New post with correct calculations but a 4500W 240V heating element will produce just 180W @ 48V
Current at 240V is 18.75A (=4500/240)
The resistance of the element is 12.8 ohms (=240/18.75)
At 48V the current is 3.75A (=48/12.8)
The power at 48V is 180W (=48*3.75)