I lost power during those really bad storms Tennessee had a week and a half ago.
My four Delaware chicks were two weeks old at the time I lost power, and I was out of power for over 12 hours.
The chicks seemed to handle it just fine. It was night when I first lost power, so I covered their cage with a woolen blanket. But by 10 am, the sun was heating up my house some, so I uncovered the two week old babies, and they did just fine without any lamp heat at all.
After that, my wife and I observed them, and they did not seem cold even after the power came back on. They seemed to just eat, drink, play and sleep normally with absolutely no sign of them being cold. So we never turned the heat lamp back on.
Of course, Delaware chicks do feather out quicker than some other breeds...