The Fire Sentry i9040 smoke alarm is a battery operated smoke alarm that utilizes ionization sensing technology. The four (4) in 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. Battery operated (9 V battery included) - easy installation and provides continuous protection even during power outages Test button - tests unit's electronic circuitry, horn and battery function Low battery indicator - low battery warning chirp alerts user to change battery Discreet design - 4 in diameter