Honestly, I ran a CS 5kW @.7ohm for almost a year with:
Stock Merc 86aH SLA battery, one run of pos and neg 1/0 to a D3100 in the rear, and a 220A alternator.
I wouldn't recommend going this small on electrical for someone who hasn't ran a good bit power before, this was just squeezing by to be honest, but anyhow I'd keep a steady voltage full tilt until about 1 1/2 minutes of slowed bass heavy music.
But because I wanted to be able to play for extended periods, and that my new competitor's organization allowed it for a certain class, I upgraded the front batt to a D3400 and and utilized two D3100s in the rear. I also added another run of 1/0 while keeping the 220A alt. I can play fullt tilt at .5ohms now, easily throughout an entire song and no heavy voltage dropping or heat buildup occurs.