The one I have is battery powered. It takes 4 AA batteries. If the batteries are dying, it will beep 10 times when you unlock it (very noticeable as different to the usual beeps). Ours lasted about 9 months on one set of batteries, it just recently yelled at me to change them.
The deadbolt takes a key as an alternative too, so each of us has a copy on our keyrings anyways (but if I bike I don't carry keys, so hubs would have to rescue me if the batteries died).