I don't have electricity in my coop. I have a roofed run attached and my chickens always sleep outside, (their choice) so I don't worry about them being cold. I decided not to put a light in after reading all the pros and cons, I want them to live as naturally as possible.
Mine have a run- but the roof is hardware cloth. But I'm happy to heat about the light. What do you use for heating the water? I am thinking the aquarium heater sounds perfect (new to chickens. Just started in May) in the "sea coast" area