We built our house in 2005 and I made sure to have a whole house generator with an automatic transfer switch wired in. We have a fairly large house and I had a 25kw generator installed and wired into both circuit breaker boxes. It runs the entire house just fine and has paid for itself on numerous occasions over the years. It is powered by a 2.5L Ford and runs on a permanent natural gas connection and we do have it on a quarterly service plan with Clarke Power in Greensboro. We can barely hear it running from inside the house as it purrs along at 1800 rpm. Never have to worry about it. I can set it up to run itself once a week or whatever interval I choose in order to exercise it, but I just go out and turn it on and let it run for about 30 minutes once a week.
It has run for about 5 -6 days during an extended power outage a few years ago and it did a great job. After the power comes back on and it switches over to Duke Energy, the generator will run about 20 minutes under no load to cool off and then it shuts down and waits for the next outage.
Mine is an Olympian 25kw. I believe it was marketed by Caterpillar back then. Here's a pic of one that is the model I have: