They are also allowed to be powered from a "local regularly-used lighting circuit". IMHO a better idea for a domestic situation as it stops the breaker being tripped to stop nuisance alarms. The local regularly used part is so that a power failure will be quickly diagnosed because the lights won't work. I think that's another good reason why this is the preferable option.
At least if the lights go off or trip, you are more likely to notice this and fix the problem therefore your smoke alarm will be working and keeping you safe.
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