How to Choose an Electric Boiler Protherm Ray – Output Based on House Area
Protherm Ray – how to choose the right output for your home
The Protherm Ray electric boiler has long been one of the most popular electric boilers on the Slovak market. That's no coincidence – it's a solidly designed appliance with modulated output, good control, and relatively simple operation. Nevertheless, customers very often make mistakes when choosing the right model. Either they choose a boiler that's too weak and can't heat the house, or – even more commonly – they oversize it unnecessarily and pay more than they need to. In this article, we'll go through the entire selection process step by step, with real values, examples from practical experience, and practical tables.
If you want to compare specific outputs across the entire range (6 to 28 kW), we recommend the article What Protherm Ray output do I need – 6, 9, 12, 14, 18, 21, 24 or 28 kW? in the same Knowledge Center. Here, we'll focus mainly on the practical calculation methodology and specific models available in our offer.
Why boiler output depends on more than just area
The first mistake people make: they take the house area, multiply it by some "rule of thumb," and consider the result final. But the required boiler output depends on a combination of several factors at once. Area is just an input variable, not the whole formula.
Key factors include:
- Heat loss of the building envelope – this depends on the thickness and type of insulation of the walls, roof, floor, on the quality of windows and entrance doors, on the compactness of the building's shape, and on exposure to wind.
- Volume of heated space – remember that a boiler heats volume, not area. A multi-storey house with high ceilings has different requirements than a single-storey bungalow of the same floor area.
- Climate zone – Slovakia is not a homogeneous whole. The design outdoor temperature in Bratislava is -11 °C, in Banská Bystrica -13 °C, in Poprad -15 °C, and in some mountain villages as low as -18 °C. This difference fundamentally changes the sizing.
- Type of heating system – underfloor heating operates at a lower water temperature (typically 35–45 °C), radiator systems at 55–70 °C. The Protherm Ray itself can handle both cases, but this is relevant for calculating heat loss.
- Age and condition of the building – a family house from the 1980s without insulation can have a heat loss 3–4× higher than a new build of the same area with quality insulation.
- Domestic hot water preparation – if the boiler is also used to heat water in a tank, you need to add the required output for hot water heating to the heating output. Protherm Ray has a built-in 5-litre expansion tank, but to heat a larger DHW tank you must have sufficient output.
Heat loss calculation methodology – the basis of correct selection
An accurate heat loss calculation is done according to the standard STN EN 12831. In practice, this is done by a designer or HVAC specialist. For an approximate selection, however, an adjusted empirical method that takes the main variables into account is sufficient.
Basic formula for approximate calculation of required output:
P [kW] = (Area [m²] × ceiling height [m] × heat loss coefficient) / 1000
The heat loss coefficient (in W/m³K) falls within the following ranges:
| Building type | Coefficient (W/m³K) |
|---|---|
| New build with excellent insulation (passive, A0–A1) | 0.20 – 0.30 |
| Low-energy house, new build (A2–B) | 0.30 – 0.45 |
| Insulated house after renovation (C) | 0.45 – 0.65 |
| Older house, partially insulated | 0.65 – 0.90 |
| Old buildings without insulation (panel, brick built before 1990) | 0.90 – 1.30 |
The result of this formula still needs to be multiplied by the temperature difference – the difference between the desired indoor temperature (e.g. 20 °C) and the design outdoor temperature in your location. Since the formula above is simplified and assumes a typical Δt ≈ 33 °C (indoor temperature 20 °C, outdoor -13 °C), adjust the result accordingly for other climate zones.
Practical example No. 1 – new build 120 m², Žilina
Family house, area 120 m², ceilings 2.6 m, new build category B (low-energy). Žilina, design temperature -15 °C.
Volume: 120 × 2.6 = 312 m³. Coefficient: 0.40 W/m³K. Temperature difference: 20 – (-15) = 35 K.
Heat loss: 312 × 0.40 × 35 = 4,368 W ≈ 4.4 kW.
Conclusion: a 6 kW boiler is sufficient for heating alone, with a margin. If we also want to heat a 200-litre DHW tank, it's reasonable to consider the Protherm Ray 9KE.
Practical example No. 2 – renovated house 180 m², Nitra
Older brick house from 1975, after façade insulation with 15 cm polystyrene, new windows, area 180 m², average ceiling height 2.7 m. Nitra, design temperature -12 °C.
Volume: 180 × 2.7 = 486 m³. Coefficient: 0.55 W/m³K. Temperature difference: 20 – (-12) = 32 K.
Heat loss: 486 × 0.55 × 32 = 8,553 W ≈ 8.6 kW.
