My fridge is on a 20 amp breaker.
My gas furnace is also on a 20 amp breaker.

I have run the fridge OR the furnace on a 3000 watt generator (about a 25 amp generator if did the math right). I did not try, nor need to, run BOTH at the same time. The idea is to run one for a couple of hours, then switch to the other. If you need to power both at the same time, then you I think you need about a 5000 watt generator, for a 120 volt system (again, if I did the math right).

Hope this helps.