Alternator amperage

For you audio guys out there do you know of a calculation or trick to know how big of alternator you need for a certain amount of wattage output?

Watts divided by voltage should show amps being drawn.
1000 watt amp divided by say 12.5 volts is drawing 80 amps. That will give you a idea because that is figuring the amp a max power constantly. There may be a better way, that's just what I was able to come up with.