I ran an orange cord to the coop, and plugged in a power strip securely mounted to the wall. I ran the cord in through the window.
I put a regular 60 watt incandescent light on a mechanical timer (the kind with the dial and little red and green pegs) to stretch the day. The light was in a standard cheap clip-on reflector. I zip-tied the clip to a 2x2 once I had it where I liked it. Then I added another, and when one lamp burned out I replaced it with a random grow-light that I bought on clearance - about 100w, with a blue film on the front of the flood. Made for pleasant light inside.
I also had the 250w red lamp I brooded them with in it's original reflector, securely zip tied to a heavy metal hook in the ceiling. I turned it on by plugging it in to the power strip, when I wanted it. I left it on 24/7 during the worst of the cold snap this winter.
I fenced off one nest box with hardware cloth and put a cheap under-desk type space heater in it. The cheaper the better - expensive ones have circuits, the cheap ones just have a thermostat. If the breaker pops and you reset it, the thermostat still works. The expensive kind (I have 2 in my trailer/office) need to be reprogrammed and such after power loss. I turned the thermostat all the way down, and it tried to keep the coop about 48 degrees.
As soon as the weather changed from sub and near zero, I reduced the amount of heat and made the chickens go outside to play. I kept the red light on so that they could rewarm when they went in to lay, but the 1500w heater was the first load to be turned off.
The advantage of the heat lamp is that it doesn't warm the air, but the Infrared light warms the skin. Trying to keep a coop at a constant temp is an expensive battle, considering how much ventilation they need.
I insulated my coop with bead foam and foil-faced foam. I used rafter-vent panels under the roof, with soffit vents at both ends to keep the coop cool in the summer. I covered the insulation with white melamine-faced masonite for a bright and easily cleaned interior.
My chickens stayed very healthy over the winter, we got plenty of eggs, and my sleep was not interrupted by rocketmom freaking out about the hens being cold.
All good!