What power of ORAVA electric boiler do I need for my household
What power of ORAVA electric boiler do I need for my household?
This is probably the most common question customers ask before purchasing an electric boiler. And it is a completely justified question – a boiler that is too weak will not heat the household even in mild winter, while one that is too powerful takes up unnecessary space, may overload the electrical installation, and paradoxically may not operate efficiently because it cycles on and off in short intervals. Correctly dimensioning the boiler's power is the foundation upon which the comfort of heating and the cost of electricity bills depend.
In this article, we will go through the entire calculation process from the basics – from a rough estimate through correction factors (thermal insulation condition of the house, type of construction, climate, building orientation) to concrete practical examples. I will show you where mistakes are commonly made in dimensioning and help you choose the right model from the range of ORAVA electric boilers.
Why is correct dimensioning so important?
From practice, I know that customers tend to choose a boiler based on two extreme approaches: either they think "more power = better" and choose a boiler with a large reserve, or they try to save money and go for a weaker model because it is cheaper. Both approaches carry their own risks.
An over-dimensioned boiler has shorter on/off cycles. This means higher temperature fluctuations in the system, greater wear on control elements and thermostats, and in the case of an electric boiler with multiple heating elements – unnecessary capacity that you will never use. Moreover, a larger boiler may require a stronger electrical supply, which increases the cost of installation.
An under-dimensioned boiler, on the other hand, runs at full capacity continuously on the coldest days of the year and never reaches the desired temperature. Rooms are cold, the occupants do not get warm, and the boiler wears out prematurely because it never "rests".
Correct dimensioning means that the boiler operates at 90–100% of its capacity when the outside temperature is around –15 °C (the design outside temperature for most of Slovakia) and only needs 50–60% of its capacity at average autumn temperatures around 0 °C. This is the ideal situation.
Basic calculation: How many kW per square meter?
The simplest rough calculation is based on the specific thermal power per square meter of the area you want to heat. This is not an accurate method – it is used only for the first rough estimate. The real number depends on many factors, which I discuss below.
The graph shows that the estimated values for common practice are as follows:
- Passive house or new construction with very low heat losses: 30–40 W/m²
- New construction meeting current standards (since 2012): 50–60 W/m²
- Older house after comprehensive reconstruction (insulation, windows, roof): 70–90 W/m²
- Older house partially reconstructed (new windows, but no insulated walls): 90–110 W/m²
- Older uninsulated house before reconstruction: 110–150 W/m² and more
Example of a rough calculation: You have a family house built in 1985, original construction, you replaced the windows five years ago, but the walls are not yet insulated. The area you want to heat is 120 m². We use the value of 100 W/m²:
120 m² × 100 W/m² = 12 000 W = 12 kW
This is your estimated required boiler power. It is now important, however, to verify and adjust this result using correction factors.
Correction factors that significantly affect the required power
1. Ceiling height
The basic calculation assumes a standard ceiling height of 2.5–2.7 m. If you have an older house with higher ceilings (3.0–3.5 m is typical for houses from the 50s to 70s), you must increase the result. Warm air rises, so with higher ceilings you need to heat a larger volume, not just a larger area.
Correction: for a ceiling height of 3.0 m, increase the calculated power by 15–20%. For 3.5 m, increase by 30–40%.
2. Building orientation and climatic zone
Slovakia is not climatically uniform. The differences between the Záhorie region (mild, average January temperature around –3 °C) and, for example, Orava or the Tatras (harsher winter, design temperature –18 °C to –22 °C) are significant. If you live in a lowland area or in a valley where frost persists for a long time, increase the power by 20–30% compared to the estimated calculation.
Also, it depends on how the house is oriented in relation to the cardinal directions. A house with a northern façade and small southern windows is thermally less favorable than a house with large southern windows – in winter months, the sun through southern windows actually provides heat.
3. Type of construction
A standalone family house (isolated) has all its perimeter walls exposed to outside temperatures. A row house in a block has neighbors on the sides, which significantly reduces heat loss – the same house in a block needs 15–25% less power than an isolated house of the same size.
4. Condition of floor and roof insulation
Floors and roofs are the second biggest sources of heat loss in older houses after windows. If you have an unheated attic space without thermal insulation on the ceiling, or an unheated basement under the living area, expect higher losses. On the other hand, a well-insulated flat roof or an insulated attic with mineral wool over 200 mm significantly improves the situation.
