Water Wigglers

I am not able to have heated bird baths in the winter as there are no outdoor electrical outlets on my older home. I have decided I do not want to install outside outlets, so I have been looking on the Internet for battery operated heating elements for bird baths. Water wigglers seem to be the only option -- do they really work for keeping the baths ice free? I live in Virginia and we have a fair number of below freezing days and nights during the average winter.