Kidde KN-COPP-3 Nighthawk Plug-In Carbon Monoxide Alarm with Battery Backup and Digital Display, 6-Pack Digital display displays the level of carbon monoxide the unit is sensing Three convenient mounting options Peak level button displays the peak CO level recorded by the alarm since it was last reset 9-volt battery backup provides protection during power outages Meets Underwriters Laboratories (UL) requirements for safety standards The digital display feature allows for easy viewing of carbon monoxide levels from 30-999PPM (parts per million)