What power output of a Thermona THERM EL electric boiler do I need for my house?

Why Choosing the Right Boiler Output Matters

When you're deciding on an electric boiler Thermona THERM EL, one of the first and most important questions is: what output do I actually need? It seems like a simple question, but practice tells a different story. In practice, we see both extreme cases – undersized boilers that can't keep the house warm even at low outdoor temperatures, as well as oversized boilers that switch on and off constantly, wear out faster, and don't operate economically at all.

Correctly sizing the output of an electric boiler isn't just about comfort. It's primarily about energy and economic efficiency. Electricity is currently the most expensive heat source on the market, so every extra kilowatt the boiler unnecessarily consumes is directly reflected in your electricity bill. Choosing the right output allows you to fully utilize stepped output control (which we also discuss in a separate article, Stepped output control in Thermona THERM EL boilers – how it works and why it matters) and maintain long, steady heating cycles, which are most favorable for both the boiler and the heating system.

Basic Principles of Calculating a Building's Heat Loss

The boiler's output must cover the so-called heat loss of the building – i.e., the amount of heat the house loses to its surroundings at the design outdoor temperature. In Slovakia, a design outdoor temperature of –15 °C is used for most areas (–18 °C to –22 °C in mountainous regions). Heat loss depends on several factors simultaneously and is never just about floor area.

An exact heat loss calculation is performed according to the STN EN 12831 standard, which takes into account every structural detail. For practical boiler sizing purposes, however, it's primarily necessary to understand the main variables that influence the result.

Factors That Determine Heat Loss

• Age and type of building – older masonry houses from the 1960s and 1970s without insulation can have specific heat losses as high as 80–120 W/m², while a new low-energy standard house reaches 20–35 W/m²
• Thickness and type of thermal insulation – an external thermal insulation composite system (ETICS) with 15–20 cm of EPS fundamentally changes the balance; the difference between an uninsulated and insulated house can be as much as 50–60% in heat loss
• Window type – old single or double wooden windows vs. plastic triple glazing (Uw ≈ 0.7–1.0 W/m²K) makes a huge difference; windows and doors account for up to 25–35% of total heat loss in uninsulated houses
• Floor height and built-up area – the boiler doesn't heat m², but the volume of air and the area of the enclosing structures
• Location and orientation of the house – a detached house loses heat from all sides, while a terraced house has a side protected by neighbors
• Altitude and climatic zone – a house in Bratislava and a house in Liptovský Mikuláš require different output even with the same type of construction
• Underfloor heating vs. radiators – affects the required water temperature, not the boiler output directly, but indirectly affects overall efficiency

Approximate Output Calculation – Practical Method for Everyday Customers

Although an exact heat loss calculation belongs in the hands of a designer, a fairly reliable empirical method is sufficient for approximate boiler sizing. The result isn't accurate to tenths of a kilowatt, but it's fully sufficient for selecting a power level from the THERM EL range.

The basic formula is simple:

Required output [kW] = Heated area [m²] × Specific heat output [W/m²] ÷ 1000

Where the specific heat output depends on the condition of the building:

• 20–25 W/m² – passive house, low-energy house with excellent insulation (window Uw ≤ 0.8, EPS thickness 20+ cm)
• 30–40 W/m² – low-energy new build or well-renovated house with 15 cm insulation, triple glazing
• 45–60 W/m² – standard post-2010 new build or older house with 8–12 cm insulation and plastic windows
• 65–80 W/m² – house from the 1980s–90s, partially renovated, plastic windows but no facade insulation
• 80–120 W/m² – old masonry house without insulation, single or old double glazed windows

Real-Life Examples – Calculations for Typical Slovak Houses

Example 1: 120 m² family house, 2018 new build, standard
The house has double-glazed plastic windows, 10 cm EPS facade insulation, and underfloor heating. We estimate the specific output at 45 W/m².
Calculation: 120 × 45 = 5,400 W = 5.4 kW
Conclusion: Thermona THERM EL 8 with a maximum output of 8 kW is sufficient, with enough reserve for domestic hot water heating (if a tank is connected) and to handle even extremely frosty periods.

Example 2: Older 68 m² apartment in a panel building, fully renovated
Windows replaced with triple glazing, facade insulation (from the outside of the apartment building), radiators. Specific output ≈ 35 W/m².
Calculation: 68 × 35 = 2,380 W = 2.4 kW
Conclusion: Here too, THERM EL 8 (8 kW) is sufficient, and the boiler will operate at a lower power level for most of the season, which is very economical.

