Car has an 88 amp alternator from the factory. Not sure what size battery is on it now.....
Anyway a possable system layout has me at about 120w for front speakers, 120w for rear, and about 600w to the subs. So about 840w total.
Or I could go crazier on the sub(s) and go about 1000w or so So that'd be about 1240w total. I imagine this power for big bass is where things may start to need upgrading.....but the temptation or bone shaking bass is hard to resist
Not sure if any other info is needed to answer this question, but ask if more is needed. Just need to take everything into account before I build a system