Fire alarm battery calculator
Turn standby and alarm loads into an amp-hour estimate.
Planning estimate. Verify before applying to a project.
How the calculation works
Capacity = [(standby A × hours) + (alarm A × minutes ÷ 60)] × multiplier
Worked example
With 0.25 A for 24 hours and 2 A for 5 minutes, the unadjusted demand is 6.167 Ah. A 1.2 multiplier gives 7.40 Ah.
What this estimate covers
This estimates capacity only. It does not select a listed battery, check charger capacity, or confirm enclosure compatibility. Verify the manufacturer’s supported batteries and required factors. Two equal 12 V batteries in series provide 24 V with the same Ah capacity as one battery.
NOTIFIER power calculation example
Keep your work with the project
Review the related fire alarm calculators and NICET practice samples. Explore calculation tools, project notes, PDF reports, and plan markups in Fire Alarm Field Pro.
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Common questions
What is the fire alarm battery calculation formula?
Add standby current multiplied by standby hours to alarm current multiplied by alarm hours. Apply the required capacity factor afterward. This calculator accepts alarm duration in minutes and converts it to hours before adding the two demands.
Does alarm current include the standby load?
Enter the total system current during the alarm condition, including equipment that remains energized. Do not enter only the extra notification current if other system loads continue drawing power. Check the manufacturer worksheet for how it groups loads.
Do two 12 V batteries in series double the amp-hour capacity?
No. Two equal batteries in series add their voltages while retaining the amp-hour rating of one battery. For example, two 12 V, 18 Ah batteries in series form a nominal 24 V, 18 Ah bank.
Can I simply round the result up to any battery size?
Choose a supported capacity at or above the verified requirement, then check charger capacity, enclosure space, listed configurations, and the manufacturer’s instructions. A capacity estimate alone does not approve a battery selection.