What power of Protherm Ray do I need – 6, 9, 12, 14, 18, 21, 24 or 28 kW?

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Protherm Ray – how to choose the right output for your home?

When a customer comes with the question "which Ray do I need?", they usually think it's enough to state the floor area of the house and that's it. In practice, it's not that simple – but it's not dramatically complicated either, if you know what to look for. Years of selling electric boilers have shown time and again that output is just one of several parameters, and underestimating or overestimating it has real consequences: either you freeze, or you pay unnecessarily for excess output that the boiler will never fully use.

In this article, we'll go through every available Protherm Ray variant – from the smallest six to the largest available model – and show who each of them is really suitable for. This isn't about marketing recommendations, but concrete numbers, examples and real-life scenarios that will help you decide correctly.

What actually determines the required boiler output?

Before we look at the individual models, we need to explain what actually affects a building's heat loss – and therefore the required output of the heat source. This isn't an academic exercise, but the foundation without which any recommendation is meaningless.

Heat loss of the building – a key concept

Every building constantly loses heat through walls, roof, floor, windows and doors. The worse the insulation, the faster this heat escapes. The boiler must compensate for these losses – and it must do so in the coldest weather that actually occurs at that location. Heat loss is expressed in watts (W) or kilowatts (kW) and is calculated for the so-called design outdoor temperature, which in most of Slovakia is −12 to −15 °C (in mountain areas even −18 °C).

Heat loss depends mainly on:

Level of insulation – an old brick house from the 1970s without insulation can have a heat loss 5–8 times higher than a new low-energy building of the same floor area
Size and orientation of windows – triple-glazed windows are much better than old single-glazed ones, but large glazed areas facing north are always an issue
Ceiling height – a house with a 100 m² floor area and 3 m ceilings has 50% more volume than the same house with 2 m ceilings, and therefore higher heat loss
Climate zone – Bratislava vs. the Tatra basin can mean a 30–40% difference in calculated output in practice
Number of floors and shape of the building – compact buildings lose less heat than sprawling ones with a large envelope surface area

A rule of thumb commonly used in practice states: 30–50 W/m² for a well-insulated new building, 60–80 W/m² for a typically renovated house and 80–120 W/m² or more for an older, uninsulated house. These numbers should be taken only as a first approximation – the actual calculation according to the standard may turn out differently.

Why isn't the formula "area × constant" enough?

I encounter this regularly: a customer comes with the calculation "I have 150 m², so I need 10 kW". This approach can work as a first approximation, but it can also completely mislead you. The owner of a two-storey family house from 1975 with original windows and no insulation may have a heat loss of 15–18 kW at 150 m². The owner of a passive house of the same floor area will manage with 4–5 kW. That's a threefold difference for the same floor area.

That's why the ideal solution is to have an energy heat loss calculation done – this can be performed by a designer, an energy auditor, or at least a quality online calculator with detailed input. If that's not possible, you'll find practical rules for individual outputs below.

Overview of available Protherm Ray outputs

The Protherm Ray series is available in several output variants. On the Slovak market, models from 6 to 28 kW are commonly available, although not every output is always in stock with every retailer. Specific models available at atria.sk include Protherm Ray 6KE, Protherm Ray 9KE, Protherm Ray 12KE, Protherm Ray 14KE and Protherm Ray 18KE.

It's important to know that Ray regulates output in stages – it's not continuous regulation, but jumps in defined steps (e.g. 2 kW). A larger model therefore doesn't mean it will always run at full output – in mild weather, even an 18 kW boiler will manage on the first stage. You can find out more about how Ray handles regulation in the article Setting and programming the Protherm Ray – regulation, weather compensation curve, OpenTherm.

Protherm Ray 6KE – who is the six for?

Protherm Ray 6KE is the smallest model in the series. With a maximum output of 6 kW, it's suitable for really narrow use cases. In practice, I encounter it mainly in these situations:

A small apartment up to 60–70 m² in an insulated apartment building with new windows – here the heat loss can be as low as 3–4 kW, so the six even has a comfortable reserve
Supplementary or backup source – the house is primarily heated by other means (heat pump, biomass), but a backup or assistance during extreme frosts is needed
A cottage or recreational cabin up to 50 m² with occasional occupancy, where full-fledged all-day heating isn't necessary
Underfloor heating in a new build to a passive or low-energy standard up to 100–120 m² – in such houses the actual heat loss doesn't drop below 3 kW even in the hardest frost

Watch out for one important limitation: the 6KE is a three-phase appliance with an input of 6 kW, which means the circuit breaker and electrical installation must be properly sized. You'll find more about installation requirements in the article Connection and installation of the Protherm Ray – requirements for electrical wiring and circuit protection.

