This calculator will tell you the required alternator rating for your vehicle at peak usage. It may not be possible to meet these specs with a single alternator or even necessary to. Many other factors come into play such as driving and listening styles. It will however give you the ideal answer and a good indication of your present alternator's ability to supply your system.  It takes into account the current draw of the OEM electrical accessories, the current draw from your amplifiers at peak draw and the current draw from any other electrical accessories (neon, fans, strobes...) in your vehicle.