Sounds like everyone else hit this one. It is a method of comparing amps. Simple fact is, one with a 30 amp fuse can take in 30 amps and one with a 20 amp fuse can only take in 20. Does that mean the 30 is better? Not really. The efficiency of any given amp can vary from 40% to 90%. That means that a really efficient amp (90% full output) with a 50 amp fuse can do 540 watts (12v). A really inefficient amp (40%) will take in over 110 amps to do the same amount of power.
