yes, in many cases, 24 hours is all you would need to wipe out all of your fish and inverts, except maybe some of the most resilient, like clownfish or some blennies that might hop on a powerhead way at the top of the tank. it will be lack of oxygen in the water that will kill them, unless it's extremely cold, of course. it's happened to me and many people i know.
this can happen even while you are home.
what would you do in a power outage that lasts a couple days?
i have a 5000 watt generator that i run my tank(s) on, and also my fridge, and anything else i need, like space heaters or window a/c units.
at the gym around the corner (they have a 150 in-wall) i have one of these-
Air-Pod Battery Back-up Air Pump | Battery Operated Air Pumps | Air Pumps & Supplies | Aquarium - ThatPetPlace.com
unfortunately it requires you to be there to turn it on, unless you find it enjoyable to use air stones in your tank, like in fresh water apps. then you can use this as your primary air pump, and when the power goes out, the batteries kick in.