What the six definitely is not: it's not a boiler for a typical family house of 100+ m², even if well insulated. In an average year it might just manage in milder weather, but at −15 °C it simply won't be enough and the house won't heat up properly.

Protherm Ray 9KE – the golden middle ground for apartments and small houses

Protherm Ray 9KE is a very popular model in practice. Nine kilowatts is an output that covers a wide range of real-life cases:

An apartment of 70–90 m² in a newer insulated apartment building – with reserve even for colder days
A family house up to 80 m² to a low-energy or well-insulated standard (built after 2010, with triple glazing, contact insulation of at least 12–15 cm)
A renovated terraced house up to approx. 60–65 m² with new windows and additional facade insulation
A single-storey house with underfloor heating and a good thermal envelope up to about 90–100 m²

A specific example from practice: a customer with a terraced house of 75 m² built in 2005, EPS insulation 10 cm, plastic windows. Calculated heat loss approx. 7.2 kW. Ray 9KE installed – works flawlessly, running at 7–8 kW on frosty days, and 4–5 kW for most of the season.

Protherm Ray 12KE – the most versatile model for typical family houses

Protherm Ray 12KE is, in my view, the best-selling model and also the one where I most often see correctly sized installations. Twelve kilowatts is an output that really covers:

A family house of 100–130 m² to a common insulated standard (new build after 2000 with standard insulation)
An older new build or renovation of 80–100 m² with good windows and at least partial insulation
A two-storey family house of 100 m² to a low-energy standard
A more spacious apartment of 100–120 m² (e.g. a family apartment in an older prefab building after complete renovation)

A practical example: a customer with a two-storey house of 110 m² from 2012, 15 cm insulation, triple-glazed windows, underfloor heating. Heat loss approx. 9.8 kW. Ray 12KE installed with reserve, fully satisfied, running at maximum output only during the harshest frosts, a few days a year.

Protherm Ray 14KE – a transitional model for larger houses or worse insulation

Protherm Ray 14KE is an interesting model that stands between the twelve and the eighteen. Fourteen kilowatts suits situations where 12 isn't enough, but 18 would be unnecessarily oversized and more expensive:

A renovated house of 100–120 m² with partial insulation (e.g. insulated facade, but old roof) – heat loss can be around 11–13 kW
A typical new build of 130–150 m² to a standard (not low-energy) level
An older house of 80–100 m² with new windows, but without facade insulation – loss here can be 10–12 kW
A house at higher altitude (600–800 m above sea level) with a design temperature of −18 °C, where about 15–20% needs to be added compared to lowland figures

An interesting note from practice: several times a customer wanted to play it safe and jump straight to the 18KE, but after calculation we found that the 14KE was exactly right. An oversized boiler unnecessarily increases starts and stops (short cycling), which doesn't benefit the boiler in the long run and makes regulation more difficult.

Protherm Ray 18KE – for larger family houses

Protherm Ray 18KE is a significantly stronger model, suitable for buildings with higher heat loss. Eighteen kilowatts really covers:

A family house of 150–200 m² with common insulation (windows replaced, facade insulated, but not to a low-energy standard)
An older villa or multi-generational house of 130–160 m² with partial renovation
An older house of 100–120 m² with no or minimal insulation – heat loss here can be 14–16 kW, and the Ray 18KE has the necessary reserve
A larger recreational building – a 120 m² cottage with poorer insulation in a foothill area
Combined installation with hot water preparation via a tank – the 18KE can handle tank heating without a significant drop in heating output

One typical scenario: a customer with a 180 m² house, built in 1988, windows replaced 5 years ago, original old facade. Heat loss approx. 15–16 kW. After discussion, we installed the 18KE with the plan that they'll insulate the facade next season – once insulated, the loss will drop to about 10–11 kW and the boiler will have a comfortable reserve even for hot water preparation.

