this stereo 1000w....and 20amps example is stupid because a 20a amplifier takes that current from the mains, at 110v or 240v..which results to 20 X 240v = 4800W. so it can give 4v at 1000amps....i mean this is not all true in real life but maths works this way.
so this could be misleading cos 1000w at the output, at 100% efficiency can be true.
so 12v line at 18 amps = 218W
and 5v line at 30 amps = 150W
and if we sum this we get 368W
at 240v we draw only 1.53amps from mains.
so image if it draws 20amps at 240v how much we get at the output at 100% efficiency.
thanks for reading