We bought the ice
and easy floating tank heater last year at a local feed store for
about 25 dollars and it has proven itself to be a very practical and
affordable way to keep outside water from freezing.
It has a built in thermostat that only kicks on when the temperature
goes below 32.
It might be the most clever name given to a farm product. I'm using
ours to keep the chicken water warm tonight so they can have something
non frozen to wake up to tomorrow.