What power electric convector do I need for my room

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What power electric convector do I need for my room?

This is probably the most common question we encounter when selling electric convectors. And at the same time, it's a question that has no single universal answer. The power of a convector depends on many factors – the size of the room, the state of insulation, its position within the building, the number of windows, ceiling height, and also what indoor temperature you actually want to achieve. This article will give you a guide on how to actually calculate the power and how not to be fooled by cheap rules of thumb that don't work in practice.

If you've ever bought a convector just based on how many square meters your room has, and then in winter you struggled in vain to warm up despite what seemed like an adequately sized unit – you're not alone. Underestimating heat losses is a classic mistake. On the other hand, overpaying for an unnecessarily powerful convector that cycles every few minutes is also pointless. The right choice is always a compromise between physical reality and practical possibilities.

Basics: what actually affects the required power?

Before we get to the numbers, we need to understand what's behind a room's heat losses. The convector must cover all the heat that the building loses in winter through walls, ceiling, floor, windows and doors. The worse the insulation, the higher the power you need. Conversely, in a new building with passive standard, a fraction of what you'd need in an uninsulated panel building may be enough.

The main factors affecting heat losses:

Room volume – not just the floor area, but the total volume of air that needs to be heated (area × ceiling height)
Insulation of the perimeter walls – an old uninsulated brick house loses several times more heat than a new building
Windows – number, size, glazing type (single, double, triple glass)
Position of the room within the building – a corner room has two exterior walls, a central room has only one or none
Vertical position in the building – a room above an unheated garage or under an uninsulated roof loses much more heat than an apartment surrounded by other apartments
Cardinal orientation – a north-facing wall is colder, a south-facing one gets solar gains
Climate zone – Bratislava and Košice have different design outdoor temperatures than mountain regions

Simple calculation: a rule of thumb for common conditions

There are several calculation methods, from simple rules of thumb to a complete thermal engineering calculation according to the STN EN 12831 standard. For an average customer, using a reasonable rule of thumb and then correcting it according to specific conditions is sufficient.

Basic rule of thumb:

Well-insulated new building: 40–50 W/m²
Older insulated house, apartment building with insulated perimeter: 60–80 W/m²
Older uninsulated house, panel building without insulation: 90–120 W/m²
Renovated old house with partial insulation: 70–90 W/m²

Example: You have a bedroom measuring 4 × 5 meters (20 m²) in an uninsulated panel building. Approximate required power: 20 × 100 W = 2,000 W. If it were a new building, 20 × 45 W = 900 W would be enough.

The difference is enormous, which is exactly why it's so important not to use a single universal rule. In practice, we've seen customers who bought a 500-watt convector for an 18-square-meter room in an uninsulated house and then wondered why the room didn't warm up above 15°C even after two hours of continuous operation.

Correction factors: when to adjust the power up or down

The basic rule is just a starting point. In practice, it's almost always worth applying correction factors according to specific conditions.

Factors that increase the required power (multiply or add)

Corner room (+15 to +25 %) – two exterior walls mean double the losses through the building envelope
Room above a garage or basement (+10 to +20 %) – the floor is another surface through which heat escapes
Attic room under the roof (+15 to +30 %) – the roof is a large heat-transfer surface, especially if not well insulated
Large window areas (over 20 % of wall area) – every window, even good triple glazing, has many times worse thermal insulation than an insulated wall
North orientation (+5 to +10 %) – no solar gains, permanently colder wall
Ceiling height above 2.8 m – the air volume is larger; for ceilings above 3 m, calculate power per m³ instead of m² (approx. 30–35 W/m³)
Houses in mountain regions or with a design outdoor temperature below -15°C (+15 to +20 %)

Factors that reduce the required power (or allow you to opt for a smaller unit)

Passive new building or low-energy house – heat loss values are actually very low, 25–35 W/m² may be sufficient
Room inside the building without an exterior wall – for example a hallway or bathroom surrounded by other rooms
South orientation with large windows – solar gains can cover part of the heat demand on sunny days
Well-sealed room with minimal cold air infiltration

Calculation using concrete examples from practice

Theory is one thing, but concrete examples say more. Here are several scenarios we deal with in practice:

Example 1: Bedroom in an apartment building from the 1990s

Room: 3.8 × 4.2 m = approx. 16 m². Ceiling height: 2.6 m. Apartment building built in the 1990s, partially insulated with 8 cm thick facade. One exterior wall (south), one double-glazed window (1.2 × 1.4 m). The room has another apartment beneath it.

