Seems to be slight misunderstanding. I was talking about the immersion style, but only planned on using it long enough to heat the water, then unplug and remove, then use the water to warm up the other buckets. My winter watering is done 2xaday inside the barn. Somehow using electric to keep a bucket warm all the time is only practical if nobody craps in it..My barn is cold, especially with no large animals providing natural heat (My current beef steer goes in the freezer in 2 weeks.) I empty water buckets when the goats and mini are done drinking, I leave the bucket with the chickens, but on the coldest days I have to alternate and bring the frozen one to the house to thaw.
I misunderstood, since you said: "Can't really use the continuosly plugged-in style because it would mean extension cords from the main outlet."
And perhaps I'm misunderstanding further, but it seems that you want to heat water (plugged into an outlet using an extension cord) while you do chores and then use that "warm" water to thaw other water buckets?
The problem I sense with that idea is that the type of water "heaters" I've seen only heat the water enough to prevent ice from forming, vice heating enough so that you could thaw out other buckets.
Our barn is also unheated. I don't "do" water in the barn. Even a 5-gallon is too heavy for me to lift (40 lbs or so?) and haul out of there, sloshing it all over myself. So our horse water is kept outside.
I've used this: http://www.miraco.com/accessories/heaters/
(the 250 watt) in a 15 gallon black rubber tub: http://www.qcsupply.com/duraflex-rubber-pan.html?utm_source=googlebase&utm_medium=cpc&utm_campaign=productfeeds&gclid=CNDKxOKO9cgCFQaOaQodQBMOKA
We use outdoor type extension cords. (I tried to find one, so I could find a photo on the internet; but couldn't, because husband out of town on the job and hid them) but the cords are blue and yellow in color. We leave the extension cords out there all winter (at the point they plug into the outlet is protected and we keep the de-icer end up off the ground, strapped to a fence post, because I don't trust the water-tight fitting enough to leave it on the ground in our rainy climate.)
We've been using those immersibles for 3 winters now.
The other one I've used is this heated bucket type: http://www.amazon.com/API-Gallon-Heated-Bucket-20FB/dp/B000HHQLHM
I think this might be similar to the one misty'smom
mentioned. We've used one of those for 2 winters. The electrical cord is wrapped in "coiled wire", which doesn't look too sturdy. We considered replacing it with armored cable, but decided it was too much trouble. We just strap the cord to the outside of the fence.
We haven't put out the de-icers yet. Just out of curiosity (because this topic roused my interest) I measured the temperature of Nicky's water this afternoon. It was 50F just filled, off our well. Several hours later, when I put them to bed, his water had dropped to 47F. I was sitting on the porch, and after he ate his hay he came out and drank it (at 47F.)