Fire Alarm Battery Calculator
Size a fire alarm panel's secondary (standby battery) supply the way NFPA 72 requires: total standby current for the quiescent period, total alarm current for the alarm period, the aging correction factor, and round up to a standard SLA battery. Seed the sheet from a verified device-current library, check it against the batteries and charger you actually have, and print an AHJ-format submittal sheet — free, no login.
Connected load
Submittal sheet header (project, panel, preparer)
Battery sizing
Standard SLA ladder
NFPA 72 (2022) 10.6.7.2.1 — 24 h standby + 5 min alarm (15 min voice/MNS); 10.6.7.2.14 — × 1.25 aging factor (was × 1.2, §10.6.7.2.1.1, 2019 and earlier); 10.6.10.3 — recharge within 48 h. Round up to a standard size, never down.
Fire Alarm Battery Calculation
Secondary power supply sizing per NFPA 72 — prepared with the NORDIX Fire Alarm Battery Calculator (nordixhq.com/tools/fire-alarm-battery-calculator)
| Project: — | Panel model / ID: — |
| Prepared by: — | Date: — |
| Standby / alarm period: 24 h / 5 min (NFPA 72 10.6.7.2.1) | Aging factor: × 1.25 (NFPA 72 2022 §10.6.7.2.14) |
| Device | Qty | Standby mA ea | Standby mA total | Alarm mA ea | Alarm mA total |
|---|---|---|---|---|---|
| FACP — Fire-Lite MS-9200UDLS (base board) | 1 | 145 | 145 | 275 | 275 |
| Addressable photoelectric smoke (SD355) | 25 | 0.3 | 7.5 | 0.3 | 7.5 |
| Addressable pull station (BG-12LX) | 4 | 0.3 | 1.2 | 0.3 | 1.2 |
| Horn/strobe 75 cd | 10 | 0 | 0 | 176 | 1,760 |
| Totals | 153.7 mA = 0.154 A | 2,043.7 mA = 2.044 A | |||
| Standby: 0.154 A × 24 h | 3.689 Ah |
| Alarm: 2.044 A × 0.0833 h (5 min) | 0.170 Ah |
| Subtotal | 3.859 Ah |
| Required capacity (× 1.25 aging factor) | 4.82 Ah |
| Batteries selected | 7 Ah (2 × 12 V in series) |
Method: NFPA 72 (2022) 10.6.7.2.1 secondary-supply durations; 10.6.7.2.14 × 1.25 aging correction (2019 and earlier editions: × 1.2 per 10.6.7.2.1.1); batteries recharge within 48 h per 10.6.10.3. Library currents are manufacturer UL-max figures (Fire-Lite / Potter worksheets, System Sensor A05-0395-007) — verify against the installed devices’ datasheets. Battery capacities per pair of 12 V SLA blocks in series. This calculation is a design aid; the adopted code edition, the panel’s listed installation instructions, and the AHJ govern.
How this calculator works
NFPA 72 10.6.7.2.1 requires the secondary supply to run the system in a non-alarm (quiescent) state for 24 hours and still operate every notification appliance for 5 minutes at the end of that period — 15 minutes at maximum connected load for in-building voice and mass notification systems. The amp-hour math is:
Ah = (standby A × standby h + alarm A × alarm min ÷ 60) × aging factor
Standby current is everything the battery feeds while it waits: the panel board, every addressable device on the SLC, annunciators, communicators, and any auxiliary load that stays powered on AC failure. Alarm current adds the notification appliance circuits at full load. The result is multiplied by the aging correction factor — ×1.25 under NFPA 72 2022 (§10.6.7.2.14), ×1.2 under 2019 and earlier — and rounded up to the next standard battery size. A 24 V panel uses two 12 V SLA batteries in series, so the answer is the Ah rating of the pair, not doubled.
Worked example
A panel totals 0.25 A standby and 2.0 A alarm on a standard (non-voice) system:
- Standby: 0.25 A × 24 h = 6.00 Ah.
- Alarm: 2.0 A × 5 min (0.0833 h) = 0.167 Ah.
- Subtotal 6.167 Ah × 1.25 = 7.71 Ah required.
