Anyone at any time is at risk of loosing their power. Most often we're at work when this happens! Having a generator on hand is the best scenario, especially one that automatically turns on in the event of a power failure but if you aren't fortunate enough to own one, you need to know what the most critical parameters are and a few things you can do to help keep your livestock...alive.
The 2 most important factors are oxygen & heat loss.
Oxygen: This is MOST important because your fish breathe & everything in your tank needs oxygen. Over time the livestock exhaust the oxygen supply. Those running an AquaCorals DSB/LR/Sump system will have more available oxygen for much longer than a mechanically filtered system. If you don't have a generator, battery operated air pumps are key to survival. They are inexpensive, especially when compared to potential livestock deaths! Everyone should have one (or more) on hand along with airline tubing and air stones. For short term a computer battery backup can help run a powerhead pump or 2. Point the pump's flow to the water surface to create agitation which will introduce oxygen into the water.
Heat loss: As your tank temp cools microscopic animals will start to perish. Fish can handle cooler water. Their metabolism actually slows down helping to further preserve the oxygen in the system. We don't want to let it drop below 72 degrees before intervening though! Heat water any way you can & fill 2 liter soda bottles (or any clean plastic or glass container you can put a cover on & seal. Sink the bottle(s) in your tank. Repeat when the bottle(s) cool. Always, always, ALWAYS keep a glass thermometer in your tank. It's the only way you're going to know what your temperature is during a power outage!
In the event of a power outage lasting more than a couple hours be sure to run new ROX carbon & Polyfilters to strip out excess nutrients that will occur from micro die-off. This will greatly help the tank from experiencing an algae bloom after the fact.