120VAC with 9vdc Battery Back-up. The BRK 9120B dual ionization smoke detector features a dual ionization sensing chamber, which enables it to detect the small particles produced by fast, flaming fires. A single button both silences false alarms and allows the user to test the alarm with ease. The smoke detector also has two power sources: 120 volt for hard wiring and a 9 volt battery backup for peace of mind during power outages. Listed to UL 217 standards, the alarm operates at temperatures from 40 to 100 degrees Fahrenheit and in humidity up to 90% and has an 85-decibel alarm.