Yeah anytime you up the voltage you cut the amps.
This is why your 1500watt heater at the house only draws around 12-13 amps.
Try that with 12v .Would be around 125 amps.
Sorry, you are wrong.
Your 1500 watt heater puts out 1500 watts of power because you have 110 - 120 volts applied across it.
Power (watts) equals the voltage times the current (amps). 120V X 12A = 1440 Watts. If the heater is drawing 12 Amps, it's resistance must be 10 Ohms. Since voltage equals resistance times current, 12 Volts across a 10 Ohm load would draw only
1.2 Amps. This is called Ohm's law, basic electronics.