Well,
You know that Watts = V X A.
And you know that Amps = V/R
If you half the voltage on a fixed resistance, you also cut the current flow in half.
So, you have half the voltage and half the current which is equal to 1/4 the wattage.
240 V. x 6 A. = 1,440 W.
120 V. x 3 A. = 360 W.
1,440 x 1/4 = 360.