Calculation: 16 m² × 70 W/m² = 1,120 W. Correction for partial insulation: fine, we stay at 70 W/m². No corner effect, no cold floor. Result: approx. 1,100–1,200 W. In this case, we go for the Protherm 1000 as the basic option, or if the customer wants a reserve and faster warm-up, we recommend the Protherm 1500 set to a lower output.

Example 2: Corner room in an old uninsulated brick house

Room: 4 × 5 m = 20 m². Ceiling height: 3.1 m. Old brick house, 40 cm masonry without exterior insulation. Two exterior walls (north + west), three single-glazed windows.

Calculation by area: 20 m² × 110 W/m² = 2,200 W. Correction for corner effect: +20 % → 2,640 W. Correction for ceiling above 2.8 m: recalculation by volume: 20 × 3.1 × 35 = 2,170 W, but this ignores wall losses, so we take the higher value from the area calculation. Correction for old windows: +10 % → approx. 2,900 W. Result: need of 2,500–3,000 W. Here we clearly recommend the Protherm 2000 plus an additional smaller convector, or consider a more comprehensive heating solution. The Protherm 2000 alone will be at the limit of its capacity on very frosty days.

Example 3: Modern new building – living room

Room: 5 × 6 m = 30 m². Family house built in 2020, 15 cm EPS insulation, triple glazing, air recovery ventilation. Ceiling height: 2.7 m. Two exterior walls, but excellent thermal parameters of the building envelope.

Calculation: 30 m² × 45 W/m² = 1,350 W. Correction for corner position: +10 % = 1,485 W. Result: 1,200–1,500 W is enough. Customers are usually surprised here – in a quality new building, the Protherm 1500 really is enough even for a relatively large room.

Example 4: Bathroom

Bathroom: 2 × 2.5 m = 5 m². Apartment building, two interior walls, one double-glazed window, exterior wall facing north. This room has a higher required temperature (22–24°C instead of 20°C) and humidity that needs to be taken into account.

Calculation: 5 m² × 90 W/m² = 450 W. Correction for higher required temperature (+2°C): +10 % = 495 W. Result: 500 W is enough. For bathrooms, the Protherm 500 is a very common and functional choice, provided the convector has an IP rating suitable for humid environments.

What "W/m²" means in practice, and why area doesn't equal area

A big mistake customers make is comparing rooms by area without regard to their thermal properties. 15 m² in a new building and 15 m² in an old cottage are completely different problems from a heating perspective. The normative heat losses of the same area can differ threefold depending on the condition of the building.

Another practical detail: always count on the fact that the convector's power should be at least 10–15 % higher than the calculation suggests. The reason is simple – the convector must be able not only to maintain the space at the target temperature, but also to reheat it after it has cooled down (for example, in the morning after a night-time setback or after coming home). Maintaining the temperature itself requires lower power, but a fast temperature ramp-up requires a reserve.

At the same time, a convector with a higher power than you need is not automatically "better". An unnecessarily powerful convector will cycle (turn on and off) more briefly, which with mechanical control can lead to greater temperature fluctuations. With electronic control this is less of a problem, because the thermostat can regulate the output more finely. You can read more about this in the article Convector with electronic vs. mechanical control: which is better in our Knowledge Center.

The effect of ceiling height: when to calculate by volume, not area

Most rules of thumb work with power per area (W/m²), which is convenient and quite accurate for common ceiling heights of 2.4–2.8 m. The problem arises in taller spaces – attic rooms with sloped walls, lofts, historic buildings with ceiling heights of 3.5 m and more, or conversely in converted basements with low ceilings.

When the ceiling is significantly higher than 2.8 m, we recommend switching to a calculation based on room volume:

Volume = area × ceiling height (in m³)
New building: 28–35 W/m³
Common older building: 38–45 W/m³
Uninsulated, old building: 50–65 W/m³

Example: An attic room of 20 m², average ceiling height (including slopes) 2.4 m → volume 48 m³. Older family house, partial roof insulation: 48 × 42 = 2,016 W. Here we would go for the Protherm 2000.

