What Size Electric Radiator Do I Need?

It’s not easy deciding on the design, make, and model of electric radiator to buy to heat your home. And once you have decided on that perfect model that suits your furnishings and decor you hit another problem. What size electric radiator do you need?

We’re not talking about the physical size of the radiator because that doesn’t really matter, well, only aestetically, not from a heat point of view. When deciding on the size of radiator you need to heat each room, it’s the amount of heat it will produce that’s important. So let’s jump straight into how to calculate what size radiator you need for each room.

How To Calculate The Correct Size Radiator For Your Needs

In very general terms, electric radiators are classified by the maximum watts they can produce. Usually the higher the watts, the more power the radiator can produce and that means the higher the heat level. However, wattage is not the only consideration that needs to be taken into account, you have to look at the efficiency of the radiator to produce heat.

We won’t go into the various types of electric radiators here because if you’re looking at sizes, you’ve already made your decision on the type of radiator you want. But if you are still undecided and want to make a much more informed decision check out our best electric radiators guide by clicking here. In a nutshell, infrared radiators are much more energy efficient but also cost more to purchase.

Electric Radiator Size Calculation


The simplest way to work this out is to work out the square footage of the room and multiply that figure by 10 watts. For instance – A bedroom that measures 12 foot by 12 foot will look like this:


This calculation assumes that the room is built to modern standards with all necessary insulation and that the ceiling is 8 foot high. If your room is not modern, insulated and has different ceiling heights you need to take that into account. Differences include:

  • 10 foot high ceiling
    If the ceiling is 10 ft high, not 8 ft high, add 25% more wattage
  • 12 foot high ceiling
    If the ceiling is 12 foot high, not 8 ft high, add 50% more wattage
  • Fully insulated home
    If your home is fully insulated up to or above recommended levels, multiply by 7.5 watts, not 10 watts.
  • Older, traditional home (with less insulation)
    Multiply by 12.5 watts not 10 watts.

So going back to our calculation, 1440 watts would mean we would need a 1500 watt radiator. If that room had a 10 foot high ceiling, the calculation would look like this


Giving a wattage of 1800 or 1.8 kilowatt. Now this doesn’t need to be one 1800 watt radiator, you could use 2 900 watt radiators if it suited your room better. And so on, by using this calculation and rounding up to the nearest wattage, you will be able to find the perfect size radiator for your needs.

Why Wattage Rating Can Be Misleading

The problem is, on most electrical appliances-radiators included, the wattage figure quoted is for energy input. That’s great for calculating how much energy the appliance is using, but it gives no indication as to the amount of heat the radiator generates. The most modern, energy efficient radiators, like infrared, are pretty good at converting energy into heat.

The older type convection heaters however, can be less efficient, and this can cause some problems. You don’t want to be paying for electricity that isn’t heating the room, but you do want to heat the room. Here’s the conundrum, energy in should equate to energy out, so how can you be sure you’re not paying for power you’re not benefitting from?

What Is An Electric Radiators Output

We’ve established that the wattage on an electric radiator refers to the amount of power going into it, (input), so how do we determine it’s output? Well, we don’t, you see as long as you are buying a modern, up to date electric radiator it will have been designed to be energy efficient. That means the input in watts should equal the output in watts too.

So using our calculation from earlier we need an 1800 watt radiator to heat our 12×12 room with a 10 ft ceiling. Which type of electric radiator will heat our room more efficiently? Any modern electric radiator will do the job, but it comes down to style and choice, and we can’t help you there. It’s a personal thing, what might suit your home and lifestyle might be completely wrong for our home and so on.

Choosing An Electric Radiator For Your Home

If you need help choosing the correct electric radiator for your home, be sure to check out our guide here where we explain all the key factors you need to take into account before buying.

Frequently Asked Questions

How do I calculate the size of radiator I need for my room?

To calculate the size of radiator you need for your room you need to multiply the width by the length and multiply that by 10 to get the total watts needed to heat that area.

What is a radiator output?

A radiators output is the amount of heat the radiator puts into the room.

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