Powered by a 2 "AA" replaceable alkaline batteries (included), works during power outages. Permanent sensor lasts up to 5 years, no expensive sensors to replace. Test/silence button silences the alarm, unit monitors and re-alarms if carbon monoxide levels persist. Loud 85dB alarm horn. No installation required, attractive design allows unit to be placed anywhere you need it, optional wall mount, no outlet required. Audible and visual low battery signal alerts you to replace the battery. Pull down action chart has convenient guide on what to do if an alarm sounds. 5 year limited warranty.