Calculator
Radiator performance calculator for the bathroom
Enter the area and height of the bathroom and the type of building. The calculator estimates how many watts the bathroom needs and recalculates what catalog performance the heated towel rail should have based on the water temperature of your heating system. This is a first estimate – the exact heat requirement will be determined by a heat loss calculation performed by a heating engineer or designer.
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Result
At water temperatures of 55/45 °C, the heated towel rail in a bathroom at 24 °C only delivers about 40% of the catalog performance. If such a heated towel rail is too large, supplement it with underfloor heating or a heating element.
Heating elements typically range from 200 to 800 W. If the bathroom requires more, supplement the heated towel rail with electric underfloor heating.
If towels will hang on the heated towel rail and it is the only source of heat, choose a performance from the upper part of the range.
Bathroom radiators at vkupelni.sk ↗
This is a preliminary estimate, not a calculation of heat losses. The exact heat requirement will be determined by a heating engineer or designer according to STN EN 12831.
How the calculator works
First, calculate the volume of the bathroom: area times ceiling height. Then multiply the volume by the number of watts per m³. A common guideline is 40 to 100 W per m³ – an insulated building is closer to the lower end, while an older uninsulated house is closer to the upper end. The calculator divides this range according to the type of building:
| Type of building | W per m³ | Bathroom 2,5 × 2 × 2,6 m (13 m³) |
|---|---|---|
| New build or insulated house | 40 to 60 | 520 to 780 W |
| Apartment in a prefab block of flats | 50 to 75 | 650 to 980 W |
| Older uninsulated house | 75 to 100 | 980 to 1 300 W |
The division is our rough estimate within the rule of 40 to 100 W per m³. A bathroom with a window, an exterior wall, a corner bathroom, or a bathroom with a high ceiling requires more heat. When calculating heat losses according to STN EN 12831, the bathroom is considered at a temperature of 24 °C, while the living room is at 20 °C.
Conversion to catalogue performance of the heated towel rail
The performance in the catalogue is according to standard EN 442 for water at 75/65 °C (supply and return) and a room at 20 °C. In a bathroom at 24 °C and with lower water temperature, the same heated towel rail will provide less heat. Example from the manufacturer Korado catalogue (heated towel rail Koralux, type KLN 1700.500):
| Water and room temperature | Heated towel rail performance | Share from the catalogue |
|---|---|---|
| 75/65 °C, room 20 °C (catalogue) | 803 W | 100 % |
| 75/65 °C, bathroom 24 °C | 718 W | 89 % |
| 55/45 °C, bathroom 24 °C | 335 W | 42 % |
The calculator will therefore divide the required performance by this share. If the bathroom needs 650 W and you are heating with water at 55/45 °C, look for a heated towel rail with a catalogue performance of approximately 1,560 W. For other heated towel rails, the share may vary slightly – manufacturers provide performance at different temperatures in technical data sheets.
Electric and combined heated towel rail
An electric heated towel rail is filled with antifreeze and heated by a heating rod. The rods typically have a performance of 200 to 800 W, and the rod's performance is also the performance of the heated towel rail. A combined heated towel rail is a hydronic heated towel rail with a heating rod – in winter, it heats with water from the heating system, and in summer, the rod dries towels. The article Electric radiator for the bathroom explains electricity consumption and safety in zones.
When one heated towel rail is not enough
- Larger heated towel rail – a taller or wider model has greater performance if there is space on the wall.
- Underfloor heating – an electric mat under the floor tiles will heat the floor and supplement the heated towel rail. More in the article Underfloor heating in the bathroom.
- Towels – if you cover the heated towel rail entirely with towels, it will heat the bathroom less effectively. If it is the only source of heat, choose a performance from the upper part of the range.
What to consider before tiling
The connection of the hydronic heated towel rail – side, bottom, or centre with a spacing of 50 mm – will be determined by the heating engineer before tiling to ensure the outlets are positioned correctly. An electrician will connect the electric heated towel rail and heating rod according to the zones in the bathroom. Always consult a professional when dealing with water and electricity. How to choose the height, width, and colour of the heated towel rail can be found in the article Heated towel rail for the bathroom.
Where to choose a radiator
Heated towel rails, electric and combined radiators can be found on the e-shop vkupelni.sk in the category Bathroom radiators – each model lists its performance and dimensions. You can also view radiators and heating for bathrooms in person at the store in Senec at Bratislavská 71.
Related: Heated towel rail for the bathroom · Electric radiator · Underfloor heating · All calculators
Sources: EN 442 (standard performance at 75/65/20 °C), STN EN 12831 (calculation of heat losses, 24 °C in the bathroom), Korado Koralux catalogue (performances of type KLN 1700.500 at various water and room temperatures), guideline of 40 to 100 W per m³ from articles on heating websites, manufacturer instructions for heating rods, e-shop vkupelni.sk, category Bathroom radiators (November 2026).
Terms in the glossary: Catalogue performance of the radiator (75/65/20 °C) · Underfloor heating · Heated towel rail
Frequently Asked Questions
What radiator performance is needed for the bathroom?
For a rough estimate, multiply the volume of the bathroom by 40 to 100 W per m³. A bathroom measuring 2.5 × 2 m with a height of 2.6 m has a volume of 13 m³ and needs approximately 520 to 1,300 W – an insulated building is closer to the lower end, while an older uninsulated house is closer to the upper end.
How many watts per m² of bathroom?
With a ceiling height of 2.6 m, the rule of 40 to 100 W per m³ corresponds to approximately 105 to 260 W per m² of floor area. A bathroom with an area of 5 m² thus needs roughly 520 to 1,300 W.
Why is the heated towel rail not heating as stated in the catalog?
The catalog performance applies to water at 75/65 °C and a room temperature of 20 °C. If the bathroom is at 24 °C, modern boilers and heat pumps heat water at lower temperatures. At 55/45 °C, the heated towel rail in the bathroom only delivers about 40% of the catalog performance.
What heated towel rail is suitable for a heat pump?
A larger one. For water at 55/45 °C, look for a heated towel rail with a catalog performance about 2.4 times higher than what the bathroom needs, or supplement the heated towel rail with underfloor heating or a heating element.
Is an electric heated towel rail sufficient for the bathroom?
Often yes for a smaller bathroom. Heating elements typically range from 200 to 800 W. If the bathroom requires more, supplement the heated towel rail with electric underfloor heating.
Must a radiator be connected by a professional?
Yes. A water heated towel rail is connected to the heating system by a heating engineer, while an electric heated towel rail and heating element are connected by an electrician according to the zones in the bathroom.
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