Well, we don't actually have pigs, but we do have sheep, horses, and dogs. Since it only stays below freezing a few weeks out of the year (typically) we use somewhat temporary stock tank heaters that we bought from our local farm store. We have some that float and some that sink, most have wire-frame guards so they are safe in plastic/rubber tanks. We also just got a few small heated buckets (a 5-gal for our outside dogs, and a 16-gal for one of our groups of ewes) from
Premier 1. The smaller heated buckets work well for a small group of animals. One problem we ran into with the larger stock tank heaters is that you can only plug one heater into a circuit, since the heaters are 1500W. We found a few 1000W units and we have two of those plugged into one 20A circuit.
Doug