Fire Alarm Battery Calculator (NFPA 72)
Add up your panel's standby current and alarm current, choose the standby duration your system requires, and get the minimum battery size in amp-hours with the 20% derating most manufacturers specify.
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Read the how-to guides
How to Calculate Fire Alarm Battery Size (NFPA 72)
24 hours standby plus 5 minutes alarm, the 1.2 derating factor, standard battery sizes and a worked example that lands on a 12 Ah set.
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Panel minimum minus wire drop must leave the last appliance above its minimum. Lump-sum and point-to-point methods, with a worked example.
The formula
Ah = [ (standby amps × standby hours) + (alarm amps × alarm minutes ÷ 60) ] × safety factor
NFPA 72 requires secondary power sized for 24 hours of standby followed by 5 minutes of alarm (15 minutes for voice evacuation systems; 60 hours for some emergency communications and where the AHJ requires). Most manufacturers add 20% to account for battery aging and temperature. Pick the next standard battery size at or above the result: 7, 12, 18, 26, 33 or 55 Ah.
Worked example
0.45 A standby for 24 h is 10.8 Ah. 2.1 A alarm for 5 min is 0.175 Ah. Total 10.975 Ah × 1.2 = 13.17 Ah. Use a 18 Ah battery (or two 7 Ah batteries in parallel is 14 Ah, which also works).
Frequently asked questions
How long must a fire alarm run on battery?
NFPA 72 requires 24 hours of standby followed by 5 minutes of alarm for most systems. Voice evacuation systems need 15 minutes of alarm. Some emergency communications systems require 24 hours standby plus 15 minutes, and certain remote supervising stations need 60 hours.
Why add 20%?
Battery capacity falls with age and cold. A 20% margin is the manufacturer's standard so the system still meets the requirement near end of life.
Where do I get the current numbers?
The panel and device data sheets list standby and alarm current in milliamps. Add them all up.
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This calculator gives an estimate for planning and is not tax, legal or engineering advice. Confirm code-related results against the code edition adopted in your jurisdiction and with your authority having jurisdiction, and follow the manufacturer's instructions.