My philosophy on it is that it depends on the likely duration of the power outage and temperature extreme possible and how many lizards.
For example, where I live, a power outage beyond several hours is highly unlikely. My lizard building is heavily insulated and even on nights where temps are 25 below zero with wind chill, as long as it has warmed up into the 60s day temps, then at night it will remain above 50 without heat of any kind.
If power remains out a day or two, and you only have a few lizards, and temps are likely to drop below 50, you can "pack" the lizards like you were going to ship them- put them in a cooler with a chemical heat pack and swap the pack out as needed.
During the day, setting the lizards up in front of a south facing window and then using blankets creatively, you can create a tropical zone in the sunlight.
50 is a pretty safe temp point to consider the danger mark for species like panthers. Veileds and most mountanous species like jacksons can go as low as 40.
Another option- you can get an electrical converter that plugs into the cigarette lighter of your vehicle for $20-$40 at walmart or similar. Run the car, use an extension cord, and you've got enough power to run lights or a small low watt heater. Cars can idle a long time on a tank of gas... This is actually my backup for my incubators- incubators are much more tricky during a power outage than lizards, at least for me...