I went back and found the article that I was referring to here on the board. Still good to know that some haven't noticed a huge rise in their electric bill, especially for those in really cold parts of the country.
https://www.backyardchickens.com/web/viewblog.php?id=1642-winter-coop-temperatures
The Cold Coop:
what to do (and not do!) about it
What about electrical heating devices?
I will be honest and tell you that I think the majority of people who run heat lamps for their chickens don't need to be doing it and probably shouldn't. A single, actual heat lamp (250 w) will cost you something like $30+ per month on your electric bill, and sometimes they cause fires. And if you live somewhere that an ice storm may knock your service out for days, chickens used to a very warm coop could find themselves in real trouble.
That said, sometimes you DO need something on top of what you can finagle out of Mother Nature. The safest and simplest thing is to run a regular ol' lightbulb, like 40 or 60 or 100w. Nearly all that wattage goes into heat production, same as a heatlamp, yet they do not cost as much to run, and because they do not get so very hot, the fire risk is less. Try one of those first and see if it suffices. In combination with a hover or partition it may well be all you need.
If you do run a heat lamp, be very careful. Attach it at two separate points, to two separate points on the coop structure, so that if one fails the other will catch it before it hits the bedding and starts a fire. Never hang a lamp just by a clamp or just by its cord. A 250w lamp should be kept at least 18" from combustible materials -- that includes walls and bedding. And use a guard to reduce the chance of chickens getting burns on their combs, which may sound unlikely but really does happen.
Fires are another thing that really do happen, and realistically it is unlikely that your chickens would survive even a small coop fire. Make very sure that your wiring is sufficient to the electrical load; that all electrical connections are tight and correct; don't use extension cords if at all avoidable, as they and their plug connections can overheat and cause fires; and it is not a bad idea to have a fire extinguisher just inside the coop door, or in a nearby outbuilding, just in case. I am not trying to scare people, and obviously MOST coops will never experience a fire, but you do want to take all avaiable precautions, because those coop fires do happen to somebody and you don't want it to be you.