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Is it cheaper to heat one room with an electric radiator?


Jo Alsop

Heating Hero

The Heating Hub

Jo answers a common question, should we just heat one room with electric or keep the boiler on?


Electric heater v gas/oil boiler cost comparison

As prices rise, many of us are searching for ways to reduce our energy bills. A really common question I am asked is whether it is cheaper to heat one room with an electric heater than to have the gas or oil boiler heat the home.

Working out whether it is cheaper will depend on the size of the room you wish to heat, commonly an office space or sitting room, and how well it is insulated compared to the size of your home and size of your gas or oil bills. We've also given some bonus guidance on wood burners and electric blankets.

Step 1 - know your current heating costs

A good place to start is with your energy bill (if you are on gas). This will tell you how many kWh you are projected to use in a year in a year (commonly 8,000kWh - 25,000kWh) and in the month just gone.

In the table we give approximate costs to heat your home for 9 hours per day depending on home many kWh of gas you use in total per year.

If you are oil, you will buy by the litre. We give a couple of examples below based on how much oil households typically buy. We need to convert it to kWh to work out the daily cost.

NB we have to account for gas inefficiency when working out the cost per kWh.

Heater output Hours No. kW heating per day Price per kWh Cost per day
Small home/flat gas boiler (8,000kWh) 9 27 12.3p (80% boiler efficiency) £3.34
Average home gas boiler (12,000kWh) 9 43 12.3p (80% boiler efficiency) £5.31
Large home gas boiler (17,000kWh) 9 75 12.3p (80% boiler efficiency) £9.27
Large home gas boiler (22,000kWh) 9 102 12.3p (80% boiler efficiency) £12.61
Average home oil boiler (1,500 litres) 9 67 13.4p £8.98
Large home oil boiler (3,000 litres) 9 151 13.4p £20.23

Step 2 - work out your electric heater output

Electric heaters come in many forms, from blow heaters (often under the desk) to large oil filled electric radiators. Their outputs vary between 500Watts (or half a kW) to 2.5kW but their efficiency does not vary.

Their kW output can vary through the day, for example the radiator may be on full for the early parts of the day as the room warms up and then turn on and off to maintain the room temperature. This will reduce the total number of kWh used.

A kWh is the kW output of the appliance x the time it is on. For example, a 1kW electric heater on for 2 hours uses 2kWh. A 1kW heater that is on for 1 hour at 1kW and 1 hour and 0.5kW will use 1.5kWh.

It is therefore difficult to calculate how much the heater will be on, and certainly it will be on less in milder weather. The price comparisons below assume it is on at full capacity for the whole day, which will be the case in colder weather

Heater output Hours No. kW heating per day Price per kWh Cost per day
500Watt electric heater - small office or well insulated room 9 4.5 34p 1.53
1kW electric heater - larger office or well insulated larger room 9 9 34p £3.06
2kW electric heater - large sitting room/poorly insulated room 9 18 34p £6.12

Step 3 - is it a clear case or marginal case?

On a desktop calculation basis, we can only say when it is definitely worth doing, i.e. small offices or sitting rooms in homes that use a lot of gas or oil. Heating a larger office with 1kW electric heater on for 9 hours costs £3.06 compared to a 22,000kWh home which costs £12.61 per day to heat or a 3,000 litre oil home which costs over £20 per day.

What if I turn all my radiator valves off except for the one room?

The boiler will run very inefficiently if it only has one radiator to heat, although it still may cost less that an electric heater. Boilers are already oversized and restricting how much it has to heat make it even more oversized which means it will cycle endlessly. Gas boilers will tolerate this better than oil boilers, but both will experience greater wear and tear. For most oil households our advice would be to go with the electric heater rather than operate the boiler in the way.

What about a log burner?

This is an even trickier question! Logs provide different quantities of heat (based on their calorific value) depending on whether its hard wood or soft wood and how dry they are.

Hard woods are more expensive because they give off more heat. Damp wood will use a lot of energy to dispel the moisture from it before it can provide much heat, but dry wood is more expensive because it has been seasoned (left to dry for 12 months or so) or kiln dried (not very environmentally friendly).

Seasoned wood (dried and good quality) costs around 10p per kWh. A 3kW stove (ignore the kW rating of the stove itself, it will be based on how much wood is in the stove) running for 9 hours will cost £2.70. 

If you can get your wood for free then even cheaper, but only use untreated wood i.e. wood without any paint or chemicals in it. Whatever you are using, it is a manual job to stack the logs when delivered and bring them in daily and load the fire. Not for the faint hearted!

What about an electric blanket?

Octopus run an electric blanket trail last winter which found households saved around £300 by using an electric blanket instead of having the heating on. Their electric blanket used just 0.1kW per hour, costing 3.4p per kWh under the current tariff or 30.6p per day, which is a huge saving over all of the above forms of heating.


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