The other end of the equation is to make maximal use of what heat you *do* have (be it from electrical heating, from a sunny window, from a solar heater, from the chickens' own body heat, from the daily high temperature, whatever, all of it).
So it is worth thinking what you can do to maximize daytime heat gain and storage, and insulate your coop well (including the roof) (and in snowy climates, remember snow is quite a good insulator if piled against strong coop walls!).
Pat