Years ago, during an emergency, I bought what I could find, and ended up with a 5,000 watt generator (2 phase, about 20 amps per phase). I wired a generator panel with an isolation switch so I can give power to the kitchen, furnace, and sump pump. It has a 5 gallon tank, and uses about 1 gallon per hour. I have used it a few times. I mention this because:
1) If you put 5 gallons in the tank, and the power comes back on after
1 hour, you have 4 gallons left in the tank. Gasoline does not keep, so you have to use it or drain it out.
2) I saw a comment a long time ago that someone had a 2,500 watt generator, and could run it all day on 1 gallon. Probably hyperbole, but you waste a lot of gas if you don't use all the power.
3) You need to balance the load on your generator. 3500 watts is still
15 amps per phase. As long as you don't start both refridgerators at the same time, you should be ok (start-up current is large, but generators do have the ability to exceed the rated output for a short period of time for just such a purpose).
4) Some time spent wiring on a nice day sure beats running cords all over the place on a lousy day when the power fails. It is so convenient, even my wife can hook it up in an emergency (just kidding about the wife part). I ran a 30 amp circuit for the generator into the Garage, where it can sit protected from the elements when running (garage door open). Hardwire is the only way you can supply generator power to the furnace.