Fire safety
Fire Alarm Battery Calculator
The BS 5839-1 battery formula in one line: capacity = 1.25 x (standby hours x quiescent current + half an hour of alarm current).
Calculator
Fill in the fields above to see results.
How it works
Enter the panel's quiescent and alarm currents from its datasheet and pick the standby period (24 h standard, 72 h for unmonitored sites). The 1.25 factor covers battery ageing.
Frequently asked questions
What standby period does BS 5839-1 require?
24 hours of standby plus 30 minutes of alarm for most monitored systems - 72 hours where the premises can stand empty over a weekend without monitoring.
Why multiply by 1.25?
Sealed lead-acid capacity falls with age and temperature. The 25% margin means the battery still meets the duty at end of life, not just when new.