This demo is based on a battery voltage of 13.8 with a class a/b amplifier 60% efficiency
The efficiency stays the same throughout the power range
In the real world the efficiency will actually vary with output
It's still a great reference point
 
If you are not using fuses, change the length of wire from the battery to the fuse to 0ft
If you are not using a  distribution block, change the length of wire from the fuse to distribution block to 0ft
Use 0 for 0 gauge.   Use -1 for Double 0 gauge
Don't forget your ground wires and amplifier rating