- 7 Ah batteries are too small — round up to two 12 V, 12 Ah in series.
This matches the worked example on our standby battery calculation chart — the calculator and the chart run on the same pinned data module.
Secondary power durations
| Parameter | Requirement | NFPA 72 (2022) |
|---|---|---|
| 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 60-hour standby some manufacturer worksheets still print for central/auxiliary/remote station systems is legacy text from pre-2002 editions — current NFPA 72 Chapter 10 requires 24 hours regardless of monitoring type. A few jurisdictions do amend longer periods back in, which is why the calculator keeps a labeled 60-hour preset and full custom durations.
The aging factor: 1.2 vs 1.25
Since the 2010 edition, NFPA 72 has mandated a margin on top of the calculated amp-hours — 20% (×1.2) through the 2019 edition (§10.6.7.2.1.1), raised to a ×1.25 correction factor in the 2022 edition (§10.6.7.2.14). The reasoning: a lead-acid battery reaches end of service life at 80% of rated capacity, and 1.25 × 0.80 = 1.00 — an end-of-life battery still covers the full calculated demand, where the old ×1.2 left it 4% short. Some manufacturer sheets express the same idea differently: Potter divides by 80% battery efficiency and Simplex applies a UL 864 “20% derating” as ÷0.8 — both arithmetically identical to ×1.25. Several fielded manufacturer Excel calculators still embed ×1.2; submitting one of those under a 2022-code jurisdiction is a documented plan-review rejection. Pick the factor for your adopted edition — and note some panel manuals required 1.25 before the code did, in which case the listing instructions govern.
Reference: the device current library
Preset currents are manufacturer UL-max figures at 24 VDC, read from the Fire-Lite and Potter official battery-calculation worksheets, System Sensor SpectrAlert Advance datasheet (DC column, temporal high for horn tones), and the Silent Knight installation manual. Currents vary by model revision and options, so every value stays editable — verify against the installed device’s datasheet. Addressable SLC devices draw the same in alarm as in standby because the panel caps how many device LEDs can latch on at once and carries that cap in its own alarm figure.
| Device | Standby mA | Alarm mA | Source |
|---|---|---|---|
| Control panels | |||
| FACP — Fire-Lite MS-9200UDLS (base board) | 145 | 275 | Fire-Lite battery-calc worksheet / DF-60601 |
| FACP — Potter PFC-4064 (base board) | 70 | 235 | Potter BatteryCalc_PFC-4064.xlsx |
| Initiating & SLC devices | |||
| Addressable photoelectric smoke (SD355) | 0.3 | 0.3 | Fire-Lite DF-52384 |
| Addressable heat detector (H355) | 0.3 | 0.3 | Fire-Lite DF-52385 |
| Addressable pull station (BG-12LX) | 0.3 | 0.3 | Fire-Lite battery-calc worksheet |
| Addressable duct smoke (PAD100-DUCT) | 0.3 | 0.3 | Potter BatteryCalc .xlsx |
| Monitor module (MMF-300) | 0.4 | 0.4 | Fire-Lite battery-calc worksheet |
| Control module (CMF-300) — NAC load it switches is entered separately | 0.39 | 0.39 | Fire-Lite battery-calc worksheet |
| Relay module (CRF-300) | 0.27 | 0.27 | Fire-Lite battery-calc worksheet |
| Fault isolator module (I300) | 0.4 | 0.4 | Fire-Lite battery-calc worksheet |
| Conventional 4-wire duct smoke w/ relay (DSD-P) | 13 | 60 | Potter BatteryCalc .xlsx |
| Notification appliances | |||
| Horn (HR, temporal high) | 0 | 69 | System Sensor A05-0395-007 |
| Horn/strobe 15 cd (P2R) | 0 | 79 | System Sensor A05-0395-007 |
| Horn/strobe 30 cd | 0 | 107 | System Sensor A05-0395-007 |
| Horn/strobe 75 cd | 0 | 176 | System Sensor A05-0395-007 |
| Horn/strobe 95 cd | 0 | 194 | System Sensor A05-0395-007 |
| Horn/strobe 110 cd | 0 | 212 | System Sensor A05-0395-007 |
