1500 watts = 12.5 amps @ 120v. I've also never seen a unit that pulls 13.5 amps and is able to plug into a 15 amp Recp. Generally things are built for continuous loads at 80% of the circuit rating max. I'm not even sure how they get away with 12.5 amps but I guess it's only 83% and everyone likes their even numbers. But even if there something screwy with the heater, the 900 watt setting should only be pulling 7.5 amps far below all Max ratings.