here's the math
watts=amps*volts
7.5 horsepower=5595 watts
for 240 volts
5595=a*240
23.31=Amperes
Because motors are NOT 100 percent efficient, we can say 25 amps respectively.
for 480 volts.
5595=a*480
11.65=amps
again, add a few amps respectively to get 13 or 14 amps.
So, it would be convenient if you had a 14 amp 480 volt generator or a 25 amp 240 volt generator that just happened to fit the alternator spot.
then you could use some kind of a converter to charge the battery in place of the alternator.
the problem with this is that the engine won't be running at peak efficiency (about 2-3 Krpm) and if it were then I think it would be making more than 15 horsepower.
that is why I think it is a better idea to use a smaller engine rather than go electric.
just a cool little thing I found.
http://www.unitconversion.org/unit_c.../power-ex.html
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