check my math enough to add 4 ch amp?

Hello. If my math is way off please forgive me... My car (Corolla) has an 80 amp alternator and will have a 36R glass mat battery. Stock 10 gauge ground wires have been replaced with 6 gauge battery cables. I am currently running a Alpine MRP-M500 (500w RMS 1000w Peak) amplifier and a Kenwood head unit. I typically run it at loud but not super loud volume, maybe 70% max? So i figure my usage is... +80 amp alt - 4 amps A/c - 3 amps blower - 6 amps cooling fans - 9 amps headlights - ~8 amps ECM, gauges, dash lights, & misc - 6 amps headunit - 30 amps sub Amp (500w RMS @ 13.5v with 15% efficiency loss & ~70% volume) _______________ +14 amps free 14 amps x 13.5 = 189w free If I add a 45w RMS x 4 channel amp... 45w RMS x 4 @ 13.5v with 35% loss (a/b) 70% volume = 12a (162w) 66A + 12A = 78A MAX... Close... but I should have enough power right? (I'm not running test tones or rap music so I assume power usage will be less than calculated.)

Monitor with a voltmeter and find out.

You are doing WAY TOO MUCH math.  You will be fine.  Just put the amp in.

Ditto. Plus you need more figures for proper calculation - eg, the average output of your alternator which is based on your driving profile (RPM etc), and if 70% is power or volume (ie, 50% power).    Even then the reality can be different because of battery characteristics and aging, alternator aging, different driving profile (wipers at night with heater). Hence my reply about the voltmeter. It indicates when you are not outputing sufficient alternator current (eg, at idle, or overloaded) and also indicates a undercharged or dying battery (at first turn on after resting). Note that the voltmeter should be across the battery - not ACC etc (though at the switched IGN relay output may be good enough). I use a small relay to switch mine. Maybe if you want to ensure you always have full voltage for the amp (ie, over 14V, not battery 12V-13V) fine, then just add your new load current to that of the existing alternator. That's the safe bet, but may be totally unnecessary - especially if only concerned with flattening the battery.