There is a persistent misunderstanding about heater wattage: that a higher-rated heater costs more to run. It does not, provided it has a thermostat.
Understanding why explains how to size one properly.
Why wattage is a ceiling, not a consumption
A 2000 watt heater draws 2000 watts while it is heating. With a thermostat, it heats until the room reaches temperature and then cycles off.
A 1000 watt heater in the same room draws half as much and has to run twice as long to deliver the same heat.
Total energy used to warm a room to a given temperature is roughly the same either way. The larger heater gets there faster and then idles more.
Where the larger heater genuinely costs more is if it has no thermostat, or if you set the thermostat higher because it can reach higher. Both are avoidable.
The practical consequence is counter-intuitive: an undersized heater is the one that costs more to run, because it runs continuously and may never reach the target at all.
The sizing rule
The working figure for a reasonably insulated room is around 100 watts per square metre of floor area.
| Room size | Wattage |
|---|---|
| 8 m² (small bedroom) | 800–1000 W |
| 12 m² (bedroom, office) | 1200–1500 W |
| 16 m² (living room) | 1500–2000 W |
| 20 m² or more | 2000 W, or two units |
Adjust upward for a poorly insulated room, single glazing, a room above a garage, high ceilings, an external corner room, or anywhere with a draught.
Adjust downward for a small, well-insulated, internal room.
A 2000 watt unit is the practical maximum on a domestic socket, and a room needing more than that needs two heaters on separate circuits or a different solution altogether.
What matters more than wattage
A thermostat. Without one, the heater runs continuously at full power and the room overheats. This is the single most important feature for running cost and comfort.
A timer, so it is not heating an empty room.
An eco or low setting, for maintaining temperature rather than reaching it.
Heaters without a thermostat are the ones that produce the alarming electricity bills people attribute to wattage.
Where the heat goes
Sizing correctly matters less than stopping heat escaping, and this is worth saying plainly because it is where the money actually is.
Draughts. A draught excluder on a door and a sealed letterbox cost almost nothing and reduce demand measurably.
Curtains closed after dark, which cuts loss through glazing substantially.
Heat the room you are in, door closed. This is the case for a portable heater over central heating in a house with one occupied room.
Nothing in front of the heater, including furniture and — a common one — a sofa placed against the wall the heater stands beside.
Insulate before upgrading. A larger heater in a leaky room is treating a symptom.
Choosing the type alongside the size
For a room occupied for hours, an oil-filled radiator sized to the room and left on its thermostat is efficient and comfortable, because retention smooths the cycling.
For a room warmed briefly, a ceramic fan heater of similar rating reaches temperature faster and there is nothing to be gained from retention you will not be there to use.
An oil-filled radiator with a thermostat, sized at roughly 100 watts per square metre, is the arrangement that heats a room comfortably without running continuously. Why a room feels cold despite a running heater is covered at Pelonis Guide.
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