In a power outage who cares about efficiency.An inverter on an idling engine will certainly work, but is a pretty inefficient generator.
Also, a maxxed out alternator at idle parked, is not a happy alternator.
I have acquired temperature data on my own alternator, and will no longer max out my alternator, at idle ,for more than a few minutes( making 50 amps) as it quickly gets to 220f, though at 65mph it stays under 140f making 121.4 amps.
100 amps at 14v, is 1400 watts
Alternators are only 55% efficient or so
Inverters are only About 85% efficient or so
Portable generators range from 10% - 20% efficiency if that matters.