What power output of electric floor heating do I need for my space?

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Why calculating the power output of electric underfloor heating is crucial

An undersized heating system won't manage to heat the space and will run continuously at full power without reaching the required temperature. An oversized one, on the other hand, will needlessly strain the electrical installation and cause an uneven floor temperature – areas above the cable will be noticeably warmer than the gaps between them. Correct sizing is therefore the foundation of every installation, not a formality.

In practice, I encounter two extremes. The first type of customer buys the cheapest and smallest cable, because "it only heats as a supplement anyway." The second type orders the most powerful available product, believing that more is always better. Both approaches lead to problems and unnecessary costs. This article will show you how to proceed correctly – from the first sketch of the room to the selection of a specific product.

Basic concepts: installed power vs. actual consumption

Before we get into the calculations, it's important to distinguish between two things that people often confuse. Installed (nominal) power is the maximum power the heating can deliver to the floor when running continuously. It is stated in watts (W) or kilowatts (kW). Actual consumption is the power you really "spend" over the course of a day – and that depends on how many hours the thermostat switches the heating on.

Electric underfloor heating with a correctly set thermostat typically runs only 30–50% of the time. This means that a system with a nominal power of 1,500 W will consume, over a day at a 50% cycle, only as much as if it ran continuously at 750 W. That's why it's worth investing in a quality thermostat – for example the HAKL TH 900 Digital Thermostat or the wifi versions HAKL TH 951wifi and HAKL TH 952wifi, which work with weekly programming and can reduce consumption even further.

Electric underfloor heating: primary or supplementary heat source?

Power calculation differs fundamentally depending on the purpose of the heating. You can find more on this topic in the article Electric underfloor heating as a primary vs. supplementary heat source; here are the brief differences:

Supplementary (comfort) heating – the goal is to warm the floor to a pleasant temperature of 26–28 °C, while the room has another main heat source. 80–120 W/m² of effectively heated area is sufficient.
Primary (full-fledged) heating – the system must cover the entire heat loss of the room. It requires 140–180 W/m² in normally insulated houses, up to 200–250 W/m² in older buildings without sufficient insulation.

Most customers in new builds or after thermal renovation manage well with comfort heating under tiles in a bathroom or hallway. The situation changes for attic apartments, ground-floor rooms above an unheated garage, or older houses with simple windows.

Step 1 – Determine the effective heating area

The first mistake in calculations is using the total room area. A cable or mat never covers the entire floor – heating is not laid under furniture (beds, cabinets, kitchen units). The reason is both practical and safety-related: under furniture, heat would not escape, would accumulate, and could damage the cable or the floor covering.

Standard procedure for determining the effective area:

Measure the total room area (length × width).
Subtract the area under fixed furniture (cabinets, kitchen unit, bathtubs, shower area).
Also subtract a strip of about 5–8 cm from the walls (the cable is usually not laid here to avoid thermal bridges to the wall).
The result is your effective area.

Example: Bathroom 3.2 × 2.5 m = 8 m². Shower area 0.9 × 0.9 m = 0.81 m². Vanity cabinet 0.6 × 0.5 m = 0.3 m². Effective area = 8 – 0.81 – 0.3 – (edge strip approx. 0.4 m²) ≈ 6.5 m². You continue working with this value.

Step 2 – Calculate the required power

Once you have the effective area, simply multiply it by the recommended power density. This density depends on the purpose of the heating and the thermal properties of the room.

Reference values for power density for electric underfloor heating:

Bathroom, comfort heating, new build: 100–120 W/m²
Living room, bedroom, comfort heating: 80–100 W/m²
Kitchen, hallway, corridor, comfort: 80–100 W/m²
Any room, primary heating, new build: 140–160 W/m²
Primary heating, older building: 160–200 W/m²
Cellar, garage, space above grade: 200–250 W/m²

Formula: Required power (W) = Effective area (m²) × Power density (W/m²)

Example 1 – Bathroom 6.5 m², comfort heating: 6.5 × 110 = 715 W

Example 2 – Living room 18 m² effective area, primary heating in a new build: 18 × 150 = 2,700 W

Example 3 – Hallway 4 m² effective area, comfort heating: 4 × 90 = 360 W

Step 3 – Choose a specific product based on power and area

Manufacturers state the total power and cable length for cables, and the power per square meter and total mat power for mats. Your task is to choose a product whose power comes as close as possible to the calculated power – ideally slightly exceeding it (5–15%) so the thermostat doesn't have to keep the heating running continuously.

