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Battery calculations size the standby batteries so the fire alarm system can operate during a power outage for the required standby period, then support alarm operation for a required duration. You sum standby currents (panel + devices) over standby hours, add alarm currents over alarm minutes, apply derating/safety factors, and select battery amp-hour capacity accordingly.
Worked example(s) Example (concept): If standby is 0.25 A for 24 hours and alarm is 1.5 A for 5 minutes, compute Ah = (0.25×24) + (1.5×(5/60)) = 6.0 + 0.125 = 6.125 Ah. Then apply safety/aging/temperature derating → choose next standard battery size above that.
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