With the 18KE, care must be taken with the electrical installation: an 18 kW input with three-phase connection (400 V) represents a current of approx. 26 A per phase, which requires appropriately sized circuit protection and supply cable. Don't do this without checking the existing electrical installation – see more in the article Connection and installation of the Protherm Ray – requirements for electrical wiring and circuit protection.

Protherm Ray 21, 24 and 28 kW – large outputs for larger buildings

Models with an output of 21, 24 and 28 kW are more specialised. In practice, I encounter them less often for typical family houses, but they have their place:

When 21 kW?

A family house over 180–200 m² to a standard insulated level, or an older house of around 150 m² with insufficient insulation. Also in cases where the boiler is combined with a large hot water storage tank (200–300 L) and comfortable hot water preparation without limiting heating is desired.

When 24 or 28 kW?

Here we're already in the realm of larger buildings – multi-generational houses over 200 m², larger recreational cabins, smaller apartment buildings (up to 4–6 apartments) or industrial spaces with heating. Also in cases where the Ray is part of a cascade installation of two boilers – but that's a topic for a separate article.

Note: with 24 and 28 kW, it's absolutely essential to ensure that the house's main circuit breaker and electricity supply have sufficient capacity. At 28 kW, the input is nearly 30 kW, which simply isn't enough for a common three-phase supply of 3 × 25 A (= approx. 17 kW). Most older family houses will need an increase of the circuit breaker from the distributor – which also has financial consequences.

Practical procedure: How to estimate the required output yourself?

If you don't have an energy calculation available, you can make a quick estimate using the following procedure. The result won't be accurate to the tenth of a kilowatt, but it will give you a real view of which output range you're in.

Find out the heated area of the house – only areas that are actually heated. An unheated garage or storage room doesn't count.
Determine the building type and insulation condition – see the specific output table above (30–50 / 60–80 / 80–120 W/m²).
Calculate the indicative heat loss: area × specific output. E.g. 120 m² × 60 W/m² = 7,200 W = 7.2 kW.
Add hot water preparation: if the boiler will heat a hot water tank, add 1.5–3 kW depending on family size and tank size.
Consider the climate zone: if you're above 600 m above sea level or in an area with a design temperature below −15 °C, multiply the result by a coefficient of 1.15–1.20.
Choose the nearest higher output from the available models – a bit of reserve is fine, but avoid unnecessary oversizing (more than 30–40% above the heat loss is counterproductive).

Where do people most often make mistakes when choosing output?

Over the years I've seen dozens of cases where the boiler was incorrectly sized. The most common mistakes are:

Underestimating the heat loss of old houses

A customer has an old house of 120 m² with original windows from the 80s and an original uninsulated roof. They say to themselves "well, I only have 120 m², 9 kW will do." In reality, they have a heat loss of 14–16 kW and the new boiler isn't enough, the house doesn't heat up properly. This is, from long experience, the most common mistake – an old house ALWAYS needs more.

Overestimating the effect of partial renovation

A customer replaced the windows and thinks the house is now as good as a new build. Windows account for only 10–20% of the envelope's heat losses – with uninsulated walls and an old roof, the effect of replacing windows is measurable, but far from transforming the house into a low-energy standard.

Ignoring altitude and location

A 100 m² house in Bratislava (design temperature −12 °C) and a 100 m² house in the Tatras (design temperature −18 °C) – with the same insulation, the difference in calculated heat loss is about 20–25%. That can be the difference between a 9 and a 12 kW boiler.

Forgetting hot water preparation

When Ray heats a hot water tank, it turns off heating (or significantly limits it) during the heating period. The smaller the boiler, the longer the tank heating takes and the longer the heating outage. With a correctly sized boiler, this time is short and you'll hardly notice the heating interruption.

Choosing based on price, not output

Sometimes a customer buys a cheaper, smaller model to save a few tens of euros. In practice, this means the boiler runs permanently at full output, has a shorter lifespan, higher consumption, and the house is on the edge of comfort. Correct sizing always pays off.

Protherm Ray and photovoltaics – boiler output in combination with solar panels

More and more customers are combining Ray with photovoltaic panels. An interesting rule applies here: with a PV system, you can afford a larger boiler than you would otherwise buy – because surplus energy from solar panels can be intelligently used precisely for heating or hot water preparation. Through relay or ripple control, Ray can use cheap or self-generated electricity. This topic is covered in detail in the article Protherm Ray and photovoltaics – how to use surplus energy from solar panels.

