Home / Tools / Battery Standby Calculator
Battery Standby Calculator
Quick answer
How do I calculate the standby battery capacity for a fire alarm system?
Add the amp-hours needed to cover the required standby period (standby current draw times standby hours) to the amp-hours needed to cover the required alarm period (alarm current draw times alarm minutes divided by 60), then multiply the total by a safety (derating) factor, commonly 1.2, to account for battery aging and temperature. Round the result up to the nearest battery capacity your panel manufacturer approves.
This tool runs entirely in your browser. Your exact readings never leave your device; only a banded, anonymous summary is sent, never a raw current draw or capacity. See the privacy policy.
Enter your circuit values
Planning tool only, not a substitute for the panel manufacturer's own battery-calculation worksheet or a stamped design. Required standby and alarm durations are left as fields the technician enters (not hardcoded), because the minimum standby and alarm periods NFPA 72 requires vary by system type (fire alarm vs. supervising station, voice vs. non-voice) and by the code edition your jurisdiction has adopted. This tool performs the arithmetic on whatever duration you confirm against your AHJ and the current adopted NFPA 72 edition; it does not assert a duration on its own. Always confirm your final battery selection against your panel manufacturer's approved battery list.
How this calculator works
The formula is Required capacity (Ah) = [(standby current in A x standby hours) + (alarm current in A x alarm minutes / 60)] x safety factor. Every term is a value you
supply from your own circuit design; nothing here is a fixed NFPA 72
figure. NFPA 72 sets the secondary (standby) power supply requirement in its power-supply provisions; this tool cites that requirement exists by name only and never reproduces code text or a specific section number it has not directly verified against the current adopted edition.
Safety factor
A 20 percent (1.2x) derating margin is standard secondary-power sizing practice across fire-alarm battery-calculation worksheets and standby-power application notes, covering battery aging, temperature effects, and connected-load tolerance. It is a sizing-practice convention, not an NFPA 72 quotation.
Standard battery capacities
Once the required amp-hours are computed, the tool rounds up to the nearest of these common capacities: 4, 7, 8, 10, 12, 18, 26, 33, 40, 55, 100 Ah. Common commercially available sealed lead-acid capacities used in fire alarm control panel secondary power supplies. Always confirm against your specific panel manufacturer's approved battery list before purchasing.
Sourcing: Standard sealed-lead-acid battery-sizing arithmetic (public engineering practice, cross-verified across multiple independent battery-manufacturer application notes), not sourced from NFPA 72 text. Verified 2026-07-30. See
src/data/PROVENANCE.md for the full sourcing note.
Also see the Voltage Drop Calculator for notification appliance circuit wiring, or start with the Level Requirements Navigator.