In years past we installed 10 watts per sq ft. With new energy requirements (attic insullation, double pane windows, etc, allow us to install only 3 watts per sq ft. I've never known an inspector to check the wattage of installed heaters. When in doubt I install on the high side of 3W/sq.ft. I believe the inspectors allow a maximum of 3,ooo watts installed on a 20 amp 240 volt circuit.Here in Seattle it does not get below 30 degrees too often. bob