If you are thinking of something like the Cozy Coop flat panel heater...don’t. I think it is mostly useless. Unless a chicken stands right next to it, and I mean practically touching it, they won’t feel anything. It won’t heat an area either.
That said, I have one in my coop that I only turn on when the temps get down below 0. It gives me peace of mind, but I know it isn’t really doing anything.
Last winter, on one of our -35f nights, the temp in the coop was -30f, and that was with the heater on. During the day, it got up to -23f, and the chickens totally ignored the heater. They spent that entire day outside in the run.
Point is, they don’t need it, and mind don’t even acknowledge it’s existence.