We built ourn8 ft by 12 ft walk in coop this summer and have electric. I used a heat lamp for about a week for the younger pullets, but otherwise only use lights and heated water bowls. (Finding a chicken perched on the heat lamp cord was a little frightening). One thing to consider is the cost of adding electric. It was a huge part of our expenses, to include the permit, renting the ditch digger (twice, failed the first inspection), electrician, underground wire (expensive!!!), and many other related expenses. I lost track of our total coop cost (got too depressing), but I think that the electricity easily added on $700-800, maybe more. Not to mention the stress of trying to get everything up to code... Good luck with your decision, electricity does make keeping chickens easier for us.
