How to size an electric radiator

In addition to picking the right type of electric radiator, it's important to choose the correct size for your space to ensure maximum comfort and energy savings. Please note that this guide deals with metric values measured in watts not imperial values measured in BTU. As a guide, 1 W is equal to 3.41 BTU/h. Therefore to convert watts into BTU/h you need to multiply your value in watts by 3.41.

Several formulas can be used to calculate the correct size of radiator and our calculator is available online. These calculators can get you results accurate to about 20%, and vary in accuracy depending on the values required. In this guide, we'll set out a simple calculation to estimate the power required from your electric radiators as well as a trickier method that accounts for heat loss and minimum temperatures where you live.

Sizing an electric radiator: the simple method

The easiest way involves multiply a coefficient of your insulation measured in watts (W) by the surface or volume you want to heat.

For more accurate results you can:

• deduct 20% if the room or home is surrounded by heated spaces (i.e. in a block of flats);

• deduct 10% if the room has a lot of direct light (i.e. a south-facing room);

• add 10% for every 500 metres of altitude (recommended);

• add 10% if your space is north-facing (recommended).

1. Room volume will provide more accurate results than surface since there can be a ceiling height difference of several metres between a loft room and a ball room!

2. Insulation quality: you won't get accurate calculations with a draughty house that throws off all estimations. A well insulated home is much more energy efficient to heat.

3. The area in which you live: a house at 300 metres of altitude in the north of the country won't have the same heating needs as a home in the south.

4. Desired room temperature: it's entirely up to you whether you prefer things brisk or tropical.

This method estimates how much heating will be required to make up for the heat loss in a home using the G coefficient, which is a common formula across Europe. The G coefficient is defined as the hourly rate of heat flow per unit volume of the interior space per degree.

This formula accounts for heat loss by taking the G coefficient of heat loss and multiplying it by room volume (V) and the temperature difference between inside and outside temperatures (Delta T).

• Heat loss = Coef. G x Volume x Delta T

Please note: it is recommended to add on a certain percentage to the results to avoid any risk of discomfort in very cold temperatures (about 20% for homes with average insulation).

Surface area

Surface area is measured in square metres (m²) and is calculated by multiplying the length of a room by its width:

• Width (W) x Length (L) = Area in m²

Room volume

Volume is measured in cubic metres (m3) and is calculated by multiplying the area of a room by its height:

• Area in m² x height (h) = Volume in m3

The heat loss of a home can be noted in watts per cubic metres and degrees Celsius (W/m3 and °C) (or BTU / h F² and °F for imperial calculations). This value can be used to evaluate the heating needs of a home.

Basic G coefficient values

Minimum temperature differences

The outside temperature to work out temperature differences in the UK is usually given as -1°C (30° F)  which is about the minimum normal temperature in winter. However, this can vary depending on where you live.

Temperature difference should be calculated using average home temperatures which vary from about 18 to 21°C, but this differs depending on room type and your own preferences.

Temperatures by room type

Room temperature refers to the desired temperature of a specific room or the set temperature for each room as set by a smart or multizone thermostat. In short you have a couple of options:

• Follow the recommended temperature guidelines to save money and energy.

• Simply set temperatures that you are comfortable with.

The World Health Organisation (WHO) advises that UK households should have a room temperature of around 18°C in the winter months. The Energy Saving Trust advises that you should always set your thermostat to the lowest comfortable temperature but that we should still aim for temperatures of around 18-21°C. Bedrooms can be kept slightly cooler overnight at around 16-18°C while bathrooms should be a bit warmer at 22 to 24°C.

Example 1 = radiators in a well insulated home

Let's take an extreme example of a well insulated home in an area with a very low minimum temperature of -14°C.

• Ceiling height: 2.5 m

• Room volume: 75 m3

• Insulation: good / G coefficient of 1

• Desired temperature: 19°C

• Minimum temperature: -14°C

Simple method

• Volume x 40 W (0.04 kW) = radiator power in watts

• (75 x 40) + 10 % = 3300 W

According to this method, a 3300 W radiator is required to heat this space.

G coefficient method

Heat loss = G x Volume x Delta T

• 1 x 75 x 33 = 2475

• 2475 x 20 % = 2970 W

According to this formula with a G coefficient of 1, a 3000 W radiator is required (rounded up).

Example 2: radiator for an extremely well insulated home

This time, let's take an extreme example of a fairly well insulated home in an area with a very low minimum temperature of -14°C.

• Ceiling height: 2.5 m

• Room volume: 75 m3

• Insulation: very good / G coefficient of 0.35

• Desired temperature: 19°C

• Minimum temperature: -11°C

Simple method

• Volume x 15 W (0.015 kW)= radiator power in watts

• (75 x 15) – 10 % = 1012.5 W

According to the simple formula, a 1000 W radiator is required (rounded down).

G coefficient method

• Heat loss = G x Volume x Delta T

• 0.35 x 75 x 30 = 787.5 rounded up to 1000 W

According to G coefficient formula, a 1000 W radiator is required (rounded up).

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