We had a big ice storm here in the midwest early this year that knocked many people out for 2 or 3 weeks. Mine was out 4.5 days and came on for good at exactly half time during the Superbowl. I have over 35 aquariums - all but of of which is in my garage. No fish died during the outage, but maybe a couple did later. I have thousands of freshwater angelfish. Here's what saved me:
1) I'm using a 30,000 BTU radiant natural gas heater in my garage fishroom which has an electric fan, but doesn't really need it. Fishroom temperature was perfect the whole time. Outside temps were low enough to keep an inch of ice on the trees and power lines for 3 or 4 days. I learned from this experience that I picked the best possible heating solution.
2) I had a couple small power inverters that could attach to my cars. One smoked immediately - I think because snow fell into its vents. That was my 400 W bigger one. I also had a 60W one that would power my 40W central diaphragm air pump that has around 50 outlets. Most tanks had marineland back filters and air driven sponge filters. I could run the air pump off the inverter attached to my car so long as I ran the car for 30 minutes every 2 hours. Eventually, I just let the car run all night the last day so I could get proper sleep.
3) The central air pump is attached to a 550 VA (or so) UPS. It can run that 40W pump for about 1.5 hours without power.
4) I didn't feed the fish at all during this time. I also didn't change any water. I didn't dump out the back filters, but I did manage to start them with the inverter a couple times for a minute or so.
5) After 3 days, I was finally able to get a generator at Home Depot. It is a 5500W LOUDLOUDLOUD beast. At $800 it is also considered somewhat disposable, unfortunately, but still works at least. I was more worried about losing a freezer full of meat than the fish, but by that time I was also wondering about water quality. A quiet generator would be nice....it would let me hear my neighbors' generators all around me!
6) In the house was a single 75 gallon freshwater moderately planted tank. I kept the living room warm with a kerosene heater and boosted the tank with fish bags full of hot water a few times a day. There's something to be said for having a gas water heater. Tank temp was 65 to 69 F. Sometimes I ran a small air pump connected to the extension cord running to our one compact fluorescent bulb in our living room lamp to airate the tank.
Once I had the generator, I could power all the equipment, house fridge, deep freezer, and most importantly, the TV and DirecTV box. OK, the fish were most important, but my wife might disagree!
There was a suggestion in our local fish forum that water level should be lowered.
Lower water level. The most oxygenated part of the water column is the water surface. As a basic principle the bigger the surface relative to the volume of water the better it is for the fish. So by making the water shallower in your tank, you have technically made your surface bigger.
Mine has gone out for 14 hours twice this month, I tried to take care of them as often as possible. But the last time it went out I was not home so it was around 2 hours before I could do anything. Then I went to sleep and woke up once to help them, I am sure they could probably survive 3 to 4 hours. JMHO
As far as UPS's are concerned, you don't need to spend over $1000. Decent quality starts around $100. Make sure to get a UPS with line conditioning! APC has a number of models available. You could even get models that connect to the network and can be monitored that way. Most filters take very little power. Heaters are the biggest draw for most followed by lights. We FW folks have it a bit easier I think since there is less to run. Bags of ice or pots of hot water can take care of heating/cooling in extended outages. Also, supplement with a small wind or solar system if you can. More affordable than a generator and always useful. Look into 12v options too.