Interconnect up to 18 units, 12 smoke alarms maximum; when one alarm goes off all units sound their alarm * The pack includes a separate large size 5-5/8-Inch mounting plate, which attaches to any standard rectangular or octagon junction box and covers drywall cutouts * This 120-Volt AC powered alarm also includes a 9-Volt super heavy duty battery backup which provides power in the event AC power is lost * The chamber and cover design protect the alarm from bugs and contaminants * The alarm sounds with its loud 85 decibel horn and the red LED indicator flashes. A green LED light indicates AC power