Sizing a heater to your room without guessing

The working rule is ten watts per square foot for a reasonably insulated room with eight-foot ceilings. A 150 sq ft bedroom needs about 1,500W to hold temperature comfortably; a 100 sq ft bathroom is happy with far less, especially for short bursts.

Three things break the rule. High ceilings add volume the rule does not count, so add roughly ten percent per extra foot. Poor insulation or single glazing bleeds heat continuously, which can mean a unit at full output never quite catches up. And an exterior wall on two or more sides makes a corner room measurably harder to heat than the same floor area in the middle of a house.

Undersizing shows up as a heater that runs constantly and never satisfies. Oversizing is less wasteful than people fear — a thermostatic unit simply cycles off sooner — but an oversized fan heater in a small room can feel gusty and dry.

For open-plan spaces, stop thinking in room area and think in zones. Place the unit on the cold side of the space, near where you actually sit, and accept that the far end of a large room will stay cooler. Two smaller units in two zones beat one large one fighting the whole volume.