5. Heating system – radiators or floor heating
The ORAVA electric boiler works equally well with any system, but what you are heating (radiators vs. underfloor heating) affects the performance and settings. Underfloor heating operates with lower water temperatures (35–45 °C instead of 70–80 °C with radiators), which is more economical but requires more precise dimensioning, as the floor area is fixed and cannot be quickly "braked down".
Accurate heat loss calculation – when an approximate one is not enough
For new buildings and comprehensive renovations, a heat loss calculation is performed according to the STN EN 12831 standard. This calculation takes into account each construction element (wall, window, floor, roof) separately with its area, thermal transmittance coefficient (U-value, W/m²K), and temperature difference. The result is an accurate value in kilowatts for each room and for the entire building.
This calculation is performed by a designer or energy auditor. An accurate calculation is not always necessary for an electric boiler – if you are buying a boiler for an existing house, where you know the history of heating, you can rely on the experience of previous winter seasons. However, for a new building or if you are planning a significant renovation, an accurate calculation is worthwhile.
From the diagram, it is clear that the most significant role is played by the external walls (about 35 % of losses) and the roof/ceiling (25 %). Windows and doors account for about 15 %, the floor for another 15 %, and natural ventilation (air infiltration) for the remaining 10 %. That is why comprehensive insulation – walls + roof + windows – can reduce heat losses by 60–70 % and significantly change the required boiler power.
Overview of ORAVA models and for whom they are intended
In the current ORAVA range, you will find several models that differ in power, type (boiler vs. convector), control, and functions. Let's look at them specifically from the perspective of dimensioning.
ORAVA EK electric boilers – for whole houses with central heating
The EK series electric boilers are intended for classic water-based heating systems – whether radiators or underfloor heating. They function as a replacement for gas or solid fuel boilers: they heat the water, which then circulates to the radiators or underfloor pipes.
ORAVA EK-2003 is a three-phase electric boiler with a power of 9 kW, suitable for well-insulated houses with a living area of up to 80–100 m². Three-phase power supply (400 V) is essential here – so you must have a three-phase connection in the house. A power of 9 kW also ensures hot water preparation, if the boiler has a built-in tank or is connected to an external storage tank.
ORAVA EK-2004 is a more powerful model with a higher thermal output, suitable for larger houses or buildings with a lower level of thermal insulation. This model can cover living areas over 100 m² with standard insulation or smaller areas with older construction and poor insulation. Again, it requires a three-phase connection and a corresponding circuit breaker.
When choosing between these two models, a simple rule applies: if your approximate calculation is close to the limit, always choose the higher model. A boiler with higher power can be regulated (modern electric boilers have step or continuous power adjustment), but an undersized boiler simply will not heat the house.
ORAVA VL convectors – for local and zonal heating
The VL series convectors operate on a different principle – they heat the air directly in the room, without water and without a distribution system. Each convector is an individual heating unit with its own control.
ORAVA VL-201 is a smaller model suitable for rooms up to 15–20 m². Typical use: bathroom, entrance, smaller bedroom, cottage or chalet, where you need to seasonally or occasionally heat the space.
ORAVA VL-202 is a more powerful convector for larger spaces – living room, larger kitchen, study or workshop. It is also suitable as an additional heat source in households where the main heating is provided by another system and the convector is used to heat problematic rooms.
A fundamental difference from boilers: convectors are suitable for single-phase networks (230 V), do not require installation of a heating system, and their installation is much simpler. If you are considering these two types and cannot decide, read the topic Electric boiler vs. ORAVA convector – which heating type is more cost-effective, where both approaches are compared in detail.
Practical examples from practice – how many kW in real households
Example 1: Apartment in a panel building, 65 m²
The customer asked whether an electric boiler or a heater would be sufficient for heating an apartment in a panel building from the 1980s, which has central heating but is underdimensioned. Panel buildings have worse wall insulation, but at the same time they are adjacent to other apartments, which significantly reduces heat loss. The real need for such an apartment is about 40–55 W/m². For 65 m², the required power is 2.6–3.6 kW. The solution here is either one more powerful heater in the coldest room or a combination of two smaller heaters in problematic rooms – a boiler is not needed or suitable here.
Example 2: Family house 150 m², built in 1978, partially renovated
The house has new windows (double-glazed, not triple-glazed), partially insulated façade (the extension from 2010 is insulated, the original part is not), and an unheated basement with an unheated cellar under the living area. The specific power is estimated at 90–100 W/m². For 150 m², this gives 13.5–15 kW. A boiler is clearly needed here, not heaters. A 9 kW model would not be sufficient – the customer should consider a more powerful model or consider whether it is possible to reduce heat losses before installation (for example, complete the façade insulation, which would reduce the required power to 11–12 kW).
