My parents have a 20 kW natural gas generator. I think it's a Generac, but I'm not sure. It cost around $10,000. It runs a/c in most of the house, fridges, outlets and lights in most of the house.

Once a week it kicks on for 10 minutes or so for a self-test. Otherwise, when the power goes out it turns on after 30 seconds or so. It's rather loud - definitely louder than the air conditioning compressors outside, but not deafening.