No batteries required. 120 volt plug-in. With silence feature, samples the air and performs self-diagnostic tests approximately every second. A microchip inside the unit stores each reading, and remembers the levels of CO it has been exposed to over time. Controls are: test/silence button, green power light, yellow service light, red alarm light. Loud 85-decibel alarm, UL listed. 5 year limited warranty. 3.3" height x 4.75" width x 2" depth, 0.24 pounds.