From an output selection perspective: if you have PV and want to actively use surpluses, it's wise to choose a model one step higher in output – e.g. considering the 14KE instead of the 12KE. Higher output allows for faster heating of the hot water tank or a buffer tank at times when the sun is shining at full strength.

Old Raja model vs. new Ray – output differences

Customers who know the older Protherm Raja series sometimes ask whether the output designation has remained the same. The answer is yes – 6KE is 6 kW, 9KE is 9 kW, etc. The main differences between the Raja and Ray are in regulation, communication interface (OpenTherm in the Ray) and control – not in the output range. So if you're replacing an old Raja with a new Ray, the same output should be the correct choice, assuming the house's insulation hasn't changed. More in the article Converting a Protherm Raja to a Ray – differences between the older and newer model.

Frequently Asked Questions (FAQ)

Is it better to buy a stronger boiler with reserve, or one precisely sized?

A small reserve (10–20%) is fine and desirable in practice – it covers sudden frosts, hot water preparation and any future extension. Too large a reserve (more than 40–50%) is counterproductive: the boiler runs in short cycles (turns on and immediately off), which unnecessarily strains it and worsens regulation. With an electric boiler there's no issue with condensate like with a gas boiler, but short cycles still put strain on electrical components and relays.

Can I have a Ray 6KE in an apartment with both radiators and underfloor heating?

It depends on the area and heat loss of the apartment. If you have a 55 m² apartment in a well-insulated prefab building after renovation, the 6KE may be enough – the heat loss of such an apartment can be 3–4 kW. If the apartment is larger or in an older uninsulated building, the 6KE isn't enough and you need to go for a 9KE or 12KE.

Do I need to change the electrical installation when switching to a larger model?

Usually yes. Each output level requires differently sized circuit protection and supply cable. For example, moving from the 12KE to the 18KE may mean changing the circuit breaker from 20 A to 25 A (or more) and possibly a heavier cable too. This should always be consulted with an electrician and verified at the distribution board. Details in the article Connection and installation of the Protherm Ray – requirements for electrical wiring and circuit protection.

What output of Protherm Ray do I need for a 150 m² house?

It depends on the insulation. A low-energy house of 150 m² will have a loss of approx. 6–8 kW, a well-insulated standard new build 9–11 kW, a renovated house with original walls 12–15 kW, and an old uninsulated house easily 16–18 kW. For most well-renovated houses of this area I recommend the 12KE or 14KE, and for older uninsulated ones the 18KE.

Can I buy a Ray 28KE for a family house?

Technically yes, but for a typical family house this is almost always an unnecessarily large output. The Ray 28KE makes sense for larger buildings – multi-generational houses over 200 m², recreational centres, smaller apartment buildings or industrial spaces. For a typical family house, such output is oversized and will cause regulation problems. It also requires significantly higher electrical power input, which may mean the main circuit breaker needs to be increased by the distributor.

What to do if I'm not sure about the correct output?

The most reliable way is to have a heat loss calculation done by a designer (cost 50–150 € depending on scope). If that's not possible, use a detailed online calculator and enter the input data as accurately as possible. As a helpful aid, you can also use the article How to choose an electric Protherm Ray boiler – output according to house area, where you'll find more detailed tables and explanations. If in doubt between two neighbouring outputs, choose the larger one – but only one step up.

Conclusion – the right output always pays off

Choosing the right output for a Protherm Ray isn't rocket science, but it does require at least a minimal analysis of the specific situation. Rules of thumb and tables are a good start, but truly precise sizing makes the difference between a boiler that calmly and efficiently heats the whole season, and one that either struggles to keep the house warm or unnecessarily switches on and off every few minutes.

If you're torn between two models, most practical experience favours the larger one – provided the output difference is one step, not two. An electric boiler like the Protherm Ray has the advantage of regulating output in stages and, in milder weather, running only at part of its maximum. A well-designed system with weather compensation control and a quality thermostat will then ensure that even a larger boiler doesn't start short cycling.

For those deciding between specific models: Protherm Ray 12KE has long been the most popular model for typical family houses up to 130 m², Ray 9KE excels in apartments and smaller houses, and Ray 18KE is the right choice for larger buildings or poorly insulated houses over 130 m². The other models each have their clear place where the specific situation requires them.

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'll be happy to help.