I think this may be the type of thing you are asking about. Years ago, I bought a 10KW generator (Generac) at Costco to use for construction on my house. Turned out I really didn't need it. At about the same time they had these load transfer boxes. I didn't need one, but I thought I might some day so bought it. It handles a total of 7.2KW. After the house was finished, I figured I might just as well use the generator and the load center to power some items during outages. I left a piece of plastic pipe in the wall for the power cable to pass through. It came as a kit with a junction box to mount on the outside wall to pass the cable through to the transfer box inside. I had an electrician I know install it. He thought it was pretty cool and was going to look into the availability of them since he said he made all of the ones he had done before by hand. At the time he charged me $350 & and said he charged $800/$900 for the ones he made & installed. The panel circuits you choose to use are wired into the transfer box and operated by the switches. With a switch at the off position the circuit is a regular line circuit. When the switch is changed to the center position, it isolates the line circuit from the house. When the switch is moved to the down position, the circuit transfers power from the generator. You can use all circuits of ones you choose as long as you don't exceed the power limit. Since we never have power out for very long, I only hooked it up to circuits for freezer, refrig, lights, some outlets, etc. All of the circuits together fall way below 7.2KW. Here are a few pics. The box has breakers for each circuit and shows the mount of power your are using. Sorry, the junction box pic is on its side. If you are going to wire the whole panel, you should be able to find a switch like each one of these to isolate the panel & switch over.
