I've been using one of these InterLock Kit devices for years to attach my generator to the entire house. I had the mechanical interlock, breaker, and outside reverse-receptacle installed by an electrician.
I can power everything in the house except for the electric water heater. I can run the water heater if I turn off a few other items.
The whole system works great as an emergency power system. I have certain breakers marked with orange dots as those get shut off when running on emergency power.
From the moment the power goes off I can have the power back on in about 6-minutes.