I have two AC units (2-3 ton) that run on my Generac 20kw, plus all my sump pumps, major appliances and some lighting, an I still have some room. As previously mentioned, you may want to stagger your AC unit start-ups due to power surge. I believe the Generac or other brand names have calculators on their sites to give you an idea of what you can run on a certain size generator. Pricing jumps once you go north of 22kw as the engines become liquid cooled, as opposed to air cooled. I purchased mine through HD and it was installed by CDS. I am very happy with the investment.
Roni - You can always stop by and check out my standby system. I am a few miles away.