In practice: If you calculated a need of 700 W for a bathroom, you would reach for a product with a power of 750–800 W. For example, the HAKL TCX 10/1230W heating cable has a power of 1230 W – this would be suitable for a larger bathroom or a space with a larger effective area (roughly 10–12 m²). Lower power options from the same series exist for smaller spaces.

It's also important to watch the cable spacing – i.e. how many centimeters will be between individual loops. The optimal spacing is 7–12 cm. Too tight a spacing increases the risk of overheating and damage to the cable, while too wide a spacing causes a striping effect – alternating warmer and cooler strips on the floor, which is unpleasant when walking barefoot.

Calculating cable spacing

Cable spacing (in cm) = (Effective area in m² × 100) ÷ cable length in m

Example: a 61 m cable over an area of 5.5 m²: (5.5 × 100) ÷ 61 = 9 cm. An ideal value. If the calculation gave 5 cm, the cable is too long for the given area. If it gave 15 cm, it is too short – the effect will be uneven.

The floor covering factor: how it affects power

Different floor coverings have different thermal conductivity. Ceramic tiles and stone cladding conduct heat best – heat from the cable spreads into the room with minimal losses. Conversely, thick carpet, PVC with a felt backing, or thick laminate act as thermal insulation and "absorb" part of the power.

Ceramic tiles (6–10 mm): Highest efficiency, the recommended W/m² value is at the lower end of the range.
Natural stone: Similar to tiles, excellent conductivity.
Vinyl/SPC flooring (4–8 mm): Good conductivity, slightly increase W/m² by 10–15%.
Laminate (8–12 mm): Significantly worse conductivity, increase by 15–20%. Note – always check the laminate manufacturer's certificate for underfloor heating.
Carpet: Electric underfloor heating under carpet is generally not recommended – losses are high and there is a risk of overheating.

You can find more about combining heating with different coverings in the article Electric underfloor heating under different types of flooring: tiles, vinyl, laminate.

The effect of thermal insulation and room location

This is a factor many customers underestimate. Two identically sized bathrooms may have completely different power requirements if one is in a new build with thick insulation and the other is on the ground floor of an older house with a floor directly above an unheated cellar.

The main factors affecting a room's heat loss:

Position relative to the exterior: A corner room has two exterior walls, a central room only one or none. A corner room may require 20–30% higher power.
Position within the building: An attic loses heat through the roof, the ground floor through the floor, the top floor through the ceiling.
Age and insulation of the building: Panel buildings from the 1980s, after thermal renovation, behave similarly to new builds. Without insulation, heat losses are 2–3 times higher.
Windows: An old single-pane window can be a source of heat loss greater than an entire wall. Double and triple glazing fundamentally change the situation.
Below the room: An unheated cellar or garage means a large heat loss through the floor. In such cases, always reach for the highest W/m² values.

Practical calculation examples from real projects

Example A: Bathroom in a renovated panel apartment

Bathroom 2.8 × 2.0 m = 5.6 m². After subtracting the shower area (0.9 × 0.9 = 0.81 m²), the vanity cabinet (0.6 × 0.5 = 0.3 m²) and the edge strip (approx. 0.3 m²), the effective area remains 4.2 m². The panel building was thermally renovated, has double glazing, and the heating will be supplementary. Power density 110 W/m²: 4.2 × 110 = 462 W. A cable or mat with a power of 450–500 W is chosen.

Example B: Living room in a family house – primary heating

Living room 5.5 × 4.2 m = 23.1 m². Corner room, house from 1985 without insulation, single-pane windows replaced with double glazing. Fixed furniture (sofa set, coffee table, sideboard) takes up approx. 6 m². Effective area = 23.1 – 6 – 0.8 (strips) = 16.3 m². Primary heating, older building, corner room: density 180 W/m². 16.3 × 180 = 2,934 W. A cable or a combination of cables with a total power of approx. 3,000 W is chosen. Note: at this power, check the capacity of the electrical connection and fuses.

