it be so nice when you buy a product for a need use and it be a lot better than you expect !! my wife could not see a small clock at night without she glass -- well , that be not a problem any longer ! this work perfect , look great , easy to operate and doesnt take up too much space . the battery back up use a 9 volt battery be a very nice feature . no , you cant tell time when the power be off , the battery keep time until the power come back on and the clock have the correct time . this be true with all battery back up , the battery CAN NOT power the light display . if you need a really BIG number clock this be it ! i search a long time and there be a lot of smaller number clock out there but no other with 2.5 inch . doesnt sound that big til you see it , WOW . dimmer work great also . do you search , there be bigger display available for 2-3 hundred dollar . this clock at 15.00 -- 20.00 be a real bargain and even have a FULL year warranty . try it , you will like it .