Kidde i9040  Battery-Operated Ionization sensor Compact Smoke Alarm - 21026056 Battery operated Ionization sensor compact smoke alarm 9 volt battery, mounting hardware and installation guide Four inch diameter discreet design Test button tests unit's electronic circuitry horn and battery function UL listed and is backed by a 3 year limited warranty Easy installation Battery operated 9V battery included Test button tests unit's electronic circuitry horn and battery function Four inch diameter discreet design Battery operated 9V battery included Test button tests unit's electronic circuitry horn and battery function Four inch diameter discreet design Easy installation Meets the UL requirements and widely-accepted safety standards