Example C: Hallway and corridor, comfort heating

L-shaped corridor with a total area of 7.2 m². No furniture, effective area after subtracting edges ≈ 6.5 m². Only comfort heating (there is also central radiator heating). Density 90 W/m²: 6.5 × 90 = 585 W. Available products fall close to this value.

Example D: Bathroom in an attic apartment

Bathroom 3.0 × 2.2 m = 6.6 m². Attic apartment, roof with minimal insulation only, exterior wall on three sides. Effective area 4.8 m². Heating will be primary, the space is thermally demanding: density 200 W/m². 4.8 × 200 = 960 W. In this case, reach for a product with a power of around 1,000 W. For example, the HAKL TCX 10/1230W heating cable with a power of 1230 W will cover even more demanding conditions, and the thermostat will regulate it to switch on only as needed.

Electrical connection and fuses: don't forget to properly size the installation

Electric underfloor heating is a continuous load – unlike a kettle, which runs for a few minutes, heating can operate for several hours at a stretch. Therefore, the electrical installation must also be properly sized.

Up to a power of 2,000 W (2 kW), a standard socket circuit of 10 A / 230 V with 1.5 mm² conductors is sufficient.
For a power of 2,000–3,500 W, we recommend a separate 16 A circuit with 2.5 mm² conductors.
Above 3,500 W (e.g. full-area heating of a large room), consider a three-phase connection or consult an electrician.
Always use a residual current device (RCD) with a tripping current of 30 mA.
A thermostat with a floor sensor must be certified for use with the given type of heating.

Power is also a reason why it's worth investing in a programmable thermostat. Wifi thermostats such as HAKL TH 952wifi allow you to set different temperatures for each day of the week and each hour of the day. The heating can then bring the room up to temperature before you wake up and switch off when you leave – this can reduce consumption by up to 30–40% compared to manual control.

Cable spacing vs. mat power: which is right for you?

Once you've arrived at the required power, you still need to decide whether to go for a heating cable or a heating mat. This topic is covered in detail in the articles How to choose electric underfloor heating: cable, mat or foil? and Heating cable vs. heating mat: which solution is right for you?. From a power calculation perspective, there is one key difference:

A mat has a fixed power per m² (typically 150–160 W/m²). If your effective area and the mat size don't match, you cannot shorten the mat – you can only cut the mesh and change direction. The power of the whole system is thus determined by the mat's size.
A cable gives you freedom – by adjusting the cable spacing you regulate how much power per m² is actually delivered. The same cable with a power of 1230 W can be laid over an area of 8 m² (154 W/m²) or 10 m² (123 W/m²).

For irregular areas, rooms with alcoves, or spaces where you need to precisely set the power density, a cable is more flexible. For rectangular bathrooms and hallways, a mat is more practical and quicker to install. More on installation in the articles Installing a heating mat under tiles: step-by-step procedure and Installing a heating cable: spacing, embedding and connection.

The most common sizing mistakes

Over the years of practice, I've seen the same mistakes repeated. Here are the most common ones:

Calculating from the total room area: A customer has a 12 m² room, buys a 12 m² mat, and forgets that half of it is under furniture. Result: overheating, cable damage, void warranty.
Ignoring thermal insulation: "Well, the bathroom is small, the smallest cable will do." But the bathroom is on the ground floor above an unheated storage room – and the result is that the heating runs continuously and barely warms the floor.
Too tight cable spacing: The customer "piles up" power by choosing a long cable for a small area. Spacing under 5 cm leads to overheating, damage to the cable insulation, and heating failure.
Forgetting a thermostat with a floor sensor: Without a floor sensor, the thermostat only measures air – the floor can overheat above the safe temperature of 27–28 °C, which damages laminate or vinyl.
Undersized electrical installation: A conductor that is too thin or an undersized circuit breaker causes outages and, in the worst case, fire.

How to reduce consumption without reducing comfort

An optimally sized system is only the first step. Real savings come from these measures:

Programmable thermostat: Set night setbacks and absence programs. A bathroom that no one is using doesn't need 26 °C.
Floor sensor vs. air sensor: For coverings with poorer heat transfer (laminate, vinyl), use a combined sensor or just a floor sensor – to prevent overheating.
Thermal insulation under the heating: Reflective insulation under the cable/mat reflects heat upward into the room instead of losing it into the concrete screed. An investment of a few euros per m² pays for itself within a season.
Wifi control: If you're unexpectedly delayed, you can switch off the heating remotely. Thermostats HAKL TH 951wifi and HAKL TH 952wifi allow this via a mobile app from anywhere.

