Circuits that are to be protected by the generator are moved to a circuit panel hooked to the generator. Commercial power is fed from the main power circuit panel to the generator panel. In my case 120Amps is fed from one to the other. When the generator panel detects that commercial power has failed, it starts the generator and flips a large relay bringing all circuits it covers back online. Typically this takes less than 45 seconds. When commercial power is restored the generator panel switches the relay back to commercial power. The generator is run for another 45 seconds or so to cool down and then it is shut down.