How to do a fire alarm battery calculation
A fire alarm panel's batteries must carry the system in standby for a set time and then run it in alarm for a set time after main power fails. The calculation is simple arithmetic once you have each device's currents from its data sheet.
The method
- Add up the standby current of every device and module powered by the panel or power supply, including the panel itself (quantity × standby amps each).
- Add up the alarm current the same way, with all notification appliances operating.
- Standby capacity (Ah) = total standby current × standby hours.
- Alarm capacity (Ah) = total alarm current × alarm minutes ÷ 60.
- Required capacity = (standby Ah + alarm Ah) × (1 + safety margin). Pick the next standard battery size up.
Many standard systems use 24 hours of standby and 5 minutes of alarm, often with a 20% margin, but voice and emergency communication systems frequently need longer alarm durations. Always check the edition your authority having jurisdiction adopted and the panel manufacturer's instructions.
Free calculator
Worked example
0.350 A standby × 24 h = 8.40 Ah. 2.100 A alarm × 5/60 h = 0.175 Ah. Sum 8.575 Ah × 1.20 = 10.29 Ah, so a 12 Ah battery is the next common size up.
Totaling 40 devices by hand gets tedious. The Fire Alarm Battery Calculator workbook keeps a row per device and suggests the battery size automatically.