It just depends on how elaborate you want to get. A generator on a transfer switch is nice! If you can afford to buy and support that generator. Also usually means placing priorities for what is automatically supported and what isn't (bound to be limited by the generator's output capabilities). You can do the same thing with battery power, switch it on automatically when the AC goes off. Also the same about setting up that battery bank, buying and supporting it (a very large 'UPS'?).
The key to any of this sort of thing is the power storage capacity. The care and feeding of such a system can be made cheaper but only with more work/effort.
A wet-cell storage bank is typically cheaper, but take much more effort to care for it properly. An AGM or glass mat type system requires less effort, but definitely will not be cheap.
How you keep that storage system 'topped off' can be done in several ways. Easiest/cheapest would probably be through the AC power systems. Then there's wind power, solar, etc, etc. The more independent you are from the AC power distribution network, the more expensive it gets, at least initially. It does get cheaper over time once you have a reliable system functioning fairly efficiently. But it will never be as inexpensive as the AC system. (Other wise, why have the AC power companies?)
If you figure out a good way of doing it on a very limited budget, I wish you'd let me know too!
- 'Doc
