"It's all about the power" Or something like that quote from the Movie Apollo 13.
I have done a couple of cars with the Racemates are either 30amp or 60amp. I have a 30amp in the Mustang, and it is not somthing you could drive on the street at night. With lights, ignition, fuel pump it is net negative and running on the battery until about 5000rpm due to the pulleys. If using underdrive pulleys expect the full power capacity of the alternator to come later in the rpm range, but not really sure how they ramp out depends on pulley and alternator.
All the juice adds up quickly depending on what you got in your car, the killer is fans and high current halogen bulbs (120watt bulb ~ 10amps each at 12volts). Fans can be huge too, not sure on the smaller ones, but I recall 7-10 amps x 2.
Fuel Pumps can be from a few amps for Holley's/Mallory style (range of 5amp) and some of the larger EFI pumps state 10+amps for the bigger (SX Perf/Aeromotive Inlines). Ignitions like MSD's 6s are roughly 1amp/1000rpm, Add AC and blower fans (??), trans cooler pump, 2500 watt boom box stereo, and neon running lights with dual nitrous with the rapid machine gun style purgers (read as sarcastic) and it adds up quickly.
Switches, Relays, Fuses and Breakers current capacity are important too, as is often overlooked wire size and connector capacity, especially from the alternator if running an alternator with 2x-3x the capacity as stock. Check some of the older threads on AC and Ignition switch issues with some of the cars.
Too much coffee today
Sandy