The Kidde p12040CA is a 120V AC, wire-in smoke alarm with 9V battery backup that can function as a stand alone unit or interconnect with up to 24 Kidde devices (of which 18 can be initiating). The Kidde p12040CA provides continuous protection against smoke and fire hazards, even during power outages. This smoke alarm uses photoelectric sensing technology. Photoelectric sensing alarms may detect visible fire particles (associated with slow smouldering fires) sooner than ionization alarms. Ionization sensing alarms may detect invisible fire particles (associated with fast flaming fires) sooner than photoelectric alarms. Leading authorities recommend that both ionization and photoelectric smoke alarms be installed to help ensure maximum detection of the various types of fires that can occur within the home. Photoelectric technology - may detect visible fire particles (associated with slow smouldering fires) sooner than ionization alarms 120 V interconnectable - will interconnect with up to 24 other devices (18 of which can be initiating), including smoke alarms (limit of 12), co and heat alarms 9 V battery backup (included) - provides continuous protection even during power outages Hush® feature - temporarily silences nuisance alarms for approximately 10 min (smoke must be present before Hush® is activated) Simultaneously tests the unit's electronics and verifies alarm operation