Quick Scoop

A simple rule of thumb is to size an oil-filled radiator at about 100 W per square meter for a well-insulated room, or about 60 W per square meter for a very well-insulated room. Larger rooms, poor insulation, or high ceilings need more heat output, and compact models can still be powerful if the wattage is high enough.

How to choose

Use your room’s floor area as the starting point, then adjust for insulation and room use. One guide suggests 500 W for very small spaces, around 1000 W for small rooms, 1500 W for larger bedrooms or medium rooms, 2000 W for bigger living areas, and up to 2500 W for large rooms or open spaces.

Fast estimate

  • Small room: about 500–1000 W.
  • Medium room: about 1500 W.
  • Large room: about 2000–2500 W.
  • Poor insulation or cold climate: go up from the base estimate by roughly 20%–50%.

Example

If your room is 12 m² and reasonably insulated, a 1200 W radiator is a good starting point using the 100 W per m² rule. If the room loses heat easily, stepping up to around 1500 W is safer.

If you want, I can also turn this into a quick room-size-to-wattage chart for your exact room.