When you need a battery calculation
Run one for every supply with its own batteries: the control panel, and each NAC extender or remote power supply. Plan reviewers expect it in the submittal, and it has to be redone whenever devices are added, swapped or reprogrammed, because a few more strobes change the alarm load. For the long version, with aging and temperature handled as separate factors, read the fire alarm battery calculation method and the secondary power supply guide.
Print a battery calculation worksheet
Press Print worksheet under the result to get one page for that supply: your panel and device currents, the durations and margin, every step of the amp-hour math and the battery size. It leaves blank lines for the project, the supply, who prepared it and where the durations came from, so you can fill it in by hand. Save it as a PDF from the print dialog, attach it to a submittal or keep it with your NICET study notes. Copy a link to this result sends the same inputs to a colleague.
How the battery calculation works
Start with one battery-backed power supply and its load list. Record standby and full alarm currents from each connected device's installation manual; include the panel, modules, annunciators and auxiliary loads supplied by that bank. Calculate other supplies separately.
Multiply each device's current by its quantity. Add those loads to the panel's current separately for standby and alarm. Use amperes (A), not milliamperes: divide mA by 1,000 to get A. Alarm currents must include the complete alarm-state load, not just the increase above standby.
- Total standby current: Iₛ = panel standby A + Σ(quantity × per-device standby A).
- Total alarm current: Iₐ = panel alarm A + Σ(quantity × per-device alarm A).
- Standby Ah = Iₛ × standby hours.
- Alarm Ah = Iₐ × (alarm minutes / 60).
- Required Ah before margin = standby Ah + alarm Ah.
- Required Ah after margin = required Ah before margin × (1 + margin % / 100).
Ah = [I_standby × hours + I_alarm × (minutes / 60)] × (1 + margin)
In the compact formula, margin is a decimal: enter 25% in the calculator to use 0.25 in the equation. Choose the smallest listed battery capacity at least as large as the unrounded final result.
The load-times-duration method appears in Fire-Lite's MS-2410BC Canadian Instruction Manual, Appendix A. That historical manual uses a 1.2 multiplier. Use it to learn the arithmetic, then select the durations and correction factors from the applicable code section or source for your system.
Worked example
These fictional loads match the starting inputs; they are not equipment specifications or an exam question. A panel draws 0.20 A in standby and 0.30 A in alarm. Twenty detectors each draw 0.005 A in standby and 0.010 A in alarm. Ten notification appliances each draw 0 A in standby and 0.10 A in alarm.
- Standby current = 0.20 + (20 × 0.005) + (10 × 0) = 0.30 A.
- Alarm current = 0.30 + (20 × 0.010) + (10 × 0.10) = 1.50 A.
- At 24 h standby: 0.30 × 24 = 7.20 Ah.
- At 5 min alarm: 1.50 × (5 / 60) = 0.125 Ah.
- Before margin: 7.20 + 0.125 = 7.325 Ah.
- With the starting 25% allowance: 7.325 × 1.25 = 9.15625 Ah, or 9.16 Ah rounded for display.
- Next tool comparison size: 12 Ah, subject to equipment compatibility checks.
Common battery calculation mistakes
- Leaving alarm minutes as minutes. 1.50 A for 5 minutes is 1.50 × 5 / 60 = 0.125 Ah. Skip the divide by 60 and you get 7.5 Ah, sixty times too much.
- Typing mA into an amp field. A detector drawing 0.3 mA is 0.0003 A. Enter 0.3 and twenty of them look like a 6 A load.
- Using measured or typical draw. Take the battery-powered standby and alarm currents from each device's data sheet or manual. A meter reading on a quiet day misses the worst case.
- Forgetting the extras. Annunciators, relay and control modules, the communicator and anything on the auxiliary output all draw from the same bank.
- Stacking factors. A worksheet that already multiplies by 1.2 does not need another 20% on top. Apply each factor once and write down what it covers.
- Rounding down. Pick the next listed size at or above the unrounded result, inside the charger range the panel manual allows.
