Many of us have experience doing these. I have done simple manual transfer units for 4500 watt portables up to 15kw and 20kw for residences. Many of which are on the side of the mountains around here (Catskills, in NY). Also 40-60kw 3 phase for commercial, restaurants and stuff.
Like unreg said, the bigger ones are fully self contained and automatic.
You need a fuel source with enough capacity to run what ever length of time you think is possible. Hurricane Floyd(??) in fall 1999 the power was out in some remote areas for 6-8 days.
What info is it you desire?