You didnt take into account the pulley ratio of 1:4.5 we are not building a direct drive unit

The 1750 rpm is the starting point for the rpm the 1:4.5 pulley ratio turns your 1750 rpm into 7875 rpm which is then enough to power the generator at full power.

A simple bathroom fan dont have enough torque that is why you use a bigger motor that does have some torque to it and if it uses 5 amps great if it uses 10 amps ok and so on.

The goal is to use the least amount of energy to get the 7875 rpm needed to turn the generator head.

The belt surface speed needs to be at 7875 rpm which is equivalent to a 47 mph wind. Now you have 83 amps of charging power instead of 500 watts of charging power that you initially started with.