I have the Genmax 9000 inverter generator. (7600 watts continuous) I run my 1.5 ton central AC (with MicroAir easy Start) and the rest of my house with ease. In fact, it might be a little too big for my application but that's okay.
You will need an Easy Start to run the central AC with almost any portable generator. The inrush amperage of a typical 2.5 ton AC condensing unit is probably in the 75 amp range. You would probably need an 18-20 thousand watt generator to start that unit WITHOUT an Easy Start.
My suggestion is to install a power inlet box on your house, with a generator interlock device mounted to your main panel, a MicroAir East Start installed on your central A/C unit, and buy one of the big Genmax inverter generators. The 9000 like mine comes in either gasoline only, or dual fuel (gas/propane) and they now also sell a 10,500 unit that is tri-fuel (gas/propane/natural gas). That is probably the cheapest legal way to run your entire house with a full 240 volt service during power outages.
A video showing MicroAir instalation and powering the entire house with a Genmax 9000 generator. Fast forward to 9:21 if you're not interested in the Easy Start installation portion of the video.