What size generator do you need to run a 7.5 hp motor?

Conversion from gas to electric should be 750 watts/hp

So 7.5*750=5625 watts

These are running watts so that is a bare minimum. Depending on the application it needs much more starting watts (up to 3 times for a compressor).

Generator should be no less than 8000 watts, maybe more if you work it hard.