Ryan Rice contacted me with some good information about the electrical aspect of heated water buckets:
“I’ve just read your article on heated buckets for horses in a barn. While I understand the concern about safety and fire hazards, there’s no reason to be worried and not use one. A 5 gallon heated bucket only uses 130 watts, while a 16 gallon bucket still only uses 260 watts. If you get a heating element for a 100 gallon Rubbermaid water bucket you might get up to around 1500 watts, but most people don’t use 100 gallon buckets inside the barn. So, if you have a 15 amp circuit, which is the smallest you could have, the maximum continuous watts would be 1440. As long as properly rated extension cords are used–preferably none–and for extra safety, a GFI outlet, you can keep your horse drinking water all winter.