1000W will keep your batteries recharged and keep basic things like lights on and plugs for low draw things like a TV or computer
2000W does all of the above + will allow you to use higher draw things like the microwave, toaster, coffee maker and hair dryer (although not more than any one of those at the same time).
3000W = all of the above + you should be able to run an A/C in most circumstances.
Of course, there are exceptions but that should be pretty close. I love my Honda 2000. Quiet, fairly easily portable and I can make the kids microwave popcorn :)