Example 3: Older 180 m² masonry house, uninsulated, old wooden windows
A typical 1960s house with 38 cm brick walls and no insulation whatsoever. Specific output 90 W/m².
Calculation: 180 × 90 = 16,200 W = 16.2 kW
Conclusion: Here the customer should choose Thermona THERM EL 23 (23 kW). At the same time, this is a strong argument for a comprehensive thermal renovation of the building – with 15 cm of EPS insulation and new windows, the required output would drop to 6–8 kW, and the electricity savings would be dramatic.

Example 4: Large 280 m² family house, 2020 new build, above-standard insulation
20 cm EPS thickness, triple glazing, heat recovery ventilation, underfloor heating. Specific output ≈ 28 W/m².
Calculation: 280 × 28 = 7,840 W = 7.8 kW
Conclusion: Despite the large area, THERM EL 8 or at most THERM EL 15 is enough as a reserve for extreme frost or when a domestic hot water tank is connected.

The Thermona THERM EL Series – Overview of Available Outputs

The THERM EL series is available in five power variants, covering practically the entire range of family houses, from small bungalows to spacious villas or smaller commercial buildings. It's important to understand that each model doesn't operate only at full output – thanks to stepped control, the boilers work at reduced output, which is key to operating efficiency.

• Thermona THERM EL 8 – maximum output 8 kW, suitable for well-insulated houses up to 150–200 m², new builds up to 280 m², apartments
• Thermona THERM EL 15 – maximum output 15 kW, typical family house of 120–200 m² in a standard condition, or a larger low-energy house
• Thermona THERM EL 23 – maximum output 23 kW, larger older house of 150–250 m² or a large modern house with a large domestic hot water tank
• Thermona THERM EL 30 – maximum output 30 kW, large buildings, older houses over 200 m² without insulation, buildings with higher hot water needs
• Thermona THERM EL 38 – maximum output 38 kW, large family houses over 250 m² with a poorer thermal insulation standard, smaller commercial premises

Domestic Hot Water Heating – An Added Load That's Often Forgotten

One of the most common mistakes when sizing an electric boiler is that the customer only thinks about heating and forgets about domestic hot water (DHW). If a DHW tank (boiler cylinder) is connected to the boiler, it must also cover this load. And here's where the problem arises – tank heating and space heating aren't normally simultaneous, but while the tank is being heated, the boiler works only for DHW and heating output may temporarily drop.

A practical rule for houses with a connected DHW tank:

• 1–2 person household, 80–120 l tank – add 1.5–2 kW to the calculated heat loss of the house
• 3–4 person household, 150–200 l tank – add 2.5–3.5 kW
• 5+ person household or 300+ l tank – add 4–6 kW; in some cases, consider separate tank heating

Example: A customer has a house with a heat loss of 7 kW and wants to connect a 150 l tank for a family of 4. Total required output = 7 + 3 = 10 kW. THERM EL 15 would be the right choice with a good reserve, while THERM EL 8 wouldn't keep up under extreme conditions.

Design Temperature and Climatic Zones of Slovakia

Slovakia is diverse in terms of climatic conditions. What applies to Dunajská Streda doesn't apply to the High Tatras. When sizing a boiler, it's important to know the design winter temperature for your location, since this value determines the output the boiler must be capable of during the coldest period.

For reference, here are the typical design temperatures for various regions of Slovakia:

• –12 °C – southwestern Slovakia (Bratislava, Trnava, Nitra – lowland areas)
• –15 °C – most of central and eastern Slovakia (Trenčín, Žilina, Banská Bystrica, Prešov, Košice)
• –18 °C – mountainous areas, basins (Liptovský Mikuláš, Poprad, Ružomberok)
• –22 °C and lower – Orava, Kysuce, high mountain areas

If your house is located in an area with a design temperature of –18 °C and the heat loss was calculated for –15 °C, it's recommended to increase the calculated output by about 15–20%. This is another reason why a slight power margin – around 10–20% above the calculated value – is always recommended when choosing a boiler.

Why an Oversized Boiler Isn't a Guarantee of Comfort

Many customers argue: "I'd rather get a bigger one, just to have a reserve." This logic has some merit, but only up to a point. A slight output margin of 20–30% is fine – the boiler has a reserve for DHW, for very cold days, and for potential expansion of the heating system. But a significantly oversized boiler brings real problems.

An oversized electric boiler operates most of the season at only the lowest power level, which isn't a problem in itself. The problem arises when the boiler is set up incorrectly and the water temperature control is set too high. The boiler then reaches the required temperature too quickly, switches off, cools down quickly, and switches on again. Short heating cycles ("short-term firing") put strain on electrical circuits and switches and cause unnecessary temperature fluctuations in the heating system.

