BTUs and watts both measure heat output. When you buy a radiator or towel rail, the size you need is worked out from the heat your room loses, and that number is given in one or both of these units. Knowing how they relate stops you buying a radiator that is too small to heat the room.
Short answer: 1 watt = 3.412 BTU per hour. Work out how much heat your room loses, then choose a radiator whose output at your system's temperature is at least that figure.
What is a BTU?
A BTU (British Thermal Unit) is the heat needed to raise one pound of water by one degree Fahrenheit. In heating, radiator ratings are given in BTU per hour (BTU/h), meaning how much heat the radiator gives out each hour. UK radiator sellers have used BTUs for decades, so you will still see them on most product pages next to watts.
What is a watt?
A watt is the SI unit of power, one joule per second. Most radiator manufacturers now publish output in watts, and watts are the standard for electric heating and for heat pumps.
- Central heating radiator: the watt figure is the heat it gives out to the room.
- Electric radiator or towel rail: the watt figure is the electricity it draws, and essentially all of it ends up as heat in the room. A 300W element gives you about 300W of heat.
Converting BTU to watts
- Watts to BTU/h: multiply by 3.412
- BTU/h to watts: multiply by 0.293
| Watts | BTU/h |
|---|---|
| 300 W | 1,024 |
| 500 W | 1,706 |
| 750 W | 2,559 |
| 1,000 W | 3,412 |
| 1,500 W | 5,118 |
| 2,000 W | 6,824 |
| 2,500 W | 8,530 |
How much heat does my room need?
The heat a room needs depends on its size, how well it is insulated, the number and type of windows and external walls, and the temperature you want. Bathrooms usually need more heat per square metre than bedrooms, because you want them warmer (22–24°C, see our ideal radiator temperature guide).
- Work out the room's heat loss. The quickest way is our BTU calculator, which takes the room size, insulation, glazing and walls into account.
- Check the radiator's output at your system temperature. Compare like with like (see the next section).
- Add a small margin. Many installers allow 10–15% over the calculated figure, especially for north-facing rooms or rooms you want to warm up quickly.
For example, if the calculator says a bathroom loses about 800W, you need a radiator or towel rail rated at 800W or more at your system's temperature. That is about 2,730 BTU/h. With a 10–15% margin, look for roughly 900W.
Why one radiator has two different ratings
Radiator output depends on how hot the water is compared with the room. Radiators are tested to the European standard EN 442 and often quoted at two conditions:
| Rating | Mean water temperature (room at 20°C) | Used for |
|---|---|---|
| ΔT50 | About 70°C | Traditional gas boiler systems, the headline figure |
| ΔT30 | About 50°C | Heat pumps and low-temperature systems |
A radiator gives out roughly half as much heat at ΔT30 as at ΔT50. A towel rail rated at 1,000W at ΔT50 will deliver around 500W on a low-temperature heat pump system. If your system runs cool, size from the ΔT30 figure. If you do not know which your system uses, ask your installer or see what flexible heat pump technology means.
Compare like with like. A 1,000W radiator at ΔT50 is not the same as a 1,000W radiator at ΔT30. Check which temperature the manufacturer's figure refers to before you compare two products.
BTUs and watts for towel rails
- Central heating towel rails are rated in BTU and watts like any radiator, but are slimmer, so the output is lower per metre of width. They are often chosen as a warm, dry spot for towels rather than the room's only heat source. See how to determine towel rail size.
- Electric towel rails run from fixed-wattage elements, commonly in the low hundreds of watts. They suit a bathroom that is already warm, or a small room, and heat towels independently of the boiler. See how electric towel rails work.
- Dual fuel towel rails can run off the central heating in winter and an electric element in summer. See what dual fuel means.
- Finish matters a little. See why white towel rails can be more efficient than chrome.
Frequently asked questions
What is the difference between BTU and watts?
Both measure heat output. A watt is a metric unit of power, and a BTU/h is the older imperial equivalent. 1 watt equals 3.412 BTU/h.
Is a higher BTU radiator better?
Only if the room needs it. A radiator that is too small will not heat the room, and one that is far too big wastes money and can overheat the space. Aim for the output your room needs plus a small margin.
How many BTUs do I need per square metre?
It varies with insulation, glazing and how warm you want the room, so there is no fixed number. A well-insulated modern room needs much less than a draughty period property. Use the BTU calculator for a room-specific answer.
Does a towel rail need to match the room's full heat loss?
Not if the room has other heating. In a bathroom with underfloor heating or a separate radiator, a towel rail may be there mainly to dry towels. If it is the only heat source, size it to the full requirement.
Do BTU figures change on a heat pump?
The rating on the box does not change, but the heat you actually get does, because heat pumps run at lower water temperatures. Look at the ΔT30 figure, or choose a larger radiator.
How do I convert my radiator's BTU to kW?
Multiply BTU/h by 0.000293. For example, 5,000 BTU/h is about 1.47kW, or 1,465 watts.
Not sure what size you need? Try our BTU calculator or browse heated towel rails. For advice on a specific room, contact us on WhatsApp.
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