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Zone 4 here.
Personally, I wouldn't rely on an incandescent bulb to heat your coop. Besides, with the southern windows, you'll likely need the supplemental heat over the night time hours, in which you wouldn't really want a light on anyways.
Will it be enough? Well, it will depend on how many chickens you have, the size of your coop, and how much heat is attained and retained by your southern windows. My 4 girls could not produce enough heat last winter to keep their coop livable. I provided 24/7 radiant heat (250 watt ceramic bulb that kept hens warm on their roost) and kept the coop around 10F. It got as cold as -5F at times (then again, it was -25F outside). Note that this was NOT warm enough to keep water fluid, so I also needed a heated water dish.
BTW, I have Buff Orps. I did not supply any additional light and they laid better over the winter than they have all this summer. Some breeds do better with supplemental light, though.