Choose durations, then account for capacity loss
The starting 24 h, 5 min and 25% entries are worked-example inputs. The 25% allowance is a study/design setting, not a verified statement of the adopted code minimum. The historical Canadian manufacturer manual's 20% allowance is manufacturer practice, not evidence of the 2022 code minimum. Do not carry them into a project calculation without checking the system type and adopted edition. For Level II study with NFPA 72-2022, open §§10.6.7.2.1–10.6.7.2.4 for the duration lookup and §10.6.7.2.14 with A.10.6.7.2.14 for the correction-factor lookup. Use your own copy through NFPA's reader.
An additive allowance and a usable-capacity fraction answer different questions. A 20% allowance gives 7.325 × 1.20 = 8.79 Ah. If a separate calculation assumes only 80% of rated capacity remains usable, solve 0.80 × rated Ah = 7.325 Ah: rated Ah = 7.325 ÷ 0.80 = 9.15625 Ah. That is a 25% additive allowance. Enter 25 in this tool to represent that factor; do not enter 20 for a 20% capacity loss.
Apply each specified factor once. This calculator applies only the entered margin. It does not determine a temperature, aging or discharge-rate correction. Obtain those inputs from the applicable code section or source and the battery/equipment documentation; do not assume one percentage covers every condition.
Using the same math for an intruder alarm panel
Battery sizing for an intruder alarm panel is the same amp-hour arithmetic: add the standby current of the panel and its devices, multiply by the standby hours, add the alarm current for the alarm period, then apply your margin. Enter the panel and device currents from the manufacturer's data and set the durations to the standby and alarm periods your standard or specification requires. Those periods differ from fire alarm practice, so do not reuse the fire alarm defaults.
Capacity options and design limits
The tool compares the result with 7, 12, 18, 26, 33, 40, 55 and 100 Ah. It is a convenience list, not a manufacturer's approved battery schedule. Other sizes exist. A result over 100 Ah returns no listed size; the calculator never rounds it down to 100 Ah.
Use the battery arrangement, capacity rating and system voltage specified by the equipment manufacturer. An Ah result alone does not specify a battery bank.
Check against your project's specifications and the AHJ / your procedure. This simplified estimate assumes constant current in each operating period. Confirm equipment compatibility, charger capacity and recharge time, enclosure limits, temperature, battery aging, discharge-rate rating, end-of-discharge voltage and manufacturer instructions. Check voltage drop separately. Account for modules, annunciators and auxiliary loads on their actual supplying battery bank; calculate separate supplies separately.
The displayed size is a capacity comparison, not a code-compliance verdict or approval to install a battery.
Fire alarm battery calculation FAQ
How do you calculate fire alarm battery size?
Add the panel's standby current to every connected device's standby current and multiply by the standby hours. Do the same with the full alarm currents and the alarm time in hours. Add the two amp-hour figures, apply the safety factor your code edition and battery maker call for, and choose the next listed battery size up. With the starting inputs on this page, 7.325 Ah becomes 9.15625 Ah at a 25% margin, so 12 Ah is the comparison size.
How many hours of standby does a fire alarm battery need?
The code edition your AHJ enforces sets the standby and alarm durations, and they change with the system type: a voice evacuation system has a longer alarm period than a horn-and-strobe system. For NFPA 72 (2022), start at §10.6.7 on secondary power. The 24 hours and 5 minutes preloaded here are worked-example inputs, so confirm them for your system before you rely on a result.
What safety factor should I use for a fire alarm battery?
Use the factor your code edition and the battery manufacturer call for, and apply it once. The method stays the same: multiply the standby-plus-alarm amp-hours by one plus the margin. Some worksheets divide by the capacity a battery keeps as it ages instead, which gives a different answer for the same percentage. For NFPA 72 (2022), §10.6.7.2.14 is where to look.
Can I install a bigger battery than the calculation needs?
Usually, as long as it sits inside the charging range in the panel manual and fits the enclosure. A bigger battery buys run time only. It does not raise the panel's output current or fix voltage drop on a long notification circuit, which is a separate check.
Check the circuit side with the fire alarm voltage drop calculator.