The advantage of installing a mains operated smoke alarm system in your house is that all the smoke alarms are interlinked i.e.  if a fire breaks out in the kitchen for e.g. the alarms will sound all over the house not just in the room affected.  All homes should have at least 3 mains smoke alarms installed - normally one in the kitchen, one downstairs in the living area and one in hall/landing. These alarms also come with a battery backup in case the fire destroys the wiring.