There are two kinds of smoke alarms - ionization and photoelectric. The ionization smoke detectors activate quicker for fast, flaming fires and the photoelectric type is quicker for slow, smoldering fires. Either one will provide you enough time to get out, but having a mix of the two types is a good idea. Models with both sensors are better than single sensor units, but of course they cost more.
Smoke alarms are powered either by household alternating current (ac), a battery, or ac with a battery that keeps it operating during power outages. The battery type is easy to install in existing homes but the battery must be changed annually. The Connecticut General Statutes for new homes require ac powered alarms with battery backup. For greater safety, older ac only smoke alarms should be replaced with ac/battery alarm, and any smoke alarm older than 10 years should be replaced.
Carbon Monoxide (CO)
Carbon Monoxide (CO) alarms are recommended when a home uses gas or oil, or has a fireplace. CO alarms are also powered by either an ac, a battery, or ac with a battery. Most CO comes from equipment that will not be working during a power outage so plug-in units are good. But if you might heat your home with a fireplace, wood stove, or kerosene heater when the power is out, you may want to use a battery-powered alarm.
The sensor element in some CO alarms must be replaced regularly. Consider the cost of the replacement element in making your selection.