See the "cold coop" page in my .sig below, for a number of things you can do (including but by no means limited to electrical appliances) to keep your coop warm(ish), also a discussion of how warm(ish) you do or do not NEED it to be.
BTW, you *do not* need a small coop for northern climates, not even if you only have a few hens -- if anything, the opposite, since they may well not want to go out during long stretches of weather and you don't want social problems and cannibalism starting. If you want to concentrate chickens' body heat around them, make an enclosure around the roost *within* their coop to do so -- that way they get the best of both worlds.
Good luck, have fun,
Pat
BTW, you *do not* need a small coop for northern climates, not even if you only have a few hens -- if anything, the opposite, since they may well not want to go out during long stretches of weather and you don't want social problems and cannibalism starting. If you want to concentrate chickens' body heat around them, make an enclosure around the roost *within* their coop to do so -- that way they get the best of both worlds.
Good luck, have fun,
Pat