Harstad and tonyb both provide great information.

At a previous home, we added a sub panel next to our primary electrical panel that would carry the load from our back up generator (B&S 7500w) to several critical loads needed in our house (well pump, refer, propane insert fan, several lights, etc.).

At our current home we added a 3.6kw solar system that we are backing up with an AGM battery bank (three sets of 48v or 12-12v batteries). This will allow our home almost normal use - except for luxury items like the A/C, garbage disposal, dishwasher, W/D. And, we can charge our batteries while providing power to our house with the back up generator that we brought with us. During several days of storms when there is no solar benefit, the back up generator should fully charge the batteries within a couple of hours giving us ample power for overnight without running the generator constantly.

It really depends on how much you want to invest into your home. A good generator with high quality extension cords (don't buy cheap ones as they don't carry loads nearly as well - read their specs! buy cords that will carry construction loads - power tools, etc.) can easily get you through a 72 hour dilemna provided you are conserving. Battery back ups can get expensive, but are a great safeguard whether tied to a solar system or charged by your power provider. Having the ability to charge your batteries during an outage with a generator is a plus too.

Regarding using your A/C during a power outage, I would say forget it! Unless you go crazy with an overkill back-up system you will not be able to carry the load. I've watched our 'net meter' go from selling power to our utility at 2.25kw per hour to buying 4.5kw per hour once the compressor on our A/C kicked on - a 6 to 7 KW swing. You will overload most standard generators or drain your battery back up in minutes if you try to run your A/C.

That's my 2 1/2 cents anyway.