What Vitotron output do I need for my house or apartment

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What Vitotron output do I need for my house or apartment?

This is a question that almost every customer faces before deciding to buy an electric boiler. And understandably so – output is a key parameter that determines not only thermal comfort, but also operating costs, the lifespan of the appliance, and overall satisfaction with heating. An undersized boiler will constantly run at full power and never manage to heat the house to the desired temperature. An oversized boiler, on the other hand, needlessly increases the purchase cost and operates inefficiently with short cycles. Choosing the right output is therefore just as important as the brand or model itself.

In this article, we will go through the whole topic from the basics – what boiler output actually means, how the heat loss of a building is calculated, what factors influence the required output, and finally practical examples so you can make a realistic estimate for your specific case. We will also cover the specifics of the various Vitotron models and when a smaller or larger model is more advantageous.

What does "boiler output" mean and why does it matter

Boiler output is expressed in kilowatts (kW) and represents the amount of thermal energy the appliance can deliver to the heating system per unit of time. For electric boilers such as Vitotron, this output is directly proportional to the electrical energy consumed – an electric boiler converts electricity into heat with an efficiency close to 100%, which is one of its main advantages over fossil fuels.

It is important to understand that the boiler output must cover the so-called heat loss of the building – i.e. the amount of heat that the house or apartment loses through walls, roof, floor, windows, and ventilation per unit of time on a design-cold outdoor day. In Central European conditions, an outdoor design temperature of –12 °C to –15 °C is commonly assumed (depending on the climatic zone of Slovakia). The boiler must be able to cover this maximum heat loss – while ideally not being too oversized, so that it works efficiently.

A major advantage of Vitotron electric boilers is smooth output modulation. For example, the Viessmann Vitotron 100 1.0 – 24.0 kW can operate within a range from 1 kW to 24 kW, meaning it perfectly adapts to current needs – in mild weather it slows down, on a cold day the output increases. This modulation is essential for heating efficiency.

Methods for calculating the required output – from a simplified estimate to a precise design

Simplified estimation method (the W/m² rule)

For an initial estimate, a simple formula is commonly used in practice: output in kW = area × specific heat loss [W/m²] / 1000. The specific heat loss depends on the quality of the building's insulation. Approximate values are as follows:

Uninsulated old houses (masonry without insulation, old windows): 80 – 120 W/m²
Partially insulated houses (after a basic renovation, double-glazed windows): 50 – 80 W/m²
Modern insulated houses (insulated facade, plastic windows, insulated roof): 30 – 50 W/m²
Low-energy houses (thick insulation, triple glazing, heat recovery ventilation): 15 – 30 W/m²
Passive houses: 10 – 15 W/m²

Example: An 85 m² apartment in a panel building after facade insulation and window replacement (specific loss approx. 60 W/m²) needs an output of: 85 × 60 / 1000 = 5.1 kW. In this case, the Viessmann Vitotron 100 0.4 – 8.0 kW would be fully sufficient, with plenty of reserve.

Another example: A family house with a usable area of 160 m², a masonry house from the 1980s, 10 cm facade insulation, plastic windows, roof not replaced (specific loss approx. 65 W/m²): 160 × 65 / 1000 = 10.4 kW. Here we would choose a larger model – the Viessmann Vitotron 100 1.0 – 24.0 kW with a reserve for domestic hot water heating or future expansion.

Detailed energy calculation according to EN 12831

For a precise calculation, the European standard EN 12831 (calculation of building heat loss) is used. This calculation is carried out by a designer or energy auditor. It takes into account:

the composition of each envelope structure (wall, roof, floor, window) and its heat transfer coefficient U [W/m²K]
the area and orientation of each structure
the climatic zone and design outdoor temperature for the given location
air exchange (ventilation, infiltration)
the indoor design temperatures in individual rooms

The result is the total heat loss of the building in kW, based on which the boiler is designed. If you are undecided, this calculation will tell you exactly what you need – and it is also part of the documentation required for the occupancy approval of a new building or when applying for a subsidy.

Factors that fundamentally influence the required output

1. Thermal quality of the building envelope

This is clearly the most significant factor. The difference in required output between an uninsulated house and a low-energy house of the same area can be 4 to 6 times. Therefore, it is not possible to say "150 m² needs X kW" without knowing the condition of the building. An old panel building without insulation and a modern low-energy new build of the same area will have radically different output requirements for the boiler.

