Calculate the heater wattage or BTU needed for a room.
Room heaters are sized on the trade rule of ten watts per square foot, which assumes an average-insulated room with an 8 ft ceiling being lifted about 20 degrees F above outside. This calculator scales that baseline on all three assumptions: multiply up for poor insulation or single glazing, for a taller ceiling, and for a larger temperature rise. A 14 by 12 ft average room lifting 25 degrees therefore needs about 2,100 W, which rounds to a 2 kW panel or a 2.5 kW fan heater. Note what the rule is for — this sizes a heater to hold a room at temperature, not to reheat a cold house quickly, and a plug-in heater above 1.5 kW needs a dedicated circuit in most homes.
Heater Size
Watts per sq ft = 10 x insulation factor x (temperature rise / 20) x (ceiling height / 8); heater watts = floor area x watts per sq ft, rounded up to 250 W
Ten as a baseline. Push toward 14 for a draughty single-glazed room or a conservatory, and down to 8 for a well-insulated modern room.
Not on a standard 120 V circuit — 3 kW draws 25 A. In North America anything above about 1,500 W needs a 240 V dedicated circuit; on a 230 V supply 3 kW is 13 A and fits a normal socket.