http://support.nest.com/article/What-is-the-difference-between-Nest-Protect-Wired-120V-and-Nest-Protect-Battery http://support.nest.com/article/What-is-the-diff

http://support.nest.com/article/What-is-the-difference-between-Nest-Protect-Wired-120V-and-Nest-Protect-Battery http://support.nest.com/article/What-is-the-difference-betwe... Looks like a couple of years with 6 AA batteries.