It depends on where your at. I personally don't advocate a heat element in the coop. I just make sure they have a place where they can get out of the direct freezing wind. Chickens grow what I like to call their own woolen underware. If you put heat in there then they tend to grow less feathers than they would have. If for some unknown reason you lose power you will probably lose some birds because they moulted off those feathers they had grown for the coming winter.
If your determined to heat the coop a simple heated waterer will help keep the edge off without over heating.
Like I said before, I personally do not advocate heating a coop. Because of a possible mid winter moult, and the danger of power outage causeing me to lose my birds because of the artificial heat.
Arklady