Moreover – a larger THERM EL 38 boiler is more expensive to purchase than a THERM EL 15. If the house doesn't actually need 38 kW, the customer unnecessarily pays significantly more for equipment without any practical benefit. That's why precise sizing is also genuinely important economically.

When Calculation Isn't Enough – When You Need a Designer

The approximate calculation described in this article is sufficient for most typical family houses and common situations. However, there are cases where it's necessary to have a professional energy audit or a heat loss calculation performed according to the STN EN 12831 standard:

• Atypical houses with large glazed areas (winter gardens, glass ceilings)
• Historic buildings with non-standard structural elements
• Mixed-use buildings (residential + commercial)
• Houses in extreme climatic conditions (altitude above 1000 m)
• Combined heating systems (e.g., THERM EL as a backup for a heat pump)
• Any building with a floor area over 500 m²

For all other common family houses, the procedure described in this article is fully sufficient for correctly selecting a model from the THERM EL range. More detailed selection criteria, including further technical parameters, can be found in the article How to choose a Thermona THERM EL electric boiler – output, space size, and other criteria.

Practical Table – Recommended THERM EL Model by Area and Building Condition

Building condition	up to 80 m²	80–150 m²	150–220 m²	220–300 m²	over 300 m²
Passive / low-energy house (20–30 W/m²)	EL 8	EL 8	EL 8	EL 15	EL 15–23
New build, good standard (35–50 W/m²)	EL 8	EL 8	EL 15	EL 15–23	EL 23–30
Renovated house, insulation + new windows (55–70 W/m²)	EL 8	EL 15	EL 15–23	EL 23	EL 30–38
Older house, plastic windows, no insulation (75–90 W/m²)	EL 8	EL 15	EL 23	EL 23–30	EL 30–38
Old house with no improvements at all (90–120 W/m²)	EL 15	EL 15–23	EL 23–30	EL 30–38	EL 38 +

Note: The values in the table assume a climatic zone of –15 °C and no DHW tank. For areas with –18 °C or colder, or when a DHW tank is connected, choose a model one level higher.

Electrical Requirements of Individual Models and Connection Requirements

THERM EL electric boilers are three-phase appliances (400 V, 3×N+PE), which is a necessary condition for outputs above 8 kW. A single-phase connection isn't possible – a three-phase connection is always required. This is one of the practical limits you need to consider before purchasing. If your house only has a single-phase connection (230 V), installing a THERM EL boiler isn't possible without first upgrading the electrical connection and installation, which needs to be arranged with the relevant electricity distributor.

Each model requires a different circuit breaker and cable cross-section:

• THERM EL 8 (8 kW) – current load approx. 11.6 A per phase, 3×16 A breaker, cable min. 4 mm²
• THERM EL 15 (15 kW) – current load approx. 21.7 A per phase, 3×25 A breaker, cable min. 6 mm²
• THERM EL 23 (23 kW) – current load approx. 33.3 A per phase, 3×40 A breaker, cable min. 10 mm²
• THERM EL 30 (30 kW) – current load approx. 43.5 A per phase, 3×50 A breaker, cable min. 16 mm²
• THERM EL 38 (38 kW) – current load approx. 55 A per phase, 3×63 A breaker, cable min. 16–25 mm²

These values are indicative, and the final design of the electrical installation must always be carried out by a person with the appropriate electrotechnical qualification. You can find more about electrical requirements and the installation process in the separate article Installing the Thermona THERM EL electric boiler – procedure, electrical installation requirements and inspection.

The HDO Tariff and the Possibility of Optimizing Boiler Size

Many households with an electric boiler use the HDO tariff (remote-controlled load switching) – i.e., cheap electricity at night, or possibly on weekends or in the evening. If you're considering this tariff, it can affect boiler sizing. If the boiler also runs at night on cheap electricity and stores heat for the whole day in the DHW tank and/or a buffer tank, you may be able to get away with a lower boiler output, which is compensated by a longer operating time at a lower electricity price.

On the other hand – on frosty days, when HDO limits boiler operation to just a few hours, the boiler must be able to heat the system sufficiently within that window. This can, on the contrary, lead to the need for a higher output. This is an area where it's worth consulting a specialist who can design the entire system, including the buffer tank and the right tariff choice.

House Renovation and Changing the Boiler Output – What Changes After Insulation

A very practical situation we encounter quite often: a customer has bought an older house, plans to renovate it gradually, and is now dealing with heating. If insulation and window replacement are still two to three years away, what now?

