Stock alternator up to the task? If not, how many Amps will I need?

The exception to this occurs when the alternator isn't producing enough power to meet the demand.

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To the OP, MRGTX's, query. The fans will be running the most at idle. Taking Rob's (Crackedback) point, the only way safe way to add the fans to the stock system is from the alternator output. For that to work OK will need an alternator that can supply enough power at idle speed. Otherwise the battery will have to send power - and it will have to go through the long circuit with ammeter. Therefore, it probably makes sense to rewire the connection direct and not use an ammeter.