Conclusion: the ideal candidate is the Protherm Ray 9KE, or the Protherm Ray 12KE if a 150–200 l DHW tank is added.
Practical example No. 3 – older panel-building flat 70 m², Banská Bystrica
Panel-building flat, area 70 m², ceilings 2.55 m, insulated panel building, middle floor. Banská Bystrica, design temperature -13 °C.
Volume: 70 × 2.55 = 178.5 m³. Coefficient: 0.50 W/m³K (insulated panel building, interior flat). Temperature difference: 20 – (-13) = 33 K.
Heat loss: 178.5 × 0.50 × 33 = 2,945 W ≈ 3.0 kW.
Conclusion: the Protherm Ray 6KE with a substantial margin; output modulation will ensure economical operation.
Overview of Protherm Ray models – parameters and suitable use
The Protherm Ray range is available in outputs of 6, 9, 12, 14, 18, 21, 24, and 28 kW. On atria.sk you'll find the complete models from 6 to 18 kW. Let's break down each model in detail in terms of suitability of use:
Protherm Ray 6KE – for small houses and flats
The Protherm Ray 6KE is the entry-level model of the range with a modulation range of 2–6 kW (in three stages: 2, 4 and 6 kW). It is suitable for:
- New builds or low-energy houses with an area up to 80–90 m² in warmer locations (southwest Slovakia).
- Insulated flats in panel or brick buildings up to 70–75 m².
- Cottages and recreational properties with occasional heating up to 60 m².
- Secondary heat sources in combined systems (e.g. fireplace + electric boiler as backup).
This model runs on single-phase 230 V power at outputs up to 3.5 kW; at higher stages, three-phase 400 V is required. Always check this when planning the electrical installation – more can be found in the article Connection and installation of Protherm Ray – requirements for electrical installation and fuses.
Protherm Ray 9KE – the most popular model in practice
The Protherm Ray 9KE is the best-selling model in practice. Output is modulated from 2 to 9 kW. It covers a wide range of households:
- Family houses of 100–140 m² after renovation (insulation + new windows) in medium climate zones.
- New builds of 120–160 m² category A or B.
- Combination of heating + heating an 80–120 l DHW tank in houses up to 100 m².
From the electrical installation perspective, it requires three-phase 400 V power and 3×16 A fuses. This is the boiler that best "fits" the average Slovak household without oversizing or the risk of insufficient output.
Protherm Ray 12KE – the golden middle for larger houses
The Protherm Ray 12KE modulates output from 3 to 12 kW. Typical deployment:
- Family houses of 150–200 m² after renovation in a medium climate zone.
- New builds of 180–220 m² in colder locations (Tatras, Orava, Kysuce).
- Houses with large DHW tanks (200 l and more) or with multiple bathrooms.
- Older houses with partial insulation up to 160 m².
Protherm Ray 14KE – for larger and less insulated houses
The Protherm Ray 14KE is advantageous when your heat loss calculation comes out between 11 and 13.5 kW. Jumping straight to 18 kW would be an unnecessary oversizing. Typical use:
- Older family houses without sufficient insulation, area 180–220 m².
- Houses with high ceilings (historic villas, old townhouses).
- Combination of heating + heating a large DHW tank (300 l) in a more demanding climate.
Protherm Ray 18KE – for demanding conditions and large properties
The Protherm Ray 18KE modulates output from 4 to 18 kW. It is the top model available in this category on atria.sk. Suitable for:
- Large family houses over 220–250 m² with below-average insulation.
- Old rural houses without insulation with ceilings of 3 m+ (area 200–260 m²).
- Properties with specific backup requirements (e.g. a cottage at high altitude with a design temperature of -18 °C).
- Single-storey administrative buildings or smaller-scale operations.
Approximate table: boiler output by area and type of house
The following table serves as a quick reference guide. The values assume a ceiling height of 2.6 m and a design outdoor temperature of -13 °C (central Slovakia). For more extreme conditions (Tatra region, northern areas), always choose one level higher model.
| House area | New build / low-energy | After renovation / insulated | Older building / uninsulated |
|---|---|---|---|
| up to 60 m² | Ray 6KE | Ray 6KE | Ray 6KE – 9KE |
| 60 – 100 m² | Ray 6KE | Ray 6KE – 9KE | Ray 9KE – 12KE |
| 100 – 150 m² | Ray 9KE | Ray 9KE – 12KE | Ray 12KE – 14KE |
| 150 – 200 m² | Ray 9KE – 12KE | Ray 12KE – 14KE | Ray 14KE – 18KE |
| 200 – 250 m² | Ray 12KE | Ray 14KE – 18KE | Ray 18KE |
| over 250 m² | Ray 12KE – 14KE | Ray 18KE | Ray 18KE (or possibly 21–28 kW) |
Why not to oversize – output modulation and its benefits
Many customers think that a bigger boiler = better assurance. In reality, the opposite is true: an oversized electric boiler operates in short cycles (so-called cycling), which increases wear, causes greater temperature fluctuations in the house, and increases energy consumption.