In this case, it makes sense to consider buying a boiler that also covers the current, non-compliant state of the building, and after renovation, operate it at lower power levels. For example, a 160 m² house in its original condition (specific output 90 W/m²) needs 14.4 kW – you'd choose THERM EL 15. After insulation (specific output drops to 50 W/m²), the same boiler will operate at only about 8 kW, i.e., at lower levels, which is still efficient operation thanks to the stepped control.

Conversely – buying a small boiler with the expectation that "I'll insulate it eventually" is a gamble. Until the renovation happens, you'll have an underheated house and the boiler will run at maximum output continuously, which isn't ideal for either the boiler or your electricity bill.

Underfloor Heating vs. Radiators – Effect on Output Selection

The type of heat-emitting surfaces (underfloor heating or radiators) doesn't directly affect the house's heat loss, and therefore doesn't directly affect the required boiler output. Heat loss is determined by the building's construction, not the method of heat distribution. However, there are several indirect effects to keep in mind:

• Underfloor heating works with lower water temperatures (35–45 °C), which is favorable for the THERM EL boiler in terms of long, calm operation. The system has greater thermal inertia and reacts more slowly to temperature changes.
• Radiators typically require water temperatures of 55–75 °C. The boiler must heat the water to a higher temperature, which takes longer at the same output. A radiator system reacts faster to changes.
• A combined system (underfloor heating + radiators in the bathroom or hallway) is common and doesn't require special sizing, just correct hydraulic setup of the system.

With underfloor heating in a well-insulated house, it sometimes happens that a customer buys a boiler with a higher output than necessary because they're worried that "underfloor heating isn't enough." This is a misguided concern – underfloor heating is very efficient when properly designed, and the THERM EL boiler works excellently with it. Details on hydraulic connection can also be found in the article Connecting the Thermona THERM EL to the heating system – wiring diagrams and hydraulic requirements.

The Most Common Mistakes in Output Selection – What We See in Practice

Over the years, we've seen almost every possible scenario. Here are the most common mistakes customers make when choosing the output of an electric boiler:

• Counting the entire gross area of the house including the garage, basement, and unheated spaces – the heated area is only where you actually want it warm
• Ignoring the insulation factor – two customers with equally sized houses may need dramatically different boiler outputs
• Forgetting about DHW – without accounting for the tank, you won't calculate the heat needed for DHW and may end up with an undersized boiler
• Choosing based on a "neighbor's recommendation" – every house is different, and a neighbor's experience with their specific house may not apply to yours at all
• Choosing based on price alone – the cheapest model may not be right for your house and could end up being more expensive to run in the long run
• Underestimating the climatic zone – a house in a mountainous area needs significantly more output than an equally sized house in southwestern Slovakia

Frequently Asked Questions (FAQ)

Can I put a THERM EL 38 boiler in a small 80 m² house if I'm not sure about the calculation?

Technically yes, but it doesn't make sense economically or practically. The THERM EL 38 boiler is significantly more expensive to purchase, requires a stronger electrical connection, and in a small house it will operate almost exclusively at the lowest power level, which isn't economical. For an 80 m² house in standard condition, a THERM EL 8 is sufficient, or at most a THERM EL 15 if the house has poorer thermal insulation or a large DHW tank connected.

How do I know if my house needs 15 kW or 23 kW – is there a big difference between them?

The difference is 8 kW of output, which is quite significant for electric boilers – it means a different power level, different electrical installation requirements, and a different equipment price. If your calculation results in a required output somewhere between 12 and 18 kW, the decision between EL 15 and EL 23 depends on whether you have a DHW tank, what climatic zone you live in, and whether you're planning a renovation. In such a borderline situation, it's better to choose EL 23 – you'll have a larger reserve and the whole system will be less strained.

I inherited a boiler from the previous owners. Is 30 kW really oversized for our 120 m² house?

For a new build or a well-insulated 120 m² house, a THERM EL 30 (30 kW) is significantly oversized – the required output is likely 5–9 kW. If the house isn't insulated, the required output could be 10–11 kW, which is still far less than 30 kW. The boiler will operate almost exclusively at the lowest levels. Operating at lower output isn't a problem in itself, but from an economic point of view, you've invested in an unnecessarily powerful device. When replacing the boiler in the future, choose the correct sizing.

I'm planning to insulate the house in two years. Should I buy a bigger or smaller boiler now?

We recommend buying a boiler that matches the current state of the house, possibly with a mod