I'm wrestling with the same dilemma, WillHunt.
From what I've read here, it sounds as though any heater will draw quite a bit of power - certainly enough that the battery would need to be replaced/rechraged nearly daily during the dead of winter. I had hoped to get a solar-type system going, but it may be way more expensive and an impractical solution.
I guess at this point, I may just see how things go this winter and change out their water 2-3 time a day (before work, after work, before bed) when we're in a cold snap. If it's too much of an issue, I will likely bite the bullet and run a conduit and wire out to the coop and get a heated dog bowl...
edit: I'm leary of simply running an extension cord - our coop is ~100' from the nearest outlet. If it comes down to it, I'll run the proper electrical wire in a conduit out to an 'outdoor' electrical outlet near the coop and have an electrician make the connections. Seems safer that way...