You might want to get a real good generator....

Your furnace has a electronic control board in it. Fry it and you
screwed with or without power!

Cheap generators provide dirty electricity, this will have a problem
with electronic equipment. Should be fine though with refrigerators
etc.

If you filter the power through a UPS, that will do ok. Otherwise
honda makes a fine line of generators that can do it. Expensive though.