To figure out the amount of amps your system is drawing you take the watts, so say 500/12 since the system should be running at 12 volts minimum right??? You also need to allow 50-60 extra amps for your car to draw off of right???
Watts/Volts = Amps and yes, usually about 50-60 amps for the car. but a better way is to figure out what amperage you are pulling for the stereo, and add that to the stock Alt's output and get a HO alt of that size. that way you'll be completely sure you have plenty of headroom.