Kidde (21007267 KN-Copp-B-LS 900-0230 Nighthawk Carbon Monoxide Alarm, Battery Operated with Digital Display Digital display shows CO level in PPM (parts per million) and updates digital reading every 15 seconds Backlit display to help see the CO level reading in the dark Peak level memory displays the highest CO concentration measured since the last reset Battery operated (3 AAs included) to provide protection during power outages Battery lockout system deters covering battery compartment without installation of batteries