Example 3: New low-energy house 120 m², built in 2019
The house meets the requirements for the low-energy standard (wall U-value under 0.18 W/m²K, triple glazing, heat recovery). The specific power will be 35–50 W/m². For 120 m², this gives 4.2–6.0 kW. A 9 kW boiler is absolutely sufficient here with some reserve – the customer will mostly use only 50–60% of the power. Full power will only be activated during extreme cold weather.
Example 4: Recreational cabin 45 m², without insulation
An old log cabin, not insulated, used occasionally in winter. The specific power can be 150–180 W/m², but since it is not heated continuously, only during stays (quick heating of a cooled space), the situation is different from permanent living. Here, heaters make more sense – you can quickly heat only the room you are in and do not have to heat the entire building. An electric boiler with distribution pipes would be unnecessarily complicated here, and in an unheated building, freezing of water in the pipes is a risk.
Electrical connection and protection – this must not be overlooked
The boiler power is not only a thermal issue – it is also an electrical issue. An electric boiler with a power of 9 kW draws current from the grid at full power, which your electrical wiring must be able to handle.
For three-phase ORAVA electric boilers, the following approximate requirements for protection and supply cable apply:
| Boiler power | Current (three-phase) | Recommended circuit breaker | Minimum cable cross-section |
|---|---|---|---|
| 6 kW | ~8.7 A | 3×16 A | 5×2.5 mm² |
| 9 kW | ~13 A | 3×16 A | 5×2.5 mm² |
| 12 kW | ~17.3 A | 3×20 A | 5×4 mm² |
| 18 kW | ~26 A | 3×32 A | 5×6 mm² |
Always check your electrical connection before purchasing a boiler – single-phase (1×230 V, max. ~3.7 kW per circuit) or three-phase (3×400 V, allows boilers of 9 kW and higher). A three-phase connection must be assessed and, if necessary, ensured by an electrician with a valid professional qualification. More about electrical installation can be found in the topic Installation of ORAVA electric boiler – procedure, requirements and what the electrician must do.
Stepped power – why it is an advantage of electric boilers
Many customers do not know that modern ORAVA electric boilers are not just an "on/off" device with one power value. They have multiple heating elements that are switched on gradually according to need. A 9 kW boiler may have, for example, three 3 kW elements – and the controller switches on 1, 2 or all 3 elements depending on how much heating is needed in the system.
This has practical implications: in autumn weather (+5 °C outside), the boiler may only use 3–4 kW, which is energy efficient. The full power of 9 kW is only required during extreme cold. This is a significant difference compared to simple electric direct heating heaters, which operate either at 100 % or not at all.
Power reserve – how much is enough?
From practice, I recommend leaving a 10–20 % reserve above the calculated value when dimensioning an electric boiler. The reasons are multiple:
- The approximate calculation is not precise – you may be more conservative in estimating the thermal insulation condition
- The reserve helps in quickly heating the house after the weekend or a holiday (if you have been using a low-power mode)
- A boiler with a reserve cycles less at normal temperatures and has a longer lifespan
- If you are considering an extension or an upper floor in the future, the reserve will come in handy
On the other hand, if the approximate calculation comes out to 8 kW and you have a 9 kW model available, that is perfectly fine and there is no reason to reach for a 12 kW model "just to be sure". The 10–20 % reserve is sufficient.
What if you are heating only part of the house?
Common situation: the customer has a family house of 180 m², but only uses the ground floor (90 m²) regularly, while the upper floor is heated minimally or not at all. In such a case, it is correct to dimension the boiler only for the actually heated area – that is, for 90 m². It is important, however, that the heating system is designed so that the unheated part of the house does not freeze (for example, with an anti-freeze thermostat set to 5–8 °C).
A similar situation arises when someone buys an electric boiler only for backup – they have a primary heat source (for example, a heat pump or a stove with a heat exchanger) and the electric boiler serves only as a backup in case of a failure of the primary source. In this case, the dimensioning is different: the boiler does not need to cover 100 % of the heat demand, 50–70 % is sufficient to maintain a basic temperature even in case of a failure of the main source.
Domestic hot water – another factor in choosing power
ORAVA EK electric boilers usually serve only for heating – domestic hot water is handled separately (boiler, flow heater). If, however, you plan to connect an external storage heater for domestic hot water preparation and the electric boiler is also to be used for its heating, you must add the power required for heating water to the calculated power for heating.
