alternative to heated water buckets?
Now that New England is finally on the Winter page, the BO has replaced the horse's regular buckets with heated ones to make sure they have something other than ice to drink. My horse, however, is One Thirsty Guy and goes through two buckets of water overnight. BO doesn't want more than one heated bucket in a stall (partly because of electricity costs, partly because of maintaining the right load on the electrical system).
So, he's got one heated bucket, but needs more water than it will hold. Any ideas for ways to keep the water from freezing without having to plug something in?