Generator and transfer switch work has exploded since the pandemic and the wave of grid outages, and it’s solid profitable work. The core requirement: you must have a transfer switch (manual or automatic) that prevents backfeed to the utility. Connecting a generator with a suicide cord (male-to-male plug) is illegal everywhere and kills people.
Sizing: calculate the running wattage of all loads you intend to power, add motor starting surge (typically 3-6x running watts for hard-start compressors, HVAC), and size the generator to at least that number. For whole-home standby, 20-22kW covers most 200A residential services with some load shedding; 32-48kW for larger homes with electric HVAC.
ATS requirements: must be rated for the loads, must disconnect the utility before connecting the generator (break-before-make), and must be listed for use with the generator. For commercial applications, NEC Article 700/701/702 each have specific ATS timing and monitoring requirements. Article 700 emergency systems have the most stringent requirements including monthly testing and annual load testing.