It worked until about 10 degrees, below that it was just a waste of electricity. I even tried a barrel wrapped in heat tape with a heater inside insulated it and inside another barrel with a heat light on a drinking cup. Other than raising my electric bill, nothing good happened.
I am still searching for the perfect winter waterer. I am actually thinking of dumping hot water into the rubber bowls and them only getting water until it freezes.
BTW Hot water, regardless of what the wives tales say, will stay unfrozen longer than cold, there are more calories to leave, at some point the hot and cold would have to reach the exact same temp on the way to becoming ice. It defies logic to think the water that was hotter once would freeze faster from that point on. I have heard that same thing about hot water freezing faster, I wonder where it started and why?