This does require electricity but has worked really well for me last year. I will be using it again this year. Submersible Aquarium Heater its plastic so no broken glass to worry about, has adjustable heat setting. I plug it into a thermo cube so it only comes on at i believe it is 34*. Ordered from good old Amazon for fairly cheap. They do have different wattage units in case someone needs some more juice. Last year i had tube type homemade waterers made from 4" PVC that held roughly 4 gals. The water inside the tube never froze but my little red drinking cups on the outside did. This year i plan on using the same heater with the standard poultry waterer that you buy from Tractor Supply 3.5 gallon ones.
Here is a picture of the heater i used:
Here is a picture of the heater i used:
