i think it would be prudent to physically test the alternator at specific speeds.
at the relatively low rpm of a vawt it's possible
 i use a musical metronome
you can adjust the beats per minute(rpm)
then turn the alt in sync and observe the output .

when you put an electrical load on the alternator the speed of the mill will be reduced.
if it is reduced to the point were the alternator is no longer producing a usable  voltage you will need increased height.