The Protherm Ray is equipped with modulated output – meaning it doesn't switch on fully or off completely, but smoothly adjusts output (in 2 kW steps) to current demand. The 9KE model can thus operate from 2 kW in mild weather up to 9 kW during frost. If you covered this house with an 18KE model, the boiler would almost always run at minimum output and its regulation would be less efficient.
Rule of thumb from practice: choose a model where your heat loss calculation comes out to 70–90% of the boiler's maximum output. You'll get sufficient reserve for domestic hot water preparation and extreme frost, but the boiler will operate in the efficient mid-range of modulation for most of the year.
Domestic hot water heating – how it affects the choice of output
The Protherm Ray does not have a built-in domestic hot water (DHW) tank – it's a boiler intended exclusively for heating and DHW preparation via an external tank. If the boiler is also used for DHW heating, the required output for heating the tank must be added to the heat loss for heating.
The output required to heat a DHW tank depends on the tank volume and the desired heating time. A practical reference value: to heat a 150 l tank from 15 °C to 55 °C in 1 hour, you need approximately 7 kW. In 2 hours, 3.5 kW is sufficient. You can set the heating time during the night tariff period (when electricity is cheaper), which is one of the great advantages of an electric boiler.
In practice, this means: a house with a heat loss of 8 kW and a 150 l tank that is heated during the night tariff (2 hours available) doesn't need the DHW output at the same time as heating. The Protherm Ray 9KE can handle both tasks alternately. If you want both functions simultaneously (which is less common), you need to add up the outputs.
Electrical connection and its impact on model selection
This is a factor that is most often forgotten in practice. Before choosing a model, always check the maximum allowed load on your electricity meter and what type of fuse protection you have available.
Protherm Ray 6KE: maximum power input 6 kW, power supply 230/400 V, fuses 1×25 A or 3×10 A.
Protherm Ray 9KE: maximum power input 9 kW, power supply 400 V, fuses 3×16 A.
Protherm Ray 12KE: maximum power input 12 kW, power supply 400 V, fuses 3×20 A.
Protherm Ray 14KE: maximum power input 14 kW, power supply 400 V, fuses 3×25 A.
Protherm Ray 18KE: maximum power input 18 kW, power supply 400 V, fuses 3×32 A.
If you have a standard household with a main fuse of 3×25 A, the Protherm Ray 14KE is a practical maximum – higher models would require an increase in the main fuse and possibly also an increase in the reserved power input from the distributor (which is an administrative and financial process). More details on electrical installation requirements can be found in the article Connection and installation of Protherm Ray – requirements for electrical installation and fuses.
Protherm Ray and photovoltaics – choosing output in the context of PV
More and more customers are combining the Protherm Ray with a photovoltaic power plant. In this case, a different output selection logic applies: the higher the boiler output, the more you can use surplus energy from the PV system for heating and water heating. The Protherm Ray supports control via an external input, through which the PV manager (e.g. SolarEdge, Fronius, or another system with a 0–10 V output or contact) can increase the boiler's output during a surplus and reduce it during a shortage.
So if you have a PV system with a peak output of 8–10 kWp, it may be advantageous to choose the Ray 12KE or 14KE model even for a house that would otherwise be fine with the 9KE – thanks to the higher output, the boiler can more effectively absorb surplus energy into the DHW tank during the summer period. We cover this topic in more detail in the article Protherm Ray and photovoltaics – how to use surplus energy from solar panels.
Practical tips and mistakes to avoid
Over the years of practice, we've encountered several recurring mistakes in choosing boiler output. Here are the most common ones:
- Forgetting about underfloor heating in the calculation – underfloor heating feeds the entire area and puts a large water volume into the hydraulic system. The system reacts more slowly, but also holds the temperature longer. The boiler output doesn't change, but you need to account for a larger tank volume in the system.
- Using the "100 W/m² rule" without considering context – this rule applied to old buildings from the 1980s. For a new build, it's an extreme oversizing (actual need may be 30–50 W/m²).
- Ignoring the influence of the house's location – a house on a hill exposed to the north wind can have a heat loss 20–30% higher than an identical house in a sheltered valley.