| Horn/strobe 115 cd | 0 | 218 | System Sensor A05-0395-007 |
| Horn/strobe 135 cd | 0 | 245 | System Sensor A05-0395-007 |
| Horn/strobe 150 cd | 0 | 259 | System Sensor A05-0395-007 |
| Horn/strobe 177 cd | 0 | 290 | System Sensor A05-0395-007 |
| Horn/strobe 185 cd | 0 | 297 | System Sensor A05-0395-007 |
| Strobe only 15 cd (SR) | 0 | 66 | System Sensor A05-0395-007 |
| Strobe only 30 cd | 0 | 94 | System Sensor A05-0395-007 |
| Strobe only 75 cd | 0 | 158 | System Sensor A05-0395-007 |
| Strobe only 95 cd | 0 | 181 | System Sensor DN-60517 / A05-0395-007 |
| Strobe only 110 cd | 0 | 202 | System Sensor A05-0395-007 |
| Strobe only 115 cd | 0 | 210 | System Sensor DN-60517 / A05-0395-007 |
| Strobe only 135 cd | 0 | 228 | System Sensor DN-60517 / A05-0395-007 |
| Strobe only 150 cd | 0 | 246 | System Sensor DN-60517 / A05-0395-007 |
| Strobe only 177 cd | 0 | 281 | System Sensor DN-60517 / A05-0395-007 |
| Strobe only 185 cd | 0 | 286 | System Sensor DN-60517 / A05-0395-007 |
| Bell, 24 V (MBA series) | 0 | 60 | Potter BatteryCalc .xlsx |
| Annunciators & communicators | |||
| LCD annunciator (ANN-80) | 15 | 40 | Fire-Lite battery-calc worksheet |
| IP communicator (IPDACT-2) | 93 | 136 | Fire-Lite / Silent Knight worksheets |
| Cellular communicator (CELL-MOD) | 55 | 100 | Silent Knight LS10146-001SK-E |
| Auxiliary loads | |||
| Door holder — aux power drops on AC loss — Counts toward neither standby nor alarm Ah — power is removed on AC failure | 0 | 0 | Potter calc / SK manual convention |
Why battery calculations get rejected
- A stale ×1.2 factor under a jurisdiction that has adopted NFPA 72 2022 — old manufacturer spreadsheet revisions are the usual culprit.
- Omitted standby loads — door holders on power that stays up on AC loss, communicators, annunciators, and aux devices all burn standby amp-hours. (Door holders on aux power that drops on AC failure count toward neither period — that’s the point of the drop.)
- Counts that don’t match the drawings — reviewers cross-check the sheet’s quantities against the device layout.
- NAC voltage drop figured from 24 V instead of the 20.4 V battery-standby voltage in the companion voltage-drop calcs most AHJs require in the same package — see our NAC voltage drop chart.
- A battery the panel can’t charge — NFPA 72 10.6.10.3 requires recharge within 48 hours, which is why every panel lists a maximum battery size. Enter it in the charger check.
Frequently asked questions
How do you calculate fire alarm battery size?
Total the standby (quiescent) current of everything the battery feeds — panel, SLC devices, annunciators, communicators, aux loads — and multiply by the standby period (24 hours for most systems). Total the alarm current with every notification appliance running and multiply by the alarm period in hours (5 minutes = 0.0833 h; 15 minutes = 0.25 h for voice systems). Add the two, multiply by the aging correction factor (×1.25 under NFPA 72 2022, ×1.2 under 2019 and earlier), and round up to the next standard battery size — never down. This calculator runs exactly that chain and shows every step.
Is the fire alarm battery derating factor 1.2 or 1.25?
It depends on the adopted code edition. NFPA 72 mandated a minimum 20% margin (×1.2) from the 2010 edition through 2019 (§10.6.7.2.1.1), then raised it to a ×1.25 correction factor in the 2022 edition (§10.6.7.2.14). The change closes a gap: end of service life for a lead-acid battery is 80% of rated capacity, and 1.25 × 0.80 = 1.00, so an end-of-life battery still meets the full demand — ×1.2 left it 4% short. Potter's divide-by-80%-efficiency and Simplex's UL 864 20%-derate conventions are arithmetically the same ×1.25. Some panel manuals required 1.25 before the code did; the listing instructions govern in that case.
