I bought a Guardian (now Generac) model 5638 liquid cooled 22kW genset about a year ago. Had it installed a few weeks before Hurricane Ike hit. Our neighborhood was pretty lucky. We were without power for only four days. The genset ran flawlessly the whole time. Mine is about 150' from my house, on the other side of my "shop" so I can't hear it run. We couldn't even tell we were on genset power, and couldn't tell when utility company power came back.
Mine is set up as a whole house genset. The ATS sits in between the meter loop and my main load center. No "load shedding" or "emergency circuits" or anything like that. At the time, it was running an inefficient 4 ton A/C (a 20 year old 10 SEER that was replaced a couple of months ago), a 1.5 hp water well pump, two refrigerators, all lights, home entertainment, several computers, garage door opener, etc. My water heater and cooktop are LPG so that helped. We purposefully didn't use the electric oven just to be safe, but my guess is I could have with no problems. I was more concerned about a timing issue with all the major power users coming on at the same time, so we eliminated the oven from that possibility. My genset has a surge capacity of 38kW, so I never had any "startup" problems. I feed the genset with a dedicated 500 gallon LPG tank. I've since bought and installed "the Energy Detective" (will move to Brultech soon) to get an idea of energy usage. At "idle", my home consumes about 1.2kW. It will jump up as high as 3kW when a refrigerator is running, coffee pot on, etc. The new A/C seems to add another 3kW or so. The peak daily power seldom goes over 8kW, and I think the highest has been under 11kW. There may be some momentary surges caused by multiple motors starting that go higher, but TED doesn't see them.
I started looking at gensets after Katrina. I live on the Texas coast, and wanted to be prepared for a major outage. The key issues I came up with were air cooled vs. liquid cooled, 3600 rpm vs. 1800 rpm, and manufacturer. Almost all air cooled gensets run at 3600 rpm. Almost all liquid cooled gensets run at 1800 rpm. The overlap between air cooled and liquid cooled starts at about 15kW and goes up to maybe 22kW(?). I think Kohler has a 30kW liquid cooled that runs at 3600 rpm which is kind of unusual. It looks like it is the 15kW model at double speed. I read somewhere that the life expectancy of an air cooled genset is 3000 hours, whereas the life expectancy of a liquid cooled model is 10,000 hours. Until recently at least, air cooled gensets needed oil changes twice as often. A genset running at 1800 rpm will probably be quieter than one running at 3600 rpm if that is a concern. It will probably generate less heat, which may be an issue in hot climates. Most liquid cooled gensets use modified "truck engines". Mine uses a 2.4L 4 cyl Mitsubishi. Generac uses the same engine in their 27kW genset, so powering a 22kW generator is really easy on the engine. It isn't downrated on natural gas like many hensets are, suggesting that the engine is nowhere near it's limit with a 22kW generator. The engine almost never lugs down.
I went with Generac because it seemed to me that they were focusing on the residential market (selection, dealer network, etc.). The 5638 was exactly the size I figured I needed, and a similar genset from the other manufacturers cost significantly more. To me, the other manufacturers offered fewer choices in the small liquid cooled models. Also, it seemed like I would have to buy the genset from a manufacturer's distributor, then contract out the install/startup to one of their recommended installers (which was usually an electrical contractor). I wanted a turnkey package and to only deal with one entity, and the Guardian/Generac dealer was the only way I found to do that. And yes, I'm sure I paid a premium for that.
They can certainly have breakdowns. A few months after installation, the main wiring harness on mine developed a problem and had to be replaced. Diagnosing that problem took the dealer (and the factory) several months. A couple of months ago, the starter went out. Both of the problems were fixed under warranty. Even though I almost never buy extended warranties, I did for this thing. I also purchased the maintenance contract. My 5638 was one of the first out of the manufacturer's door, so that may have caused some problems, too.
I have maybe eight UPS's of various sizes (most between 1000 and 1500va) running at all times. I have not tweaked their sensitivity. Many are fairly old APC models that have a reputation for being very sensitive and not liking genset power. My dealer even warned me about this, and it's mentioned in the genset manual. During my four day "Ike outage", I heard UPS beeps maybe three or four times and only one or two beeps each time. My guess is that was when the A/C kicked on while the water well pump was running, or vice versa. That tells me two things...1) the genset power is very clean, and 2) it's able to keep the voltage/frequency correct even when a big surge hits it.
This is a good forum for Guardian/Generac gensets.