- Choosing output based on the old gas boiler – the previous gas boiler was usually oversized. If you're replacing gas with electricity, always do a new calculation, don't rely on the output of the old boiler.
- Not considering the need for regulation and OpenTherm – the Protherm Ray supports control via OpenTherm, an equithermal curve, and an outdoor sensor. Proper regulation setup can save 10–20% energy, but requires a compatible thermostat. More in the article Setup and programming of Protherm Ray – regulation, equithermal curve, OpenTherm.
Ray in combination with another heat source
The Protherm Ray is very well suited as a supplementary or backup source in combined systems. A typical configuration is a heat pump as the main source + Protherm Ray as a bivalent backup source for extreme frosts. In this case, it's not necessary to size the Ray for the entire heat loss of the house – it's enough to cover the peaks that the heat pump can't handle (usually from -10 °C downward).
If, for example, you have a heat pump with a COP of 3 and an output of 10 kW, and the house has a heat loss of 12 kW at -15 °C, a backup Ray 6KE or 9KE is sufficient – in this context, you don't need to size it for the full heat loss.
Frequently Asked Questions (FAQ)
How do I find out my house's heat loss without a designer?
You can't determine it exactly without a designer, but you can manage an approximate calculation yourself using the formula in this article: house volume (m³) × heat loss coefficient by building type (W/m³K) × temperature difference (K) = heat loss in watts. For a house where you're unsure of the insulation type, use a higher coefficient value – it's the safer side. For an accurate calculation according to STN EN 12831, we recommend an HVAC designer, who also takes into account thermal bridges, ventilation, and infiltration.
Can I use a Protherm Ray 18KE in a house where my calculation comes out to only 8 kW?
Technically yes, but practically it's not advisable. The boiler would operate in short cycles at minimum output (4 kW), which is less efficient and increases wear. In addition, it costs more and requires stronger fuse protection. Ray's modulation does help, but optimal operation occurs when the load corresponds to 70–100% of rated output. For a house with a heat loss of 8 kW, the correct choice is the Ray 9KE or at most the Ray 12KE.
Ray 9KE or Ray 12KE – how to decide when my calculation comes out right on the border?
If your calculation comes out between 8 and 10 kW and you're also planning to heat a DHW tank (100–200 l), choose the Ray 12KE. If you don't need a DHW tank or have a separate storage water heater, the Ray 9KE is sufficient, even with a small reserve for colder days. Also, if you live at an altitude above 600 m or in an area with a design temperature of -15 °C or lower, always choose one level higher model.
Will the choice of boiler output affect my insurance or electrical connection?
Yes, significantly. Each Protherm Ray model has different fuse protection requirements. For the Ray 18KE, you need a 3×32 A circuit breaker and an appropriate cable (typically CYKY 4×10 mm²). Before purchasing, always check the main circuit breaker in your distribution board and compare it with the values in the technical sheet of the selected model. If you need to increase the main circuit breaker or reserved power input, contact your electricity distributor – the approval process can take several weeks.
Does the approximate table also apply to recreational cottages and cabins?
For recreational properties, it applies with an important adjustment: cottages are usually less well insulated and used intermittently – a cold building needs to be heated up quickly. This increases the required output during the start-up phase. For cottage use, we therefore choose an output 20–30% higher than would result from a static calculation for permanent residence. In addition, if the cottage is in a mountain location with a design outdoor temperature of -18 °C, this value must be taken into account carefully.
What if my house's area increases due to an extension? Should I buy a bigger boiler right away?
If you're planning an extension within 2–3 years, yes, it's reasonable to consider the future area when choosing a boiler. The Protherm Ray isn't easily changed – it just needs reprogramming, not a boiler replacement, but the output is determined by hardware. If you buy a Ray 9KE today and add a 40 m² extension in three years, it may turn out that the Ray 9KE won't be sufficient and you'll have to replace it with a 12KE. Investing in a higher model now could save you costs later.
Conclusion – how to correctly choose a Protherm Ray
Choosing the right output for the Protherm Ray electric boiler isn't rocket science, but it does require a systematic approach. It's not enough to rely on rough rules like "100 W per square metre" – these rules lead either to oversizing (and unnecessary costs) or undersizing (and a cold house). The correct approach is: determine the actual volume of the house, estimate the heat coefficient according to the condition of the building, insert the design outdoor temperature for your location, and add the output for DHW preparation if needed.
Then choose the model from the Protherm Ray range whose maximum output corresponds to 110–130% of your calculated heat loss. Output modulation will ensure the boiler operates efficiently even in milder weather. And if you're not sure – write to us or call us, we'll be happy to help you make the right choice before purchasing.
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 advise.