I buy this because I want a small , maintenance-free alarm clock . it be not as small as I would like -lrb- it be about the size of two VHS tape -rrb- . it claim to be solar power but it come with a cr123a lithium `` back-up '' battery . the manual read , `` this battery should provide a back up power supply to you clock for up to 8 year or even longer . '' I have not have it long so I can not verify that claim . it do set the time automatically , which be cool .