Approximately: a 100-liter tank requires about 5.8 kWh to heat from 10 °C to 60 °C. If you heat it in 2 hours, you need an additional power of about 2.9 kW. This is not a dramatic change, but in some combinations (small boiler + large tank) it can be decisive. Therefore, I recommend always consulting a designer or seller if you plan a combined heating and domestic hot water system.
Frequently asked questions (FAQ)
How many kW of electric boiler do I need for a 100 m² house?
For a well-insulated house from new construction (after 2012), 5–6 kW is sufficient. For an older partially renovated house (new windows, un-insulated walls), calculate with 8–10 kW. For an un-insulated older house, you may need up to 12–15 kW. The result always depends on the specific condition of your property, not just on its area.
Can I connect an ORAVA electric boiler myself?
The hydraulic part (connecting the boiler to the heating system) can be done by a skilled person according to the instructions, but the electrical connection of the boiler – that is, connecting the cable to the distribution board, installing circuit breakers and the grounding conductor – must be done by an electrician with professional qualifications according to Decree No. 508/2009 Coll. Without this, the boiler is not only dangerous, but also out of warranty. More details can be found in the topic Installation of ORAVA electric boiler – procedure, requirements and what the electrician must do.
Is it better to buy a boiler with higher power and regulate it, or to dimension it precisely?
Ideally, combine both – a more accurate estimate of the heat demand and an appropriate 10–20 % reserve. A boiler with too large a reserve (e.g. 18 kW for a house needing 6 kW) will most of the time operate only with one heating element, the rest will be idle. This is not a problem in terms of consumption (you pay only for the energy actually consumed), but with some types of boilers with less quality regulation, excessive cycling can occur. In the case of ORAVA boilers, the stepwise control of power is well designed, so a larger model is not a major problem, but it is not an optimal solution.
Is an electric boiler worth it at today's electricity prices?
Electric heating has higher operating costs than gas or heat pumps, but zero fuel costs (no need for fuel supply), zero need for a chimney, minimal maintenance and zero emissions at the point of use. The economics are significantly improved by the night tariff (HDO), regulation via thermostat and good insulation of the house. For a more detailed comparison, see the topic ORAVA electric heating and energy consumption – how to reduce heating costs.
Can I use a combination of two ORAVA boilers instead of one larger one for a bigger house?
Technically it is possible, but practically more complicated – it requires hydraulic connection, common regulation or at least coordination of thermostats, and two separate electrical circuits. For most households, a single sufficiently powerful boiler with multi-stage regulation is a cleaner solution. A combination of two boilers makes sense, for example, during a phased renovation, where the first boiler covers one zone and the second another.
How can I find out what thermal losses my house has, without an expensive energy audit?
The simplest way is to look at the consumption history – if you previously heated with gas or electricity and have the bills, you can calculate the annual energy consumption and derive the average power from it. Example: annual electricity consumption of 15,000 kWh for heating with 2,000 hours of heating season means an average power of 7.5 kW – but this is an average, not the maximum power. The maximum power (for boiler dimensioning) will be higher, typically 1.5–2× average, i.e. in this case 11–15 kW. This estimate is quite rough, but for an existing building with a history, it is a better basis than just an approximate calculation from the area.
Conclusion: step-by-step selection process
To avoid getting lost when choosing the power of an ORAVA electric boiler, let's summarize the whole process into simple steps:
- Step 1: Find out the total heated area of your house or apartment.
- Step 2: Determine the thermal insulation condition of the property (new build, insulated older house, un-insulated) and select an approximate value in W/m² accordingly.
- Step 3: Multiply the area and the specific power – you get an approximate required power in kW.
- Step 4: Apply correction factors: ceiling height, climatic zone, type of building, orientation of the house.
- Step 5: Add a 10–20 % reserve.
- Step 6: Check your electrical connection – for boilers over 3.7 kW you need a three-phase connection 3×400 V.
- Step 7: Choose an ORAVA model that best matches the calculated power – ORAVA EK-2003 for houses up to 100 m² with good insulation, ORAVA EK-2004 for larger houses or poorly insulated buildings, ORAVA VL-202 or ORAVA VL-201 for local heating of individual rooms without a water system.
If you are still unsure about the selection after reading this article, also see the topic How to choose an electric boiler or ORAVA convector – what to focus on, where the differences between individual models and types of heating are explained in more detail, or the topic Frequently asked questions about ORAVA electric boilers and convectors, where you will find answers to further practical questions before purchase.
Do you have a question about this topic?
Not sure how to decide or dealing with a specific situation in your household? Write to us – we are happy to help.