I don't have electricity in some of my coops so a battery is the way to go for those birds. A battery charge lasts at least 5 days. Eventually I'll have my solar chargers going so I don't have to mess with the batteries at all.
I have a battery powered heater that heats a 5 gallon container to around 60 F with outside temps as low as 4 F. I don't know how low temps can go and still have it work but we typically get temps down to -18 F so we'll see. My birds love it. I don't think I'd want the water much warmer, though...