Concur with Mitchwelt, the Honda gens are small and quite. But as to the orginal posters question:
The answer depends on your circumstances, i.e., are you in a condo/apartment or in a house. If in a house, then you really really really should look into a generator for you tank plus key house components like the refrigerator. If in a condo/apartment, a small generator for same is you best bet, but you can also get by with a battery backup powering only the minimum low wattage items. One of the most electrically efficient would be old school air pumps, several of them, running continuouslly. Low draw powerheads like the korelia's positioned to maximize surface agitation (and thus oxygenation) would also work. Do not expect to run chillers, lights or even main pumps for long on a battery back up.
As for how long power is likley to be out, that is totally dependent on the seriousness of the hurricane damage, the specific location you are in, and a host of other factors. During the bad ones, some areas can be out for two weeks, but that is getting close to the worst case scenario at least for the last decade. FPL has improved their emergency response, so a couple of days is ore likely, but it is almost certain that some areas will be far worse off than that if we get hit hard.