On mine, the electrician just mounted a box on the house near the generator, and put an interlock on my existing panel.
I think the whole deal was a couple hundred bucks.
Oddly, my well pump runs off a separate box (main house service), which I didn't know at the time.
So, even if the gen is supplying the house, it's "after" the well pump.
Not a big deal for me, though I wish I'd known that; I just keep water (5-gal bucket) to refill the toilet and keep a few gallons of drinking water.
My main service would be a pain to get to in a decent snowstorm anyway,
The well pump is run separately so you can fight a fire if your house (panel) is burning. Probably for an equal amount of time you are more likely to burn your house down during a power outage (using candles, too much power for the generator...) and even if they were the same risk you still want to have your water to fight a fire, so it would be a good idea to do something about it. I have a slow well (2GPM) and an 1850 gallon cistern, I like having a gasoline powered pump handy to pump right out of the cistern in a total loss of power.
