The best way to tell what size alt you need is to check your amp draw from your stereo and then match your alt (if possible) to your system draw. I can tell you from experience the 70 amp wont replenish your batteries at all if you are running your stereo at max capacity for any length of time. It was a good move using the trojan t-105s they last a decent amount of time and can be depleted many times without damage. PM me if you'd like to know how I have mine set up. I'm running a 5 amp system, 6 hlcd's on the tower 6 subs and 4 in boat wetsound 6.5s. I'm pulling around 200 amps at 75% max volume.....