First, trap the heat you already have
A vivarium cools from the moment the power dies, and everything you do in the first half hour decides how slowly. Close the doors or lid and resist opening them to check on the animal, because every check lets out a lungful of the warm air you are trying to keep. Drape blankets, towels or a duvet over the outside of the enclosure to insulate it, keeping fabric clear of lamp fittings so nothing scorches when the power suddenly returns. If the enclosure can be moved safely by two people without sloshing water bowls or collapsing decor, shift it into the warmest room in the house, away from external doors and draughty windows; a small room that the household is living in, warmed by people and candles used safely, holds heat far better than a spare bedroom.
Add gentle backup heat the animal can move away from
Disposable hand warmers and hot water bottles are the outage workhorses, but they must be used like the blunt instruments they are. Wrap them in a towel or cloth so no surface the reptile touches gets properly hot, and place them against the outside of the glass or in one corner inside, never underneath the whole enclosure and never spread everywhere: the animal still needs a cooler end to retreat to, because overheating in a sealed box harms faster than cooling does. For a small lizard or snake, your own body heat is a legitimate backup: the animal in a cloth bag or secure tub, kept against you under a jumper, stays at a safe, steady warmth for hours. Replace cooling bottles and spent warmers as they fade, and use a battery-powered or simple dial thermometer to check the cool end rather than guessing.
What not to do while the lights are out
Do not feed the animal, however normal the hour feels: digestion runs on heat, and a meal sitting in a cold stomach causes real harm, while a healthy reptile misses food for days without consequence. Skip misting and baths too, since water on the animal and in the air pulls warmth away. Never bring barbecues, camping stoves or generators indoors to warm the room; the fumes are lethal to you and to the reptile long before the warmth helps. And do not unplug or rearrange the heating equipment in the dark: when the power returns, you want the original thermostat-controlled setup to come back exactly as it was.
Know the point at which you relocate the animal
Most healthy reptiles tolerate a slow drift down to cool room temperatures for some hours without lasting harm, especially if they were warm and well fed before the outage. Act earlier if the outage stretches on, the house itself becomes bitterly cold, or the animal is very young, unwell or a tropical species with no tolerance for a chill: pack it in its insulated tub with warmers and take it to warmth rather than waiting beside a cold glass box. That warmth is often closer than you think, because outages are patchy street by street. Through KinsLe's Circle of Love, your free account connects you to nearby pet lovers, and a fellow keeper whose power is already back can offer the warm spare room your reptile needs.