My power rarely fails for more than a few minutes at a time. If I were to loose power for an extended period, chances are my flock would be fine and not even notice that the lights went out early. My coop is well insulated and averages 15-25 degrees higher in the sub freezing times. I only use a 150 watt heat bulb, only at night and only when the temp drops below 35 degrees. I attribute most of the heat in the coop to the 50+ small space heaters that I keep there. In the event of prolonged power failure, my main concern would be keeping the water liquid.
I built my Chicken Palace from scratch and planned for extreme weather conditions that have been known to hit this area from time to time (like every year). I am motivated by low maintenance hobbies. I would rather spend a few pennies more a month to run a 150 watt bulb and a thermostatically controlled water heater, than to spend money on antibiotics for infected frostbitten combs, replacing dead egg machines and the the number of trips to the coop to refill or replace the frozen fountains.
To each their own though. I do not think my chickens need the heat bulb to survive. I do believe that because the coop is comfortable during the winter, my egg production stays consistent to summer time numbers. I do believe the girls like the heat bulb as evidenced by the cluster that sits in it's glow. They don't hang out there, but use it somewhat like the proverbial water cooler. They gather there from time to time, exchange gossip, and then disperse. I am unwilling to unplug the lamp to toughen my birds in the off chance that we will loose power for an extended period of time. That would be akin to me turning off the heat in the kids room to toughen them for the same event. After all...man survived the ice age w/o electricity.
Just my technique...