Quick reference table: power by room

Room type	Effective area (example)	Supplementary (W)	Primary (W)
Small bathroom (up to 5 m² eff.)	4 m²	400–480	560–800
Medium bathroom (5–8 m² eff.)	6.5 m²	650–780	910–1,300
Hallway / corridor	5 m²	450–500	700–1,000
Bedroom (eff. area 10 m²)	10 m²	800–1,000	1,400–1,800
Living room (eff. area 15 m²)	15 m²	1,200–1,500	2,100–3,000
Kitchen (eff. area 8 m²)	8 m²	640–800	1,120–1,600

Frequently Asked Questions (FAQ)

Can I use a higher power than I calculated – will the heating be more powerful?

Yes, but only within reasonable limits. If you exceed the recommended power density (e.g. 200 W/m² for comfort heating), the floor may overheat above 27–28 °C. This will damage laminate or vinyl coverings and make walking uncomfortable – a floor that's too hot isn't comfortable. Tiles and stone have a higher tolerance, but even so: a thermostat with a floor sensor will protect you from overheating, but that's not a reason for extreme oversizing.

What if I have an irregular area – an L-shape or alcoves?

Divide the area into regular sections, calculate each one separately, and add up the total power. Then choose either one cable with sufficient total power and length, or two mats for each section separately. If two mats or cables are connected in one room, we recommend a single shared thermostat with a floor sensor placed at the point farthest from the power supply point.

Does a bathroom need special power or IP rating?

A heating cable or mat embedded in screed or adhesive under tiles is well isolated from water. The cable itself in the floor doesn't have an IP classification issue – it is physically protected by the construction. What does require increased attention, however, is the thermostat – it must be located outside the zone of direct water splashing (min. 60 cm from the shower/bath, zone 2–3 according to ČSN 33 2000-7-701). We recommend a thermostat with an IP21 protection rating or higher for bathroom environments.

How do I check whether my electric heating is working correctly after installation?

The basic check is measuring the cable's resistance with a multimeter before embedding and after the screed hardens. Compare the measured resistance with the value on the cable's label – a deviation of up to ±10% is acceptable. After startup, monitor the thermostat: if the heating runs more than 70–80% of the time and still doesn't reach the set temperature, the system is probably undersized or there is an insulation problem. The topic of faults is covered in detail in the article Common faults in electric underfloor heating and how to fix them.

Can I shorten the cable if it's too long for my space?

No. A heating cable must not be shortened or lengthened – its resistance is designed for a specific length, which determines its power. Shortening it increases power density per meter, decreases total resistance, and increases current. The cable will overheat and become damaged. If the cable doesn't fit your area with the correct spacing, choose a different model with a shorter length or lower power. Manufacturers offer cables in many length variants precisely for this reason.

Is the power calculation the same for foil heating as for cable?

The procedure is the same – effective area, power density, total power. The difference is that foil heating is used exclusively under floating floors (laminate, wooden panels) and has a lower power density (typically 60–100 W/m²), because it isn't embedded in screed. For primary heating under laminate, foil is less suitable – the thermal resistance of the laminate reduces efficiency. More in the article How to choose electric underfloor heating: cable, mat or foil?

Conclusion: three steps that guarantee correct sizing

The whole calculation can be summarized into three steps that you can do with paper, a pencil, and a tape measure in under an hour:

1. Measure the effective area – total area minus furniture, fixtures, and edge strips. Be precise, every square meter counts.

2. Determine the power density – based on whether you want comfort or primary heating, and the thermal condition of the space. Use the table above as a guide.

3. Choose a product with power as close as possible to the result – ideally 5–15% higher than the calculation. Check the cable spacing and don't forget a thermostat with a floor sensor.

You can find an overview of available products in the electric underfloor heating category. If you're unsure about your choice or have an atypical space, don't hesitate to contact us – we help customers with similar calculations every day.

Do you have a question about this topic?

Can't decide or are dealing with a specific situation in your home? Write to us - we'll be happy to help.