I really hate to admit we had zero smoke detectors in our home. We have natural gas so co2 was important to monitor as well. These do both, without having to have separate alarms. They came in with all parts including batteries. Very easy to determine if it's working just hit the test button. Also you'll see it flash randomly that shows it's on and batteries are working. Smoke detector hasn't been very sensitive to my cooking so that's a bonus. I can promis it's loud though!