Engineering Guide

Fire Alarm Battery Calculations: A Step-by-Step Guide

FUEGOAI Engineering Team · August 2026 · 7 min read

Every fire alarm submittal needs a battery calculation, and it's one of the first things a plan reviewer checks. NFPA 72 requires fire alarm systems to have a secondary power supply that can carry the system through a power outage and still perform its job at the end of it. This guide walks through the standard calculation method step by step, with a worked example.

Step 1 — Inventory every device on secondary power

List everything the panel and each power supply must support: the control panel itself, addressable devices (detectors, modules, pull stations), notification appliances, annunciators, communicators, and auxiliary loads. For each, pull two numbers from the manufacturer's datasheet: standby current (normal supervision) and alarm current (full alarm). Quantity matters — 42 detectors at 300 µA is a very different load than 8.

Step 2 — Sum standby and alarm currents separately

Multiply each device's current by its quantity, then total the standby column and the alarm column separately. Remember that notification appliances typically draw nothing in standby but dominate the alarm column, while the panel electronics dominate standby.

Step 3 — Apply the required durations

NFPA 72 defines how long secondary power must carry the system. The commonly applied baseline is 24 hours of standby followed by operation in alarm — 5 minutes for conventional notification, with longer alarm durations (commonly 15 minutes at maximum load) for in-building emergency voice/alarm communication systems. Verify the requirement for your system type and edition.

Step 4 — Convert to amp-hours

The core formula:

Step 5 — Apply a safety margin and select the battery

Multiply the result by a derating factor — 20% (× 1.2) is the widely used industry margin, and some manufacturers' calculation sheets require it — to account for battery aging, temperature, and tolerance. Then round up to the next standard battery size (7, 12, 18, 26, 33, 55 Ah…), checking the panel's charger is rated for the batteries you select.

Worked example

Device	Qty	Standby (A)	Alarm (A)
FACP (base panel)	1	0.150	0.300
Smoke detectors	42	0.013	0.013
Pull stations	6	0.001	0.001
Monitor modules	8	0.003	0.003
Horn/strobes (NAC)	24	0.000	1.440
Total	81	0.167	1.757

Standby: 0.167 A × 24 h = 4.00 Ah. Alarm: 1.757 A × 5 min = 0.15 Ah. Subtotal 4.15 Ah × 1.2 = 4.98 Ah → select two 12 V 7 Ah batteries (24 V system).

The mistakes that get submittals bounced

• Forgetting separate calculations per power supply. Every NAC power supply needs its own battery calc — not just the main panel.
• Not updating after revisions. Add a dozen strobes in an addendum and the alarm current changes; reviewers cross-check counts.
• Using generic instead of datasheet currents. Currents vary by model and candela setting.
• Ignoring the charger limit. A panel that can only charge up to a certain Ah caps your selection regardless of the math.