The Fire Sentry i9040 smoke alarm is a battery operated smoke alarm that utilizes ionization sensing technology. The four (4) inch unit is smaller in diameter giving it discreet design with the same quality you would expect in a Kidde manufactured product. The i9040 is powered by a 9V battery, providing continuous protection, even during power outages. This easy to install unit includes a test button to verify the detection electronics are working as intended. Benefits The Fire Sentry i9040 battery operated, single station smoke alarm provides an early warning signal to you and your family in the event of a fire hazard. The i9040 operates on a 9V battery power source that provided continuous protection even during a power outage, Installation is easy, no wiring required. This alarm uses ionization sensing technology. Ionization sensing alarms may detect invisible fire particles (associated with flaming fires) sooner than photoelectric alarms. Photoelectric sensing alarms may detect visible particles (associated with smoldering fires) sooner than ionization alarms. Kidde strongly recommends that both ionization and photoelectric smoke alarms be installed to help insure maximum detection of the various types of fire that can occur within the home.