Neither loss nor restoration of the primary power should cause an alarm signal. The batteries must meet all power requirements for at least one year, including routine testing. The unit must also be capable of producing the alarm signal for four consecutive minutes during the seven days of trouble signal. Battery-operated smoke detectors have two advantages and one disadvantage when compared with detectors operated from the dwelling's electrical power. One advantage of battery-powered units is that if the electrical power in the dwelling should be off due to an interruption from the utility company or from a fire within the dwelling, the detector will still function.
Some manufacturers are now manufacturing units with a battery backup in case power is lost.
The other advantage is that you can install a smoke detector in an existing dwelling where an appropriately placed electrical outlet is not available. The major disadvantage is that the batteries need to be replaced once a year. If hallways are longer than 40 feet between the sleeping and living areas, use two smoke detectors. You might consider installing a photoelectric unit in the bedrooms of any family members who smoke. Heat detectors or additional smoke detectors can supplement the basic system.