Where do you live?
IMHO, unless you have really delicate breeds (like seramas), a heat lamp for adult chickens is more of a disservice than a kindness. Kept in a draft-free coop chickens can do just fine at some pretty cold temps. Keeping a heat lamp on them prevents them from acclimating naturally to their enviroment. Not acclimated and the power goes out, you'll have some chickens that are ill-equipped to deal with the cold. They can get chilled and chilled chickens frequently turn into sick chickens.
Again, JMHO.