The laws of thermodynamics, the heat source in the center of the bottom of the bottle will cause the warmer water to rise, creating a continuous "current", as warm water is displaced with cold, thus circulating the water without the need for mechanical assistance.
No need to have a "large" tin or surface area, just sufficient energy to keep 5 gallons above 32F regardless of ambient temperatures.
I know a 38W bulb will keep ~2.5 gallons of water liquid till at least -9F, using a metal double walled waterer and a cookie tin heater.
Heat will not conduct through plastic as efficiently as it does through metal.
Nor does cold ...
Condensation that is formed due to the hot and cold surface temperatures, creates a continuous bond from the surface of the tin, to the pizza pan, to the bottom of the water bottle. This condensation creates a fairly efficient transfer of heat from the source to the water.
As there are multiple barriers between water and electricity, safety concerns are minimized. This assumes all electric power is GFCI protected, and the heat source is properly constructed.
Birds dribble water, wet litter...
Add Pizza pan to catch dribble.
Birds drink any dribble from pizza pan.
Litter is now completely dry!
Everything sits on Popcorn Tin Heater. Update: Everything sits on my Forever Water Heater. I am not fond of mixing electricity and water, this method keeps everything well separated, and dry.
Easy fill funnel, usually covered with a 5 gallon fill bucket to keep dust and debris out.
Everything disassembles for ease of cleaning.
I will continue to use a 38w bulb.
I have higher wattages on hand, should the water ever freeze.
I will update as more cold data is acquired.
Update 1/5/15: Temperature in coop had a low in the high single digits Fahrenheit.
Used a 50W flood in the Forever Water Heater. Water and nipples were warm to the touch. Water in the drip pan was warm also!
There was no additional insulating material around the water container.
System worked as designed, flawlessly!
Note: This will also work well with a simple 5 gallon bucket with horizontal nipples.
Thanks for viewing!