The Platform of Fire Engineering built this FACP battery calculator for fire alarm designers, commissioning engineers, maintenance teams and consultants across the Middle East and beyond. It turns the fire alarm battery calculation from a spreadsheet exercise into a four-step check, and it is one of the free field tools we develop alongside our professional training in fire alarm systems, NFPA 72 and CFPS exam preparation.
Worked example: fire alarm battery calculation
Here is the same formula the calculator uses, worked by hand. The currents are example values only. Always use the currents from your own panel and device datasheets.
Example system: total standby (quiescent) current of 0.50 A for the panel and all devices, and total alarm current of 2.50 A with all sounders and strobes active.
| Step | NFPA 72 | UAE Fire & Life Safety Code |
|---|
| 1. Standby: 0.50 A × 24 h | 12.00 Ah | 12.00 Ah |
| 2. Alarm: 2.50 A × alarm time | 5 min (0.083 h) = 0.21 Ah | 30 min (0.5 h) = 1.25 Ah |
| 3. Total before margin | 12.21 Ah | 13.25 Ah |
| 4. Add 20% safety margin (× 1.2) | 14.65 Ah | 15.90 Ah |
| 5. Select the next standard battery size | For example 18 Ah | For example 18 Ah |
Under BS 5839-1 the same system gives 13.25 Ah × 1.25 = 16.56 Ah, so an 18 Ah battery would also be the next standard size.
What the example shows: standby uses about 98% of the capacity in the NFPA 72 case. So the biggest saving comes from lowering the quiescent current of the panel, modules and detectors, not from the alarm load. When a design is close to the next battery size, check each device's standby current first.
NFPA 72 standby and alarm time, explained
When mains power fails, the batteries (the secondary power supply) must keep the whole system working. NFPA 72 sets two periods: 24 hours of standby at normal quiescent load, and then 5 minutes of alarm (15 minutes for an in-building voice evacuation system). The idea is simple: after a full day without mains power, the system must still detect a fire and warn everyone in the building.
Two practical points for site engineers:
- Use the alarm current with every notification appliance on, not only one zone.
- Check the local code. The UAE Fire & Life Safety Code and BS 5839-1 use a 30-minute alarm time, which this calculator applies when you pick those codes.
Learn how batteries, power supplies and the rest of the system fit together in the free fire alarm course. Need a panel's default access level code for commissioning? Use the Fire Alarm Panel Password Finder. For commissioning tips, read smoke detector dust covers and commissioning. Preparing for certification? See the CFPS exam preparation course.
Battery calculator FAQ
How do you calculate FACP battery capacity?Add up the standby current of the panel and every device, then multiply it by the standby time (24 hours). Add the alarm current multiplied by the alarm time: 5 minutes under NFPA 72, or 30 minutes under the UAE Fire & Life Safety Code and BS 5839-1. Apply the safety factor (20% for NFPA 72 and the UAE Code, or the 1.25 ageing factor in BS 5839-1) and select the next standard battery size in ampere-hours (Ah).
What standby and alarm times does NFPA 72 require for fire alarm batteries?NFPA 72 requires 24 hours of standby at quiescent load followed by 5 minutes of alarm, or 15 minutes for in-building voice evacuation (EVACS), with a minimum 20% safety margin on the calculated battery capacity.
Which fire alarm control panels does the calculator cover?Simplex 4100ES and 4010ES, Notifier NFS2-3030, NFS2-640 and NFS-320, and Gent Vigilon Plus and Vigilon Compact. Any other load, such as door holders, beam detectors or aspirating units, can be added as a custom row.
Can I use the result in a design submittal?Use it for training, pre-design checks and site verification. Device currents are typical datasheet values, so confirm them against your project datasheets. The final FACP battery calculation must be prepared by the responsible design engineer and approved by the authority having jurisdiction.
