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Fire alarm battery calculator

Required battery capacity = (total standby current × standby hours + total alarm current × alarm minutes ÷ 60) × (1 + safety margin). The usual figures are 24 hours of standby, 5 minutes of alarm and a 20% margin. They come from the code edition your AHJ has adopted, so each one is editable here.

Code basis: the standby hours, alarm minutes and margin are inputs, not built-in code values. The defaults are the widely required figures (24 hours, 5 minutes, 20%). Not affiliated with or endorsed by NFPA. Last checked 14 October 2026.

Enter each device type once, with its quantity and its currents from the datasheet, in milliamps.

Durations and margin

Separate sizes with commas, for example 7, 12, 18.

Result

Add a device with its currents to see the required amp-hours.

AlarmAmps runs this offline on iPhone and saves the job.

Worked example

4.29 Ah

Computed by the same code as the calculator above.

A small conventional system: ten smoke detectors, four pull stations and the control panel, at 24 hours of standby, 5 minutes of alarm and a 20% margin. The currents are example datasheet figures, not values for any real model.

DeviceQtyStandby eachAlarm each
Smoke detector100.50 mA30.0 mA
Pull station410.0 mA150.0 mA
Control panel1100.0 mA250.0 mA
  1. Total standby current: Sum of (standby current × quantity) for every device = 0.145 A
  2. Standby amp-hours: 0.145 A × 24 h = 3.48 Ah
  3. Total alarm current: Sum of (alarm current × quantity) for every device = 1.150 A
  4. Alarm amp-hours: 1.150 A × 5 min ÷ 60 = 0.10 Ah
  5. Subtotal: 3.48 Ah + 0.10 Ah = 3.58 Ah
  6. Derate 20%, applied to the subtotal: 3.58 Ah × 1.20 = 4.29 Ah

With 7, 12, 18 and 35 Ah batteries on the truck, the smallest that meets 4.29 Ah is the 7 Ah battery.

Limits, and when to ask your AHJ

  • Not a substitute for engineered design. The designer of record is responsible for the final calculation. Verify against the code edition adopted by your AHJ.
  • Currents come from your own datasheets. There are no built-in device tables, because currents vary by model and revision.
  • It sizes the battery only. It does not check NAC voltage drop, whether the panel’s charger can recharge the battery you pick, or the largest battery the panel accepts. Check the panel’s installation manual.
  • Ask your AHJ if you are unsure which standby or alarm duration applies to the occupancy or system type, or if the system has functions with their own duration requirements, such as emergency voice communication.

Save jobs and get the full tool in AlarmAmps

NAC voltage drop, saved projects and a PDF submittal sheet for the AHJ, offline on your iPhone. Free download with a one-time unlock.

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Questions

How long must a fire alarm battery last?

The common requirement is 24 hours of standby followed by 5 minutes of alarm, and those are the defaults here. Some systems need longer, such as 15 minutes of alarm where a longer evacuation signal is required. The AHJ’s adopted code decides; change the inputs to match.

Where does the 20% margin go?

On the sum of standby and alarm amp-hours, not on one term. Applying it to standby alone undersizes the battery. In the worked example the subtotal is 3.58 Ah, so the requirement is 3.58 Ah × 1.20 = 4.29 Ah.

Where do I find the current values?

On each device’s datasheet or installation sheet, usually in milliamps: one figure for standby (supervisory) and one for alarm. Enter the figures for the exact model and revision you are installing.

Which battery should I pick?

The smallest battery whose rating is at or above the required amp-hours. List the sizes you stock and the calculator picks the smallest that fits, or tells you none of them does.

What does the AlarmAmps app add?

The app runs this calculation free, plus voltage drop for one circuit. Submittal Pro, a one-time $4.99 purchase, adds the PDF submittal sheet, multiple projects, multiple circuits and booster panels, and a saved device library.

More free tools

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