Good question. Unfortunately, there isn't a good answer. If you knew what the average current draw from you system was then you could select a proper size alternator. Assuming that the big use of current is your stereo system then the average is really hard to establish. Knowing the power of the amps would tell you the peak current, but unless you listen to a constant tone at full volume all the time the average will be much lower. A good way to tell that you don't have a big enough alternator is to crank the stereo up to your "typical" volume, run the engine at about 2000 RPM and measure the voltage at the battery using a Multi-meter. If you aren't getting at least 13.2 volts, you need a bigger alternator.
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