The correct way is to have an electrician install a transfer switch for essential circuits and an outside connection port. Wouldn't you also want to power other circuits, like for your WiFi, refrigerator/freezer and a light or two?

Yes, Randy, that would be the best. In my case, there are two 200A load centers. One has only the furnace, the associated control components, and a couple of other basement circuits which I added, so to put that on a transfer switch would be the easiest. In my area, long term power outages are extremely rare. When I asked locals "do you have any long power outages", the answer was something like "yes, back in '07, there was a really long one. It lasted maybe 2 or 3 hours"! My service is connected to the main line connecting a substation with the town hospital, and that line is sourced from two directions. That's why I haven't done this earlier.

On Long Island, where my primary residence is, long usually means over a week, such as during Sandy, when we were out for 10 days.

You could consider tranferring desired loads to the one 200amp load center of choice (with the floor heating) for backup powering - e.g. WiFi, refrigerator, security systems/critical lights etc.