Kidde i9040 Fire Sentry Battery-Operated Ionization Sensor Compact Smoke Alarm (6 Pack) CustomerPackageType: Standard Packaging Size: 6 Pack, Model: i9040-6, Tools & Outdoor Store 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