You need to see this as two entirely different systems, the output being dependent on the input for raw energy (watts), but not for frequency.

What comes off an automotive *alternator* is speed-to-voltage dependent AC, which goes through a diode-block and regulator and becomes chopped DC. Which, in turn is *smoothed* by the mother of all capacitors called a "Battery".

Which then goes into the Inverter - which does not really care whether it is alternator power, or battery power. It takes the DC at some voltage between ~11 VDC and ~15 VDC, and converts it to AC by its internal magic.

Peter Wieck
Melrose Park, PA