Size the Systems as a Set, Not One at a Time
The systems in a home interact more than most people realize. A furnace and an air conditioner share the same ductwork and blower, so their sizes should be chosen together against the same heat load. An electric water heater and an electric dryer are two of the biggest draws on your panel, which is why a load calculation matters before you add either. And every one of those loads becomes a generator-sizing question the first time the power goes out.
Start wherever your project starts, then follow the cross-links. If you are replacing a furnace, run the furnace BTU calculator, confirm the AC tonnage matches, and check the panel has room with the electrical load calculator. If an outage just sent you shopping, the generator size calculator will tell you which circuits you can honestly back up. All of these tools are free, with no sign-up, and each one explains the math instead of just returning a number.