Did you ever come up with anything? I was wondering the same thing--would solar work for heating the chicken's water?
You may get some IDEAS and advice from a recent thread.
https://www.backyardchickens.com/th...ater-heater-light-electric-coop-door.1313185/
If you do have a possible access to electricity from the home,,,, it is the easiest option. Not sure how much winter weather you get since you do not have location in your profile.

All that is generally needed is APPROXIMATE LOCATION. This way respondents can offer advice to questions based on climate factors.
WISHING YOU BEST,,,,,,,,,,,,,,,,,,,,,,,,,,, and