Required temperature: every extra degree costs energy

Standard calculations are based on a required indoor temperature of 20°C and a design outdoor temperature of -12°C to -15°C (depending on the climate zone in Slovakia). If you want a higher temperature or live in a colder region, you need proportionally higher power.

How it works in practice: every extra degree Celsius in the required indoor temperature increases heat losses and the required power by roughly 4–6 %. A bathroom where you want 24°C instead of 20°C needs about 20 % more power than you would expect from a plain area calculation.

On the other hand, in bedrooms many people prefer 18°C or 19°C instead of 20°C. This allows for a somewhat smaller unit and also saves energy. Healthy sleep in a slightly cooler room is also a professionally confirmed benefit.

Convector as a supplementary or main heat source

The power calculation also depends on the role the convector plays. If it is the primary heating source in the room, you need full power covering the maximum heat losses. If the convector is only a supplement to a central heating system (for example, to warm up a hallway or temporarily heat a guest room), you can go with a lower power – the room will never cool down to the outdoor temperature.

This topic is covered in more detail in the article Electric convector as main vs. supplementary heat source in our Knowledge Center. In short: if you heat an entire apartment or house with convectors and have no other heat source, take the calculation seriously and rather leave a reserve. If the convector only supplements a boiler or heat pump, you can be more lenient.

How control affects the choice of power

The convector's power and its control method are very closely related. A convector with continuous electronic control can operate at partial power – meaning that even if it's nominally 2,000 W, in practice it may run longer at 800 or 1,200 W, maintaining a more constant temperature with lower consumption. You can safely choose such a unit slightly stronger than the calculation suggests.

Conversely, a convector with a mechanical thermostat usually only operates in full power / off mode. There, too much power is more of a drawback – it cycles quickly, the room overheats and cools down, control is worse, and consumption can paradoxically be higher. With mechanical control, therefore, make sure not to oversize the power by more than 20–25 %.

Power, voltage and fusing: technical limitations

When choosing a high-power convector, you must also take the electrical installation into account. A standard 230 V / 16 A socket can handle a maximum of 3,680 W. Most common household electric convectors have a power of 500–2,500 W, which is fine. If you needed higher power (over 3 kW), this would typically be solved with two convectors on two circuits, not one super-powerful device.

When installing a larger number of convectors (for example, an entire house on electric heating), it is essential to consult with an electrician about the total load and the capacity of the main circuit breaker. You can read more about installation in the article Mounting an electric convector on the wall: step-by-step procedure in this Knowledge Center.

Mobile vs. permanently mounted convectors: the power is the same, the use is different

The convector's power is not affected by whether it is wall-mounted or stands on wheels. The physics of heat loss is identical. Mobile use, however, brings the advantage of flexibility – you can use the same convector once in the living room, once in the guest room. If you're considering a mobile solution, remember that a wheel set for Tesy is available for most Tesy convectors, turning a wall unit into a mobile one. You can read more about this topic in the article Wheels for electric convectors: when they're worth it and what to know.

In terms of power, one important note applies: a mobile convector is moved between rooms with different heat losses. If you have one convector for multiple rooms, design the power for the most demanding one (largest area or worst insulation).

Energy and costs: higher power = higher bill?

Logically, it seems that a more powerful convector consumes more electricity. But it's not that simple. A 2,000 W convector running only 30 % of the time consumes the same as a 1,000 W convector running 60 % of the time – in both cases it's 600 Wh per hour of real time. What matters is the building's heat loss, not the device's power.

In other words: the convector's power determines the speed of heating and the maximum temperature you can achieve, but monthly consumption depends primarily on the building's thermal insulation and the temperature you're heating to. That's why insulation and building airtightness are a much more important investment from the perspective of long-term costs than the choice of a specific convector model.

The most common mistakes when choosing power

Over years of practice, we've seen the same mistakes repeated. Here's an overview of the most common ones:

Using one number for all building types – "50 W/m² is enough" may be true for a new building, but in a panel building it's tragically insufficient.
Ignoring ceiling height – in an attic with a ridge at 4 m and an average height of 2.4 m, the air volume is substantially larger than the area would suggest.
Forgetting about the room's position – a corner apartment on the top floor without roof insulation is an extremely unfavorable combination.
Buying the smallest available unit due to lower price – an underpowered convector runs constantly at full power, fails to heat the room, and wears out faster.
Buying according to manufacturer's "room recommendations" without context – these recommendations are usually for ideal conditions.
Ignoring the window factor – large glazed areas (French windows, panoramic glazing) are the thermally weakest part of the envelope.

