If you've already got a generator, you might be better-off getting a deep-cycle lead-acid battery and an inverter.
Run the generator for a while at its max-efficiency power to charge the battery, then use the battery to power the fan via the inverter.
Did that in the winter at the shack in the mountain when we were snowed in and didn't get sunlight. 1 hour of genset a day would charge the battery bank enough to last about a day and a half.
That takes a good charge controller in the inverter. I was using an old 2.4kva Trace unit.
/johnny
Good advice. Thanks.