Intruder Alarm Battery Calculator: engineer's guide
Intruder alarm standby is longer than fire alarm, up to 60 hours per EN 50131 Grade 3, so batteries are typically larger.
Open the Intruder Alarm Battery calculator →
What this calculator does
The calculator applies EN 50131 grade-based standby and alarm durations and computes minimum battery Ah with aging derate.
Formulas used
- Battery Ah:
Ah = (Isb × tsb + Ial × tal) / Kage
Variables
- tsb = 12 h Grade 1, 12 h Grade 2, 60 h Grade 3, 60 h Grade 4
- tal = 15 min for external siren; typically 15/60 h
Worked example
Scenario: EN 50131 Grade 3 panel, Isb 0.15 A, Ial 1.5 A external siren.
- Ah = (0.15 × 60 + 1.5 × 0.25) / 0.80
- Ah = (9 + 0.375) / 0.80 ≈ 11.7
- Select 12 Ah battery
Common mistakes to avoid
- 12 h standby for a Grade 3 panel (needs 60 h)
- Missing external siren driven from separate battery
- Ignoring keypad backlight standby load
Frequently asked questions
Grades explained?
Grade 1: low risk. Grade 2: retail. Grade 3: banks, jewellers. Grade 4: high-security military/government.
Central station comms?
Grade 3+ needs redundant communications (dual-path IP + GSM).
Standards and references
EN 50131-1, NFPA 731, IEC 62642
Related tools on this site
Open the calculator that pairs with this guide: Intruder Alarm Battery Calculator.
Verification note
This guide is planning support only. Final design must be verified against the applicable project specification, authority requirements, manufacturer data, and the standards cited above. Consult a licensed engineer for construction-issue calculations.