The sub and amp will shorten the life of the alternator if drawing too much, but remember a alternators rating is NOT it's constant output, it is a peak reading, typically a 70 amp alternator will not be happy at any more than about 45 amps, a larger battery will give more reserve.
Watts/volts as said give the amps but volts should be nearer 13.5 -13.8.
tabetha