I have multiple tiers setup that provide pretty good coverage.
First is a pure sine-wave computer battery backup, the pure sine wave is important since many AC pumps don't like stepped sine-wave in the cheaper backups and will chatter and not run well. I keep an MP-10 and a Jaebo RW-4 plugged into it along with a battery powered air bubbler, NO return pumps, no lights, bare minimum to keep display and frag tank water circulating. This is a 1500VA unit and gives an hour to 2 hours runtime depending on pump settings.
When that runs out, the battery bubbler senses the outage and kicks in to keep some oxygenation going for the fish. Hopefully by then I can be home and use the ghetto generator AKA my car with an inverter hooked up. Once my car runs out of gas, out comes the pitcher to provide manual aeration as a last ditch effort.
Also, water bottles are kept frozen in the freezer that can reduce temps when the A/C dies and temps start rising. (this has come in handy when the A/C had problems and room temps hit high 80's.
If all this fails, I have to concede defeat and realize I did about as much as I could within my budget.