking for a specific model, check out the full range of ELÍZ electric boilers and the detailed parameters of individual models, such as the ELÍZ EQH 802. For further questions about installation, setup, or combination with photovoltaics, we also recommend reading other articles in the Knowledge Center – for example, Setting and programming the control of an ELÍZ electric boiler or Maintenance and servicing of an ELÍZ electric boiler: how to extend its lifespan.

Do you have a question on this topic?

Can't decide or dealing with a specific situation in your household? Write to us - we'll be happy to help.