Heater size for a 12 m² bedroom
A 12 m² bedroom needs roughly 720-1,200 W of electric heat — 60-100 W for every square metre of floor, and more if the walls are uninsulated. Which type to buy depends on how fast you want the warmth and how long it should linger after switch-off.
| Heater type | Power draw |
|---|---|
| Fan heater | 1,500-2,400 W |
| Oil-filled radiator | 1,500-2,500 W |
| Halogen heater | 400-1,200 W |
| Immersion water heater | 2,000 W |
| Frost-protection setting | Fraction of rated power |
Roughly 720-1,200 W, at 60-100 W per square metre of floor area — more if the walls are uninsulated.
The sizing rule is simple multiplication: take the floor area in square metres and allow 60-100 W for each one. For 12 m² that gives a band of 720-1,200 W. A well-insulated room with modern windows sits near the bottom of the band; a draughty room with uninsulated walls sits at the top or above it.
Insulation is the single biggest correction. The same 12 m² bedroom in an uninsulated wall construction can need noticeably more than 1,200 W to hold temperature, which is why two identical-looking rooms can need different heaters.
Oversizing is tolerable because of the thermostat. A 1,500 W heater in a room that only needs 900 W will reach the set temperature and then cycle on and off instead of running continuously, so the average draw settles near what the room actually loses.
Typical times for portable heaters rated between 400 and 2,500 W.
For overnight bedroom use an oil-filled radiator is the steady choice; fan and halogen heaters solve different, shorter-term problems.
A fan heater rated 1,500-2,400 W moves 200-400 m³ of air per hour and warms a room in 5-10 minutes. The trade-off is retention: the warmth fades within minutes of switching off, so it suits a quick warm-up before bed rather than all-night heating.
An oil-filled radiator rated 1,500-2,500 W needs 15-30 minutes to bring its fins to about 60-90 °C, then holds the temperature by cycling on the thermostat. It is slower to start but steadier to sleep next to, with no fan forcing air around the room.
A halogen heater rated 400-1,200 W works by infrared: it warms people and objects within 1-2 seconds instead of heating the whole air volume. It is a spot heater — effective where you sit, not a way to raise the temperature of a whole bedroom.
Running cost is watts multiplied by hours multiplied by your tariff — nothing else enters the equation.
The arithmetic is fixed: 1,000 W running for one hour is 1.0 kWh. A 2,000 W immersion water heater running for 30 minutes uses exactly 1.0 kWh, which at a tariff of 0.15-0.30 USD per kWh costs 0.15-0.30 USD per use. The tariff varies by country, so the same heater costs twice as much to run in one place as in another.
A 2,000 W room heater at full power for one hour uses 2.0 kWh, or 0.30-0.60 USD at the same tariff range. A correctly sized 900 W average draw on a cycling thermostat costs less than half of that over the same hour.
Frost-protection mode is the cheapest way to keep an empty room safe: it holds 5-7 °C — just enough to stop pipes freezing — and draws only the fraction of rated power needed to maintain that floor.
| Heater type | Power draw | Time to feel warmth | What it heats best | Warmth after switch-off |
|---|---|---|---|---|
| Fan heater | 1,500-2,400 W | 5-10 minutes for room air | Whole-room air, moved at 200-400 m³/h | Fades within minutes |
| Oil-filled radiator | 1,500-2,500 W | 15-30 minutes to fin temperature of 60-90 °C | Steady, even whole-room heat | Thermostat cycles; fins release stored heat |
| Halogen heater | 400-1,200 W | 1-2 seconds | People and objects in the infrared beam | None — radiant heat stops instantly |
| Immersion water heater | 2,000 W | Heats water, not rooms | Water in a tank or bucket | 1.0 kWh consumed per 30-minute use |
| Frost-protection setting | Fraction of rated power | Maintains rather than warms | Empty rooms and exposed pipes | Holds 5-7 °C continuously |
Figures are drawn from manufacturer ratings for portable heaters, the EU space-heater energy label rescale of March 2021, and standard 230 V electrical arithmetic.