Smoke alarm with battery back-up. Features a photoelectric sensor， which is more effective at detecting smoldering fires that smolder for hours before bursting into flame. 120V (volt) AC (alternating current) inter-connectable (with maximum 11 compatible devices). Power indicators show which unit in the series initiated the alarm. Quick plug-in power connector for easy installation. 9V battery back-up for peace of mind during power malfunction. Battery life varies according to use and application. Optipath 360 technology allows 360 degree access to the layer of air on the ceiling or wall that carries the fire signatures (smoke， combustion particles， and volatile components). Single test/silence button which both silences a nonthreatening alarm or low battery warning and allows you to test the unit's functionality. Locking feature for tamper resistance. 10-year limited warranty.