NFPA 72, National Fire Alarm and Signaling Code specifies the power requirements for fire alarm systems.
A system must have either two sources of power (primary and secondary) or a single Uninterruptible Power Supply (or UPS). Where primary and secondary power supplies are used, the secondary supply can consist of batteries or batteries plus a standby generator.
NFPA 72 requires that the circuit disconnecting means be labeled “fire alarm”. The marking must be red in color.
For a basic fire alarm system that uses primary power with batteries only as secondary power, the battery capacity must be sufficient to operate the system in a non-alarm condition for a minimum of 24 hours.
The Code requirement for 15 minutes of full load should be considered in conjunction with performing a risk analysis to determine if a larger capacity is needed.