Fire-Alarm / Security Standby Battery
Required battery amp-hours from standby and alarm loads times the aging/derate factor, with the next standard size (NFPA 72 §10.6).
Formula and source
Ah = [(I_standby * h_standby) + (I_alarm * h_alarm)] * derate, with the alarm minutes converted to hours; the next standard battery size is the smallest at or above the requirement.
Secondary (standby) battery sizing for a fire-alarm or security control unit - standby amp-hours plus alarm amp-hours times the aging/derate factor - per NFPA 72 National Fire Alarm and Signaling Code §10.6 (secondary power supply) and the panel manufacturer's battery-calculation worksheet, by name.
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