The losses going from generating the power to then run the rear wheel are massive even using the best of the best, on flat ground you will move maybe, first slope you will stop.
Its been attempted many times over the years.
Oh and putting out 300w for any length of time is pushing it for the average cyclist especially when you figure in the losses to the alternator (just googled 50 to 60% at partial load) that means 600w to get 300w output and then losses to the motor lets be generous say 80% efficient, so 240W to the ground from 600w imput.
I have used a power meter when riding, you will not put out 600w, 100w is meant to be an average fit adults output.
Whoops I forgot the inverter, charger, battery and gearing up to working Rpm's of the alternator losses.