Fire Alarm Battery Calculator 🔋

How to Calculate Fire Alarm Battery Size (NFPA 72 Method)

Every fire alarm control panel needs a secondary power supply — batteries that keep the system alive when mains power fails. NFPA 72 defines exactly how much: the batteries must run the system in normal standby for 24 hours and, at the end of that period, still operate every notification appliance in full alarm for 5 minutes (15 minutes for emergency voice/alarm communication systems). Undersize the batteries and the installation fails inspection; oversize them and they may not fit the panel cabinet or exceed the charger's capability.

The NFPA 72 Battery Formula

Where I_standby is the sum of all device quiescent currents (panel + detectors + call points + modules), and I_alarm is the sum of all device alarm currents including every sounder and strobe. The 1.20 factor is the 20% derating applied in NFPA 72 practice to cover battery aging and temperature — the system must still meet the requirement near the battery's end of life.

Worked Example: Addressable System, 120 Detectors

Panel (150/300 mA), 120 smoke detectors (0.3/1 mA), 20 call points (0.3/1 mA), 8 modules (0.4/5 mA), 40 horn-strobes (0/80 mA), standard 24 hr + 5 min, 20% derating:

• Standby current = 0.150 + 0.036 + 0.006 + 0.0032 ≈ 0.195 A → × 24 = 4.69 Ah
• Alarm current = 0.300 + 0.120 + 0.020 + 0.040 + 3.200 = 3.68 A → × 0.0833 = 0.31 Ah
• Subtotal = 5.00 Ah → × 1.20 = 6.0 Ah → select 2 × 12V 7 Ah batteries

Notice the pattern: standby dominates the calculation — 24 hours of quiescent load is almost always far larger than 5 minutes of alarm load, which is why cutting standby current (fewer LEDs, efficient devices) shrinks batteries more than anything else.

Design Tips

• NAC/booster power supplies need their own calculation. If sounders and strobes run from separate notification appliance circuit power supplies, size those batteries with the same 24hr + 5min method — don't lump them into the panel battery.
• Voice evacuation = 15 minutes. EVACS/PAVA amplifier loads at 15 minutes of alarm change the numbers significantly — and amplifiers often have large standby draw too.
• Check charger and cabinet limits. Panels list a maximum battery size they can charge (often 18–75 Ah); bigger batteries need an external battery cabinet and possibly a larger charger.
• Local codes may differ. BS 5839-1 practice uses 24 hr standby + 30 min alarm; some authorities require 60 hr standby for unmonitored premises — always confirm with the AHJ.
• Use real datasheet currents. The defaults in this tool are typical values for preliminary sizing; final calculations submitted for approval must use the exact manufacturer figures.

Frequently Asked Questions

Disclaimer: This tool provides preliminary sizing using typical device currents. Battery calculations submitted for authority approval must use exact manufacturer datasheet values and comply with the applicable edition of NFPA 72 / local codes.