I posted something like this a while ago. I am off the grid completely, and there will be NO electricity in my coop- extension cord or no. I would have to run a generator for one of those water de-icers. That will not happen. We aded solar panels and we're hoping that this wnter the generator will not have to run as much this year *fingers crossed* I went with an ecoglow 14W brooder because a 100W bulb would have been the most inefficient thing in our house if left on 24-7.
I am new to keeping chickens, and this will be my first winter doing this. I will be using the black rubber flexible buckets that you find in the farm supply stores. Depending on the volume of water, They will freeze less quickly, but water will still have to be changed out a few times a day. Just pop the iceblock out, or switch it out and bring the other one inside to melt. I would say that a 5 gallon pail would have to be surrounded by a platform and might break from the freezing. Metal is a bad idea all around, and those double wall founts would break apart very easily.
For any helpful suggestions that didn't involve an extension cord, I searched the sufficientself.com forum (A BYC sister site). Someone there suggested a cooler type drinking fountain that has insulated sides (igloo makes them). Just fill with water and build a platform for it to sit in so the chickens can drink from the top. I will be trying that.
Another person suggested a car battery and battery heating pad, here is what they posted. "One thing you can do, is to use a car battery, a metal oil pan and a battery heating pad. Do what is needed to plug the heater to the battery. Use a good silicone to adhere the heater to the inside of the pan. Cut a small hole on the side for the cord.
Turn the pan over, set your water dish on top and plug in the heater. Provide a step for the chickens to reach the water if needed." I may just try this, I have and extra solar little panel, and already have one hooked to a water pump. If it drains the battery too quickly, I'll just hook up a bigger battery. If I can't keep the bigger battery charged with my setup, I'll just go back to the igloo drink cooler and/or the rubber buckets.
Good luck!