Kidde 900-0234 Nighthawk Carbon Monoxide Alarm, Long Life AC Powered with Battery Backup and Digital Display Digital display shows level of carbon monoxide the unit is sensing (updates reading every 15 seconds) Peak level button displays the peak CO level recorded by the alarm since it was last reset or unplugged 9-volt battery backup provides continuous protection even during power outages Tamper-resist feature goes into constant alarm mode if unit has been unplugged or has come loose from the outlet 7-year end-of-life warning alerts user to replace CO alarm (40 percent longer than other CO alarms)