I tested the concept and it does work. I took a "floodlamp" light bulb (think it is 100 watt) and a small piece of 2" styrofoam. Made a hole and screwed the bulb through the hole into a light cord so that the styrofoam now creates a float. (wood would also work) Placed the bulb in a 1/2 plastic barrel waterer with about 20 gallons of water, just outside , in the snow with no cover. Plugged it in and this morning it was not only ice-free, the water temp was +43 F and , while the temp was -5 F. For colder temps the efficiency could be improved with insulating the barrel/trough, setting it on insulating base or using higher wattage bulb. Trick is to only turn light on while in the water. All my horses have ice-free waterers now, but will keep on hand and could readily devise safe use for the horses.