I will add some to this discussion....
IN THE EVENT OF AN OUTAGE, DON'T THINK THAT FISH CAN LIVE FOR A DAY IN A BAG SO THEY WILL BE FINE IN YOUR TANK. I once had a 6hr outage that killed about $600 in fish because I thought it would be fine overnight. The problem is that in the bag, the fish is the only thing consuming oxygen. In your tank, every living organism will be consuming it. The more live rock and sand you have, the quicker all that bacteria will consume the oxygen and starve out your fish.
If you plan on an inverter from the car, keep in mind that it will take a lot of gas to keep it going. Also, TEST IT!!!! The last thing you want to do is plan on doing this and then find out that your inverter doesn't put out enough power to run what you want it to.
Another idea is to get a cheap generator from Harbor Freight. They make some small ones that often go on sale for as low as $77. Be careful when buying though. Harbor Freight often will run two or more sales on the same product, at the same time. They will show one price online, one in a newspaper ad, and another in the store. You will only get it for the price you show them or what's in the store. They will not tell you if it's cheaper online and all you have to do is show them the webpage to get that price. I have one of these and will tell you that they work fine. They don't put out enough power to run a shopvac, but will run a pump in a bind. Also, keep Stabil in it, cause they are bad about not starting if they sit with fuel in it too long.
Another idea... a computer battery backup. As with any backup, you have to consider how much power you will be taking to run what you want to run. It may say it provides 100hours of power, but that may be with one light bulb on it also. My advice is to get a regular timer that plugs into any outlet. Put your pump and aerator on a timer so it runs only 50% of the time. Maybe 30mins off and 30 mins on. This will make the power last longer.
Another idea... whatever you choose to run on backup is important also. For example, if you have a hob skimmer on your tank, you may want to run it to aerate the water. You can't just depend on circulation without aeration. Depending on the size of skimmer and size of tank, you may want to run a powerhead and the skimmer. My setup right now consists of a big MRC2 skimmer ran by a mag24 on a 150g stock tank. With that much flow through the skimmer, I could get by with just running the skimmer alone, during a power outage. It would provide fair circulation while also aerating.
Another idea.... If you're home and want to really conserve power: Raise a power head to where it's close to the surface and begins to suck in some air and create bubbles. This will allow one device to aerate and circulate. Combine this idea with the battery backup on a timer and you should be able to make your available power last as long as possible.
