Holds 20 AAA batteries. This holder is designed with a cavity behind which I used to set the mounting system to the door. I placed it onto the door of a cabinet so the that the batteries are available on opening the door. To connect it to the door I used 3M Dual Lock. This is the greatest stuff and worth purchasing if you don't already have some. It is essentially velcro on steroids. The adhesive backing on this stuff is permanent and heavy duty. See http://solutions.3m.com/wps/portal/3M/en_US/Adhesives/Tapes/Products/~/3M-Dual-Lock-Reclosable-Fastener-SJ3870?N=5927706+3294281433&rt=rud for more details. The gap in the back will perfectly fit the Dual Lock so that the unit sits flush with the wall or door that it is mounted on.