The Ei 146 smoke detector runs on either AC power or a 9V battery, and lasts up to 5 years. When the detector detects smoke, it produces an audio signal of 85 decibels. Since the smoke detector runs on AC power, you can connect it directly to the default socket in the ceiling. The batteries serve as back-up in case of a power outage. Use the test button to check whether the smoke detector is still working.Note: Can only be connected wirelessly when you own the Ei 168RC base.