Key parameters include:

Thickness and type of facade insulation – the difference between an uninsulated facade and 15 cm of polystyrene is enormous
Window type – single glazing (U ≈ 5.8 W/m²K), double glazing (U ≈ 1.2–2.0), triple glazing (U ≈ 0.5–0.8)
Roof and top-floor ceiling – thermal bridges through an uninsulated roof are a major source of losses
Floor and basement structures

2. Climatic zone of Slovakia

The design outdoor temperature varies by region in Slovakia. Bratislava and south-western Slovakia use –12 °C, mountain regions –18 °C to –22 °C. For a house in the Tatra region, you therefore need a 30–50% higher boiler output than for an identical house in Bratislava. Customers often underestimate this.

3. Shape and orientation of the building

A long, narrow house has a larger envelope surface for the same floor area as a compact square house – meaning greater heat losses. A northern orientation of the main rooms increases losses compared to a southern one. This is also related to the number of corners, parapets, terrace doors, and other structural elements.

4. Floor height and total volume of the heated space

Volume, not area, is the physically correct quantity. A house with a floor height of 3 m has about 20% greater losses through ceilings and floors than an equally sized house with a height of 2.5 m. For a simple estimate, area is sufficient, but for a precise calculation, volume is needed.

5. Domestic hot water heating

If you want to use the Vitotron boiler to also heat domestic hot water (DHW) via a storage tank, you must add the output needed for water heating to the heating output. A 150–200 liter tank for a family of 4 needs roughly 3–6 kW of additional output for heating (depending on the desired heating time). Models designed for combined heating + DHW include the Vitotron 100 0.4–8.0 kW with 3-way switching valve or the Vitotron 100 1.0–24.0 kW with 3-way switching valve and tank sensor. More about this configuration can be found in the article Vitotron with 3-way valve and tank sensor – when is it worth it.

6. Heating mode and duration

If you heat continuously (24/7), the boiler works evenly and a lower output is sufficient. If you only heat during certain times – for example morning and evening – the boiler must be able to reheat the cooled space more quickly, which requires higher output. Holiday homes with intermittent heating need relatively higher output than permanently occupied homes of the same size.

Practical examples – what Vitotron output for different types of buildings

Example 1: 65 m² panel-building apartment, insulated panel

An apartment in a panel building from 1980, on the 5th floor. The facade was insulated with 8 cm of mineral wool in 2010, and plastic windows were replaced in 2008. Heated area 65 m². The apartment is surrounded by other apartments on all sides except two exterior walls and the roof (corner apartment on the top floor – i.e. a more demanding case). Specific heat loss approx. 55 W/m².

Calculation: 65 × 55 / 1000 = 3.6 kW. For this apartment, the Viessmann Vitotron 100 0.4 – 8.0 kW would be sufficient with a large reserve. DHW in a panel building is usually solved centrally or with a separate water heater outside the boiler. Result: Vitotron 100 up to 8 kW is ideal.

Example 2: 120 m² family house, masonry, uninsulated, central Slovakia region

A house from 1975, 38 cm brick without insulation, original wooden windows still in place, roof not replaced. Heated area 120 m², design outdoor temperature –14 °C. Specific heat loss approx. 105 W/m².

Calculation: 120 × 105 / 1000 = 12.6 kW. For this house, a Vitotron 100 with a range up to 24 kW would be sufficient. However, note – if the customer plans to insulate the house within 2–3 years, the heat loss will decrease to approximately 6–7 kW after insulation. In this case, it is worth considering whether to invest first in insulation and then in a boiler with a corresponding output, or to buy a modulating boiler with a wide range (1–24 kW) that will work correctly even after the losses are reduced.

Example 3: 180 m² new build, low-energy standard, Bratislava

A house built in 2020, EPS facade insulation 20 cm, triple glazing, insulated roof 30 cm, air heat recovery ventilation. Heated area 180 m². Specific heat loss approx. 22 W/m².

Calculation: 180 × 22 / 1000 = 3.96 kW. For this house, a boiler with an output of 4 kW would theoretically be enough! If the customer also wants to heat DHW via a 200-liter tank, we add 3–4 kW, bringing us to a requirement of approximately 7–8 kW. The Vitotron 100 0.4–8.0 kW with 3-way valve would be an ideal solution for such a new build.

Example 4: 80 m² holiday cabin, mountains, intermittent heating

A masonry cabin from 1990, uninsulated, design outdoor temperature –18 °C, heated only on weekends. Area 80 m². Specific loss approx. 130 W/m² given such a low design temperature and poor construction quality. Intermittent operation requires a correction factor of 20–30%.

