I'm looking for clarity about electric towel rails/radiators and wattages. Hope you can help.

I want to connect a heated electric only towel rail into a circuit that has a smart home 'puck' switch rated for a maximum of 400 watts.

Our bathroom requires roughly 1700 BTU (500W), according to online calculators, for a short period. There is a limited amount of underfloor heating so I'm keen to meet as much of this as I can with a towel rail, balanced against size and style.

I've read about how the delta rating affects the stated heat output of radiators in BTUs. The BTU figure is then often converted into watts. The products I'm looking at range from 1503-1649 (439-482W).

However this isn't the required input wattage, that is clearly governed by the element inside.

I've learned that the appropriate element is given by diving the (delta50) output wattage by 1.81 https://www.designerradiatorshowroom.co.uk/product-category/accessories/elements/ https://www.onlyradiators.co.uk/blog/buying-guides/dual-fuel-towel-rail-buying-guide

My question is basically is this right, why, how is it possible?! It seems bizarre to me to use a 250W element to meet a 500W heating need. Presumably the radiator never gets close to it output rating? I guess the only way it does this is by being on twice as long? What am I missing? Or, to look at it another way how does a 250W element product over 482W of heat?

I presume this has something to do with BTU (energy) and W (power) and time but I'm baffled. Will my 400W switch be ok?

Please can someone explain?

Writing to the radiator vendors and makers has so far not gotten me an answer!

Thanks in advance for any thoughts you have.

A supplier has told me

"The Delta/BTU only relate to a radiator that is on central heating and wouldn't apply to an electric radiator. A 400w would simply build up too much pressure and put your radiator at risk of blowing. The element won't change the heat that is generated in the radiator. The watt of the element is due to it being a closed system and nowhere for the excess pressure to be released. The element will produce the heat output that is determined by the rad surface area, not on the size of the element. It will build temperature to the maximum without causing to much pressure and resulting in the radiator leaking or even worse bursting! A 150W element is recommended for this electric only radiator."

I'm still baffled by this logic and would really like to understand it.

  • 2
    Your question is VERY difficult to follow and you are stating things as fact that are, in fact, incorrect. For resistive heating elements, the input power in Watts is very nearly equal to the heat output in Watts. Like where else does the power go? Your assertion that 250W input gives 482W output is like perpetual motion. In other words, it's not true.
    – jwh20
    Commented Dec 7, 2020 at 21:41
  • @jwh This is precisely my point and why I am confused. Sorry if its hard to follow. The specifications of those radiators say over 400W output. Yet I have been told by email and in those other links that the elements that go in them are roughly half that. It makes no sense to me.
    – JamesPB
    Commented Dec 7, 2020 at 22:58
  • 2
    Model # of the towel rack? Commented Dec 8, 2020 at 0:29

2 Answers 2


These are dual-fuel radiators. Meaning they actually can use heat from two different sources, which is why the numbers aren't adding up the way you expect.

In their standard mode of operation, they work just like a traditional radiator on a hot water heating system -- water is heated in a central boiler, then pumped through radiators throughout the building. This is what the output heat spec is referring to -- if you run near-boiling water through this radiator, it'll output that much heat (482W in your example). In this mode, electricity is not used at all.

Where the electric element comes into play is when the central boiler is shut off -- in the summer, for example, you may not need or want nearly as much heat as the central boiler creates, but you may still want some. So for this use case, an electric heating element is turned on. This element could theoretically be any size, but since the use case for electric is for when only a little heat is needed, the manufacturer decided to use elements 1.8x smaller than the radiator's output when using hot water. So, when using electricity only, your example heater will only put out 250W. And since the electric mode only uses 250W, your 400W rated smart switch should work fine.

  • I was really encouraged by your logical answer, thanks. However I've written to one one supplier and they said " The Delta/BTU only relate to a radiator that is on central heating and wouldn't apply to an electric radiator. A 400w would simply build up too much pressure and put your radiator at risk of blowing. A 150W element is recommended for this electric radiator. " So I'm still baffled Any thoughts?
    – JamesPB
    Commented Dec 9, 2020 at 23:24
  • I think what they're saying is that IF you're planning on using these radiators as electric heaters only, and not connecting them to a central boiler, you still have to follow the lower electric element limit. So, if you purchased their 400W heater and put a 400W electric element in it against their advice, it would blow up -- that heater is only designed to dissipate 400W when it's used with a central system. If you actually want 400W of electric heat out of this kind of dual-fuel radiator, you'd probably have to buy one rated for ~1000W on a central system.
    – Nate S.
    Commented Dec 10, 2020 at 0:12
  • 2
    Of course, if you're not planning on connecting your towel rack to a hot water heating loop at all, you'd likely be much better off buying one that's electric only. For example, very different style, but here's a 500W one: wayfair.com/home-improvement/pdp/…
    – Nate S.
    Commented Dec 10, 2020 at 0:26

I seem to have answered the same question elsewhere, but...

A 250W element will only give 250W of heat, whatever radiator you put it in.

But you want to make sure that the power of the heating element is less than the amount of heat that the radiator is designed to dissipate to the room. Otherwise, everything could get dangerously hot.

  • This makes total sense but why does a supplier say that a radiator rated for over 400W when supplied with water from a boiler, can only have a electric element of less than half that wattage? They say a 400W would cause over pressure and burst. It seems an electric radiator can only be at best half as effective in terms of its output as central heating one?!
    – JamesPB
    Commented Dec 9, 2020 at 23:41
  • @JamesPB purpose-designed electric radiators are usually oil-filled, as the oil won't boil. The limitation comes as you retro-fit an element into a water-filled radiator.
    – Simon B
    Commented Dec 10, 2020 at 8:44
  • and as well as 'retrofit' and old radiator, as in this case use a brand new radiator intended for water, with only electric?? And then I presume you'd only get the lower amount of heat out of it not the full advertised output??
    – JamesPB
    Commented Dec 10, 2020 at 9:45

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.