[I_Am_Nous]

It's related to Ohm's law, but if you have a specific load you need to power (say 8 watts) a 12 volt system can run that using 0.6 amps (at 18 Ohm) while a 48 volt system can run that same 8 watt load at 0.16 amps (at 288 Ohm).

Another way to look at it is a little easier with something like a 24 volt PoE injector vs a 48 volt PoE injector. The 24 volt supplies the needed energy at 1 amp, but the 48 volt supplies the same energy at 0.5 amps. Both work out to 24 watts (volts*amps=watts) but a wire carrying 48 volts doesn't generate as much resistance, which would be lost as heat into the wire carrying the load. If it loses too much heat because the resistance is too high, well, that's how you make a heating element -- cram amperage into a wire until the resistance makes it hot.