I think ImportCarTuner hit this right on the head. Your car should be running when you're using an audio system like this. If not, be prepared to buy a new battery on a regular basis - even Optimas have a life. While running, the alternator is doing the work. Add caps for bass hit supplement.

If you really want to do this right, add up the current draw of everything non-stock. For the audio amps, you'll have an average. Add that to the stock alternator output and the sum will be the size aftermarket alternator you'll need. Going 5 amps over wont hurt.

Spy