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The iron won't use any more wattage than it is rated for. With that being said what you are actually doing in this circuit is adding resistance, which in any circuit when one variable changes there is either a rise or fall of another of the 3 ( Voltage, Resistance, or Amperage). The voltage of this will remain at 110-125 VAC, while the amps will change resulting in a rise or fall of wattage. Hope this helps you understand a bit more about what you are actually doing by using this with whatever you have plugged into it. If not do some research on the relationship between Voltage, Resistance, and Amperage. Happy building.
Great instructable, works very well.