How long must a fire alarm battery last?
NFPA 72 10.6.7.2.1 requires 24 hours of standby operation followed by 5 minutes operating all notification appliances — 15 minutes at maximum connected load for in-building voice/mass-notification systems. With an automatic-starting standby generator as part of the secondary supply, the battery portion drops to 4 hours. The 60-hour figure some manufacturer worksheets still print for central/auxiliary/remote-station systems is legacy pre-2002 code text, though a few jurisdictions amend longer periods back in — this calculator carries presets for all of these plus custom durations.
What standby and alarm current do I use for each device?
Use the manufacturer's UL-max figures from the datasheet, not 'typical' values. Addressable SLC devices (smokes, heats, pulls, modules) draw a fraction of a milliamp — around 0.3 mA — and the same in alarm, because the panel caps how many device LEDs latch on and carries that load in its own alarm figure. Notification appliances draw nothing in standby and their alarm draw varies with candela: a SpectrAlert Advance horn/strobe runs from 79 mA at 15 cd to nearly 300 mA at 185 cd, and low-frequency 520 Hz sounders draw 2–3× more. This calculator seeds those numbers from a verified library and leaves every value editable.
Do door holders count in a fire alarm battery calculation?
Usually not — and that is by design. Door holders are typically wired to auxiliary power that drops on AC failure, so the doors close and the holders draw nothing during the entire battery period: they count toward neither standby nor alarm amp-hours. But if they sit on power that stays up on battery, their hold current burns standby amp-hours for the full 24 hours, and omitting that load is one of the documented reasons AHJs reject battery calculations. Check which way the panel's aux power is configured before you delete the row.
What size batteries do fire alarm panels use?
Sealed lead-acid (SLA/VRLA) batteries in standard sizes — commonly 4, 7, 12, 18, 26, 33, 40, 55, and 100 Ah. A 24 V panel uses two 12 V batteries in series, so the rating you buy is the rating of the pair, not doubled. Round the required amp-hours up to the next standard size. Most compact panels house and charge up to two 18 Ah batteries internally; beyond that you need an external battery cabinet, and beyond the charger's listed maximum you need an external charger or power supply.
Why would the AHJ reject my battery calculations?
The recurring rejection triggers: a stale ×1.2 factor from an old spreadsheet under a jurisdiction that adopted NFPA 72 2022; omitted standby loads like communicators, annunciators, or battery-backed door holders; device quantities that don't match the drawings; NAC voltage-drop calcs figured from 24 V instead of the 20.4 V battery-standby voltage; and a selected battery larger than the panel's listed charger can recharge in the required 48 hours (NFPA 72 10.6.10.3). This calculator cites the factor and durations on its printable sheet and checks your battery against the charger limit to head those off.
Can this calculator produce a submittal sheet for the fire marshal?
Yes. The print output follows the format plan reviewers see from manufacturer worksheets: project and panel header, a quantity × per-unit standby/alarm current table with totals, the explicit math chain from amp-hours through the aging factor, the code sections relied on, and the selected battery with a pass/fail adequacy line. Fill in the header fields, click Print, and submit it alongside your NAC voltage-drop calculations — most AHJs want one calculation set per power supply, including NAC boosters.
Method: NFPA 72 (2022) 10.6.7.2.1 secondary-supply durations, 10.6.7.2.14 aging correction factor (2019 and earlier: 10.6.7.2.1.1), 10.6.10.3 recharge requirement. Device currents are manufacturer UL-max figures — verify against installed-equipment datasheets and the panel’s listed installation instructions. Rechargeable secondary-supply batteries must be listed or component-recognized effective Jan 1, 2024 (2022 edition). This is a design aid, not a substitute for the adopted code edition, the panel manual, or the AHJ.
Pair with the standby battery calculation chart, the SLA battery size chart, and the NAC current draw chart for the tables behind the tool.