How the battery calculation works
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 20% in the calculator to use 0.20 in the equation. Choose the smallest listed battery capacity at least as large as the unrounded final result.
This current-times-duration method is described in Potter's Advanced Fire Training Manual, Battery Standby Calculations, printed page 7-53. Its example uses a 1.2 multiplier. Here the margin is editable so the designer can choose the value appropriate to the project.
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 a chosen 20% margin: 7.325 × 1.20 = 8.79 Ah.
- Next listed typical size: 12 Ah, subject to equipment compatibility checks.
Durations and margin are project inputs
Set these from your project's specs and the AHJ. The editable 24 h and 5 min defaults are common design inputs, not quoted requirements. NFPA 72 sets secondary-power duration requirements; check your own licensed copy of the edition your AHJ enforces for the applicable system and conditions. No code-book text or tables are reproduced here. NFPA provides code information and free read-only access.
The 20% default is a designer's choice. An additive margin is different from an efficiency derating: adding 20% multiplies by 1.20, while allowing for 80% usable capacity divides by 0.80, equivalent to a 25% additive margin. Follow the factor and method specified for your equipment; this tool does not automatically apply a separate aging or temperature factor.
Typical sizes and design limits
The comparison list is 7, 12, 18, 26, 33, 40, 55 and 100 Ah, typical sealed lead-acid sizes. 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.
Capacity is the bank's Ah rating at the required system voltage. For two equal 12 V batteries connected in series for 24 V, voltage adds while Ah stays the same: two 12 V, 12 Ah batteries form a 24 V, 12 Ah bank, not 24 Ah. Use the manufacturer's approved battery arrangement.
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.
Published method source
Potter Electric Signal Company. Advanced Fire Training Manual, 8700055, Rev H (May 2017), Battery Standby Calculations, printed page 7-53 (PDF page 57). Checked 17 October 2026. Used for the amp-hour method and series-bank explanation, not as a source of current code requirements or a universal margin.