Which alternator? It depends on the amp load you want to power. Mopar and Denso alternators make a relatively low amount of power at idle. Like Nacho says, only about 65 amps at the most, which I find to be a generous number for that alternator at idle.
If you have a lot of electrical add-ons like a powerful fuel pump, high-powered stereo, A/C, Power windows and a adequate electric fan or fans; you can easily exceed 65 amps.
I had such add ons in my '70 E-Body. I bolted on a GM CS140 alternator that had been rewound and upgraded with heavy duty bridge rectifiers. It puts out over 100 amps at a low idle, and well over 200 amps at a fast idle. It is self regulating and very reliable. Here's a photo of it installed above my A/C compressor. Of course my wiring was upgraded to handle the amp load too.
1970 Plymouth 'Cuda #'s 440-6(block in storage)currently 493" 6 pack, Shaker, 5 speed Passon, 4.10's
1968 Plymouth Barracuda Convertible 408 Magnum EFI with 4 speed automatic overdrive, 3800 stall lock-up converter and 4.30's (closest thing to an automatic 5 speed going)
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