A 1,500 watt heater draws 12.5 amps at 120 VAC, but the same heater operating on 240 VAC draws just 6.25 amps.
120 VAC times 12.5 amps:1,500 W
240 VAC times 6.25 amps:1500 W
That part was true, the rest wasn't. It draws half the current at 240 VAC because current was halved at the same resistance/impedance for the SAME WATTAGE. Don't reduce the resistance for the higher voltage by having double the windings or whatever and see what happens. It will then be destroyed from overheating.
I assume that heater has a switch for 120 VAC or 240 VAC operation. That makes it simple to double the windings for 240 VAC or add resistance or whatever the switch does.
But these days, more is possible with various types of tricks, especially in switch-mode power supplies.
For an example, I have an electric motorcycle that charges at 1.4 KW if 120 or 240 VAC is used.
I have another where if I charge with 120VAC it will charge at 1.4KW, but with 240 VAC it will charge at 2.8KW.
The first example has a switch mode power supply type of charger with some very tricky circuity to reduce the current as the voltage increases to keep the wattage the same. The 2nd example has no such special circuity, but still uses a switch-mode power supply. So then the power is allowed to increase.
The reason the first example will not allow the wattage to increase is because the small power supply would have to waste that power in heat and would then blow itself out. The 2nd example has a much larger power supply and can allow more than double the wattage with 240 VAC.
So, there are times when some items these days can SEEM to be going against Ohms law, but in reality, they are not. They are just being tricky.
In fact, if I use the OBC (On Board Charger) with the electric motorcycle I have with me now, there is NO difference in charge time or wattage with any AC voltage from 90 VAC to 250 VAC. But the heat and current draw will then be MUCH less at the high end of the voltage range.
But I also have two 2.5 KW external power supplies with me for the same bike. They will be at 1.4 KW with 120 VAC but 2.5 KW if I use 240 VAC.
Do I have you confused yet?
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-Don- Mayo, FL