Fire Alarm Standby Battery Calculation
How to size the secondary (battery) supply for a fire alarm system, per NFPA 72 §10.6.7 (2022 edition): 24 hours of standby plus 5 minutes of alarm (15 minutes for voice), multiplied by the 1.25 aging factor and rounded up to a standard battery size. The method and a worked example are below; the appliance current draws that feed it are on the related charts.
Secondary-power requirements
| Parameter | Requirement | NFPA 72 § |
|---|---|---|
| Standby (quiescent) period | 24 hours | 10.6.7.2.1 |
| Alarm period — standard fire alarm | 5 minutes at full alarm load | 10.6.7.2.1 |
| Alarm period — voice / mass notification | 15 minutes at full alarm load | 10.6.7.2.1 |
| Aging / derating factor (2022) | × 1.25 (was × 1.2 pre-2022) | 10.6.7.2.14 |
| Generator secondary source | 4 hours standby in lieu of 24 h battery | 10.6.7 |
The formula
The battery has to carry two very different loads. For almost the entire 24 hours it carries only the standby (quiescent) current — the panel, detectors, and modules sitting idle. Then, for 5 minutes, it carries the alarm current — every notification appliance firing at once, a much larger draw. Size for the sum of both, in amp-hours:
Ah = [ (standby A × 24 h) + (alarm A × 5/60 h) ] × 1.25
Use 15/60 hour in the alarm term for a voice or mass-notification system. The 1.25 multiplier is the aging factor — a sealed lead-acid battery delivers well under its rated capacity after a few years of float service, so the code builds in margin. Round the result up to the next standard battery size.
Worked example
| Step | Calculation | Amp-hours |
|---|---|---|
| Standby load | 0.25 A × 24 h | 6.000 Ah |
| Alarm load | 2 A × 5/60 h | 0.167 Ah |
| Subtotal | standby + alarm | 6.167 Ah |
| Derating × 1.25 | 6.167 × 1.25 | 7.71 Ah |
| Battery size | round up to next standard size | 12 Ah |
Getting the currents right
The calculation is only as good as the two current figures. Standby current is the sum of every device's quiescent draw from the panel's battery-calculation sheet; alarm current is the sum of the panel in alarm plus every notification appliance at its rated candela. The appliance side is usually the larger unknown — the NAC current draw chart gives typical horn/strobe values by candela for early planning, but the final number comes off the actual device data sheets and the panel's own current draw.
This is a look-up of the method, not a substitute for the manufacturer's battery-calculation workbook, which the AHJ will expect to see. Confirm the edition your jurisdiction enforces — the 1.2-versus-1.25 factor is the difference between a passing and a failing calculation on the same system.
Common questions
How do you calculate fire alarm battery size?
Add the standby (quiescent) current × 24 hours to the alarm current × the alarm time (5 minutes = 5/60 hour for a standard system, 15 minutes for voice), then multiply the total by the aging factor and round up to the next standard battery size. Under NFPA 72 2022 the aging factor is 1.25. For example, 0.25 A standby and 2 A alarm gives (6.0 + 0.17) × 1.25 = 7.71 Ah, so you install a 12 Ah battery.
Is the derating factor 1.2 or 1.25?
Both — it depends on the edition. The 2022 edition of NFPA 72 raised the aging factor from 1.2 to 1.25 (§10.6.7.2.14), because a sealed lead-acid battery delivers less than its rated capacity as it ages. Use 1.25 under the 2022 edition and 1.2 under the 2019 or earlier editions your jurisdiction may still enforce.
How long does a fire alarm battery have to last?
The secondary supply must run the system for 24 hours in standby (normal supervision) and then still sound a full alarm for 5 minutes — 15 minutes if the system has voice or mass-notification. An automatic generator can substitute for the 24-hour battery with a 4-hour battery instead.
Do you round the battery size up or down?
Always up. The calculated amp-hours are a minimum; you select the next standard battery capacity at or above it. Rounding down would leave the system short of the required standby and alarm time at the end of the battery’s service life.
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