Basic calculation: 80 × 130 / 1000 = 10.4 kW. With correction for intermittent heating: 10.4 × 1.25 ≈ 13 kW. A model with higher output is a suitable solution. More about the operating costs of such a system can be found in the article Vitotron operating costs – how much does heating with an electric boiler cost.

Overview of Vitotron 100 models and their output ranges

The Vitotron 100 range offers four main configurations, which differ in output and equipment:

Vitotron 100 0.4 – 8.0 kW – a compact model for apartments, smaller houses, and low-energy new builds. Modulation from 0.4 kW allows for very fine adjustment, so it doesn't run short cycles even in mild weather.
Vitotron 100 1.0 – 24.0 kW – a powerful model for larger houses, older buildings, or a combination of heating + DHW. The 1–24 kW range covers the vast majority of family houses in Slovakia.
Vitotron 100 0.4 – 8.0 kW with 3-way valve and tank sensor – identical output to the first model, but with integrated provision for direct DHW heating via a storage tank.
Vitotron 100 1.0 – 24.0 kW with 3-way valve and tank sensor – the most powerful and best-equipped model for large houses requiring water heating.

Additionally, the Viessmann EKCO.TM 10–30 is available, designed for larger installations and commercial applications with an output of 10 to 30 kW – for example for larger apartment buildings, businesses, or industrial premises where one Vitotron is not enough.

How to choose correctly between models and what else to focus on when choosing is described in the article How to choose a Viessmann Vitotron electric boiler – what to focus on.

Why output modulation changes the rules of the game

Traditional simple electric boilers operate in a binary mode – either full output or off. Vitotron works differently: its output smoothly changes according to current demand. This feature, called cascade modulation, has several consequences for choosing the output:

A boiler with a wide range is more flexible: the 1–24 kW model can handle both a small apartment in January and a large house in a harsh winter. In practice, however, buying a 24 kW model for a small apartment doesn't make economic sense – the purchase price is higher, and control may be less precise at very low load.
Minimum output matters: during transitional periods (spring, autumn) you only need a minimum amount of heat. The 0.4–8 kW model can heat at a level of 400 W, which is very fine control. The 1–24 kW model stops at a minimum of 1 kW.
You are choosing a range, not a fixed point: when buying a Vitotron, it is important that the maximum heat loss of your building falls somewhere in the middle of the boiler's output range – not right at the top. An ideal reserve is 10–20% above the calculated loss.

Mistakes people make when choosing output

In practice, dozens of customers repeatedly make the same mistakes. Here is an overview so you can avoid them:

Buying too large a boiler "just to be safe": the logic "bigger is better" doesn't apply here. An oversized boiler runs in short cycles, doesn't heat the system properly, wears out faster, and also increases the purchase cost without added value.
Ignoring planned insulation upgrades: if you are planning to insulate the facade and replace windows, buy the boiler according to the future condition, not the current one.
Not including DHW heating in the output: if the boiler is also used for water heating, that is additional output that needs to be factored in.
Forgetting about the climatic zone: a house in a high mountain village and a house in Bratislava of the same area may need a 40–50% difference in output.
Relying on a "per m²" number without context: a rough W/m² estimate is only an orientation starting point, not a conclusion.

Electrical connection and fuses – what to check before buying

Choosing the boiler output is directly related to the electrical capabilities of your building. This is an aspect customers ask about less often, but it is essential. An electric boiler with an output of 8 kW needs a current of approximately 36 A (230 V) on a single-phase supply, which is at the limit of typical fuse protection. Therefore:

Boilers up to approx. 3.5 kW can be connected to a single-phase 230 V network
More powerful Vitotron models (typically above 3.5–4 kW) require a three-phase 400 V supply, which is standard for family houses
The Vitotron 100 1–24 kW is always three-phase
Check the capacity of your electrical connection and main circuit breaker – a larger Vitotron may require an upgrade of the distribution board

Installation and connection of the boiler is described in a separate article, Installing a Viessmann Vitotron electric boiler step by step. Remember that installation may only be carried out by a person with electrical qualifications.

How Vitotron combines heating with photovoltaics

More and more customers are considering combining an electric boiler with a photovoltaic system. Here, the smooth modulation of the Vitotron 100 is a huge advantage: a PV controller (e.g. via a 0–10 V input or SG Ready contact) can adjust the boiler's output in real time to the available output of the PV panels. When the sun is shining and the PV system produces surplus energy, the boiler draws more energy and heats the tank or heating water further. When production drops, the boiler reduces its output. This is one of the reasons why an electric boiler with modulation like Vitotron is a more logical choice for PV installations than a heat pump without such flexibility. For correctly setting up the system with PV, it is generally advantageous to choose a model with a 3-way valve and storage tank – a larger hot water tank serves as a thermal battery for storing solar energy.

