How much of the 20KW are you actually using? Peak? Average? Unless you know, then it doesn't matter how big the generator is.
The OP lives in a rural setting in NY. Chances are, he will survive without AC.
I'm sure AC is nice to have in your location, but unless you have a compelling medical condition, you would also survive.
My 6500W generator will run my house. Well pump, water heater, lights, microwave, refrigerator and small chest freezer. I cook with propane, heat with wood and sunshine, and it would be a long outage before I would really miss using the washer and dryer. With some manual power management, the dryer could be used. It wouldn't surprise me if the generator breaker tripped if the well, water heater and refrigerator all ran together. That would not be hard to manage either.
No point in overspending for generator capacity that will never be used.
Sigh, why is it when someone here posts something like "I've done the math, and I need 10KW", some expert has to tell them "oh no, I get by with 7KW, so you should be able to as well"?
The OP said he's estimated that he needs 10KW....let it go.
I had two electricians do a survey of my house, and they both said an absolute minimum of 17KW to run most things, including the AC, but that would still require being careful. I ran the numbers myself (I'm pretty familiar with Ohm's law), and cross-checked that on Generac's site, and other sites that have calculators for figuring how much you need. The low end was 17KW, and the high end was 22KW, as I recall. The difference between a 17KW and 20KW unit was around $300, so it would have been stupid to go with the smaller unit. I would have gone with 22KW, but they don't make an air-cooled model that large, and the liquid cooled units get big, and expensive.
Survive an outage in high temps? Probably, but as usual, the folks making comments like this don't know the facts that they're commenting on. My wife primarily works from home (this week she's in DC), in a loft office that would have been simply unbearable if the AC wasn't available the last time we had an outage (24hrs). She's a senior VP at an international company, and is paid accordingly, so she can't just say "oops, our power is out". She absolutely has to be able to work normally regardless of power outages....cell phone backup, mobile 4G hotspot, etc, etc are all in place so she can stay connected. Her company made a major concession keeping her as a remote employee, so we have to do our part. In addition, I sometimes have to travel for work, and am literally on-call 24/7/365. If things go really bad, I may not be home for several days, or longer. The closest gas station is 20min away, and I wouldn't have my 105lb wife trying to drag a generator out, and deal with trying to keep the tank full, and gas cans topped off, on top of working all day....it's simply not worth it.
We've had outages as long as four days here in the last few years, and several hours, to a day, is pretty common....we're in the sticks, and the last to get things fixed. I'm much happier having a unit that isn't taxed 100% to just barely keep things going. In fact, during the last outage, it was very hot (as I mentioned), and there were times when the AC would kick on, and it was obvious that the fan was spooling up slower than usual....short version, we were using all the generator had to give, and we were being careful about what we were using.
There's no point in spending a bunch of money on a generator if it still won't do everything you want it to do, and a little extra capacity won't hurt anything. It's like buying a 40gal hot water heater when you really need a 50gal unit, and wind up taking cold showers....dumb, and false economy.