What to do if you're not sure: practical procedure

If after reading this article you're still not entirely sure, we recommend the following procedure:

Calculate the base power according to the area and condition of the building (as described above).
Identify whether you have any unfavorable factors (corner room, cold floor, north orientation, old windows).
For each unfavorable factor, add 10–20 % on top of the base calculation.
Round the resulting number up to the nearest available model.
If you're still torn between two neighboring power ratings (e.g. 1,500 W vs. 2,000 W), choose the higher one – the convector won't always use it fully, and the control will take care of comfort.

A more detailed guide to selection (not just in terms of power, but also other criteria such as dimensions, control type, installation) can be found in the article How to choose an electric convector: power, control and other criteria in this Knowledge Center.

Frequently Asked Questions (FAQ)

Can I use one powerful convector instead of two smaller ones for the same room?

It depends on the shape of the room and the placement of the convector. One stronger convector placed in an optimal spot (under the window) can be just as effective as two smaller ones, and it's also a simpler installation and control setup. The problem arises in long or irregularly shaped rooms – there, two units in different locations may be more advantageous for even heat distribution. If a room is, for example, L-shaped or longer than 8–9 meters, two smaller convectors are a more sensible choice.

Is a 500 W convector enough for a 12 m² children's room?

It depends on the condition of the building. In a new building with good insulation: 12 × 45 = 540 W – so 500 W is on the borderline, but it will manage (with somewhat slower heating). In an old uninsulated house: 12 × 100 = 1,200 W – there 500 W is absolutely not enough, you won't heat the room to 20°C in severe frost. For a children's room, we always recommend an extra 5–10 % reserve due to the higher required temperature (22°C) and safety.

Why does my convector run non-stop, but the room still doesn't warm up?

This is a classic symptom of an underpowered unit. The convector runs at 100 % all the time and still can't keep up with the heat losses. Solution: either a bigger convector, or improved thermal insulation (insulation, window replacement, sealing). Never leave a convector permanently running at maximum – this accelerates wear and is expensive. If this situation persists for a longer time, also check out the article Common electric convector faults and how to fix them.

Can I install a 2,000 W convector even if I only have a 10 m² room?

Technically yes, and there's no safety issue – the convector has a thermostat and won't start "scorching" the room. But it's unnecessary. In a small room, a 2,000 W convector will switch on and off very briefly, which with mechanical control leads to significant temperature fluctuations and worse comfort. With electronic control this is less of a problem. Ideally, keep the power appropriate to the room – an unnecessarily large convector is also unnecessarily large (physically on the wall).

Does the power calculation differ for a bathroom compared to a living room?

The calculation procedure is the same, but you need to take two things into account: first, in a bathroom we usually want a higher temperature (22–24°C), which increases heat losses. Second, the air humidity in a bathroom is higher, which for some types of convectors (intended for dry spaces only) poses a safety issue. For bathrooms, always choose a convector with an appropriate IP rating, typically at least IP24 or IP44. Then adjust the basic area calculation with a correction for the higher required temperature.

How do I check the heating power after installation?

The simplest way: during frosty weather (outdoor temperature around the design value for your region, i.e. -10°C to -15°C), measure what percentage of the time the convector actually runs (on/off ratio per hour). Ideally it should run 60–80 % of the time. If it runs 100 % and the room temperature still drops – the convector is underpowered. If it switches on only 20–30 % of the time even in severe frost – it's probably oversized, which is not a safety issue, just an economic and comfort one.

Conclusion: calculating power is an investment, not a formality

Choosing the right power for an electric convector isn't just a technical formality – it's a decision that directly affects your everyday comfort, monthly electricity costs, and the lifespan of the device. Five minutes spent on calculation before purchase can save you years of an unheated room or unnecessarily high bills.

The basic rule is: measure, take the condition of the building into account, apply corrections according to the specific conditions of the room, and don't forget a 10–15 % reserve. If you're still unsure, check out the entire category of electric convectors – with each model you'll also find approximate recommendations for room area, which you can compare with your calculation. And when in doubt – better one level higher than one lower.

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 help.