How do I heat one room cheaply?

Which heater type costs least to run, why the answer is usually 'the one heating the smallest space', and the free measures first.

Difficulty: beginner
Time: 30 min
Read: 2 min
Safety: warning

Short answer

All electric heaters cost the same per unit of heat — the differences are in how they distribute it. Heating the person rather than the room is what actually saves money, so a heated throw costs a fraction of any heater.

The most misunderstood point about electric heating: every electric heater converts electricity to heat at essentially 100%, so a 2kW fan heater and a 2kW oil radiator cost exactly the same per hour. Choosing between them is about comfort and distribution, not efficiency.

Safety

Never use a gas hob, oven, barbecue or any fuel-burning appliance not designed for indoor heating to warm a room — this produces carbon monoxide and kills people every year. Never cover an electric heater or leave one unattended near fabric. Do not run heaters from extension leads or multi-way adapters, and always use a carbon monoxide alarm if you have any fuel-burning appliance.

Step by step

Draught-proof the room first.A one-off cost of a few pounds that reduces heating need permanently. Doing this before buying a heater is the actual saving.
Close the door and the curtains.Free, immediate and surprisingly effective. Curtains closed at dusk cut a significant amount of heat loss through glass.
Understand that all electric heaters cost the same to run.Per kilowatt-hour of heat, they are identical. Adverts implying one type is cheaper to run are describing distribution or thermostats, not efficiency.
Match the heater type to how you use the room.Fan heater: fast, noisy, good for short bursts. Oil-filled radiator: slow, silent, good for sustained background heat. Halogen or infrared: heats objects and people directly, good for immediate localised warmth.
Use a thermostat and a timer.The genuine saving in any heater. One that runs constantly at full power costs far more than one cycling to hold a temperature.
Heat the person, not the room.A heated throw or electric blanket runs on a fraction of the power of any room heater. For sitting still, this is by far the cheapest option.
Compare with turning the central heating on.If the boiler can heat just that room using thermostatic valves, gas is usually cheaper per unit than electricity. Turn the other radiators down rather than assuming a heater is cheaper.
Never improvise a heat source.Gas hobs, ovens and outdoor appliances used indoors produce carbon monoxide. This kills people every winter and is entirely avoidable.

Tips

A thermostat matters more than the heater type. It is the difference between running constantly and running a third of the time.
Reflective panels behind radiators on external walls are cheap and do help, particularly on uninsulated solid walls.
If the room is cold because of draughts, no heater fixes it economically. Address the draughts first.

Common mistakes

Believing one electric heater type is cheaper to run — They are all essentially 100% efficient. The differences are speed, noise and distribution, not running cost per unit of heat.
Using a gas cooker to heat a room — It produces carbon monoxide in an unventilated space. This is a genuine and regular cause of death.

Questions people ask

Which electric heater is cheapest to run?

None — they all convert electricity to heat at essentially the same rate. What varies is how quickly and where the heat goes, and whether there is a thermostat.

Is it cheaper to heat one room or the whole house?

One room, generally. But if you have central heating, turning down the radiators elsewhere is often cheaper than an electric heater, since gas usually costs less per unit than electricity.