Bingo!
Automotive alternators in general are about 40% effecient.
The big generator heads your see on standby gen-set are approx 92 - 94% efficient.
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The alternator on your car makes AC power ,, same as your standby genset ,, however in the car one half of the “AC waveform” is discarded to create DC current ,, therefore the most efficient a car alternator can be is 50% efficient if it was “perfect” ... now if you could tap into the current before the diode in the voltage regulator discards half your electricity and you use the wasted electricity then you have recaptured energy.