Table: Quick guide to output by building type

Building type	Area (m²)	Specific loss (W/m²)	Estimated output (kW)	Recommended model
Panel building, insulated, corner apartment	60–80	50–65	3–5.5	Vitotron 100 0.4–8 kW
Masonry house, improved insulation	100–140	55–75	6–10.5	Vitotron 100 1–24 kW
Old house without insulation	100–150	90–120	9–18	Vitotron 100 1–24 kW or EKCO.TM
Low-energy new build	150–200	20–35	3–7	Vitotron 100 0.4–8 kW (+3WV if DHW)
Passive house	150–200	10–18	1.5–3.5	Vitotron 100 0.4–8 kW
Cabin, intermittent heating, mountains	60–100	100–140	8–17	Vitotron 100 1–24 kW

Frequently Asked Questions (FAQ)

Is it enough to know the area of my apartment to choose the right Vitotron output?

Area is a good starting point, but it's not enough on its own. Equally important is the thermal quality of the building – the type and thickness of insulation, window type, roof condition. Two 80 m² apartments may need an output of 3 kW and 9 kW respectively, if one is in a low-energy building and the other in an uninsulated panel building. Use the approximate formula area × specific loss / 1000, estimating the specific loss according to the building's condition (see the table above).

What if I'm not sure – should I choose a higher output just to be safe?

Not necessarily. It's advisable to add a 10–20% reserve above the calculated heat loss, not 100%. An oversized boiler operates inefficiently – short cycles don't heat the system properly, cause shock loads on the electrical network, and the boiler wears out sooner. The modulating nature of the Vitotron 100 further reduces the need for this reserve, since the boiler always adapts to current demand.

Can Vitotron also heat domestic hot water? Do I need to increase the output?

Yes, Vitotron can also be used to heat DHW via a storage tank. When sizing the output, you need to add the output required for heating the tank to the building's heat loss – approximately 3–5 kW for a 150–200 liter tank. Models with an integrated 3-way switching valve and tank sensor are designed for combined heating and DHW. Details can be found in the article Connecting Vitotron to a system with a boiler or DHW tank.

I have an old house and plan to insulate it in 2–3 years. What output should I choose now?

It's best to either buy a boiler according to the future (insulated) condition and supplement heat some other way in the meantime, or choose a model with a wide output range – for example the Vitotron 100 1–24 kW – which can handle today's high loss as well as the future lower loss after insulation. Buying, say, a 15 kW boiler for today's condition and then replacing it with a 7 kW one after insulation doesn't make economic sense.

What is the difference in output between the Vitotron 100 up to 8 kW and up to 24 kW? Is the bigger one worth it?

The 0.4–8 kW model is sufficient for apartments and well-insulated houses up to approximately 100–130 m² in a mild climatic zone. The 1–24 kW model is for larger houses, older buildings with higher heat loss, or a combination with DHW heating. The larger model's price is higher, so it's not worth buying a 24 kW unit for an apartment that needs 4 kW. If you're on the borderline – e.g. a need of 7.5 kW – it's better to choose the 1–24 kW model for sufficient reserve and comfort.

Can I use Vitotron for a commercial building or a larger building?

For larger buildings needing more than 24 kW, the Viessmann EKCO.TM 10–30 is available, which can deliver up to 30 kW of output. For even greater needs, a cascade of multiple boilers is used, providing higher total output while maintaining modulation. Each such installation, however, requires an individual design.

Conclusion: output is not a number, it's the result of a calculation

Choosing the correct output for a Viessmann Vitotron boiler is not a matter of intuition or chance – it is the result of specific input data about your building. Heated area, envelope condition, climatic zone, usage pattern, and the requirement for water heating are the parameters from which the correct choice is calculated, not estimated. The approximate tables and formulas in this article will help you make a meaningful initial estimate. For a final decision on larger or atypical installations, it is always wise to consult a professional or have a thermal calculation prepared according to EN 12831.

At the same time, Vitotron 100 output modulation gives you much greater comfort than a fixed-output boiler – the system finds the right operating point between minimum and maximum on its own and heats exactly as much as needed. This reduces costs, extends lifespan, and ensures even thermal comfort throughout the entire heating season.

Do you have a question on this topic?

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