Carbon monoxide alarm powered by a 9-volt replaceable battery, included. Battery operation allows alarm to work during power outages. Easy access battery compartment makes changing the battery simple. 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 85 decibel alarm horn. No installation required, attractive design allows unit to be placed anywhere it is needed with optional wall mount. No outlet required. Audible and visual low battery signal alerts you to replacethe battery. Pull down action chart has convenient guide on what to do if an alarm sounds. 5-year limited warranty. UL (Underwriter's Laboratories) listed.