Fire alarm secondary power calculator
Primary power is usually the building AC supply to the fire alarm control unit or power supply. Secondary power is the standby source—almost always batteries—that must carry the system for a required standby time and then support alarm (or emergency communication) operation for a required alarm time after primary fails. This page turns those currents and durations into an estimated amp-hour requirement.
The method
- Sum standby current (A) for the panel and every device powered from that secondary source (quantity × standby amps from data sheets).
- Sum alarm current (A) the same way, with notification appliances (and voice channels, if applicable) operating as required.
- Standby amp-hours = standby current × required standby hours.
- Alarm amp-hours = alarm current × required alarm minutes ÷ 60.
- Required secondary capacity = (standby Ah + alarm Ah) × (1 + safety margin). Select the next common battery size at or above that value, then confirm with the panel manufacturer and the code edition your authority having jurisdiction has adopted.
Many conventional systems use long standby and short alarm durations; voice and mass-notification style systems often need longer alarm or evacuation times. This calculator does not tell you which durations apply to your project—enter the values required for your occupancy and adopted criteria.
Free calculator
Defaults are example inputs only. Use device data sheets and the standby/alarm durations required for your system type and adopted code edition.
Worked example
0.420 A standby × 24 h = 10.08 Ah. 3.200 A alarm × 5/60 h = 0.267 Ah. Sum 10.347 Ah × 1.20 = 12.42 Ah, so a 18 Ah battery is a common next size up (a 12 Ah pack would be short of the margin in this example).
For a related walkthrough focused on battery sizing language, see How to do a fire alarm battery calculation. Totaling dozens of devices by hand is tedious; the Fire Alarm Battery Calculator workbook keeps a row per device and suggests battery size automatically.