Power out, how long can tank be ok?

Just wanted to update, I actually wasn't even at home when the power went out. I was away for the weekend. My parents called and let me know what was going on. The power was out for about 4 hours and the good news is that everything seems ok (just got back today). All corals are open and fish are swimming around. After this scare, I am going to look into a small generator. Thanks for input from everyone.

 

Generators are good. I Have a nice little honda unit that will run a tanks essentials very well. As a backup, I also have a 120V inverter hard wired into my truck, so all I have to do is fire up the truck, pop the hood, and run an extension cord. Its a good back-up to a generator.

 

Glad all was ok in the end