Battery standby calculator

Check whether a fire alarm panel's standby batteries are big enough, using the minimum capacity formula from BS 5839-1:2025 Annex E.

Based on BS 5839-1:2025 Annex E · Last reviewed October 2026

Your system

Results update as you type.

Installed battery capacity

The rated capacity of the batteries fitted to the panel.

System load

A mA

Standby current (I₁)

Total quiescent current. Include any always-on network module — an IP communicator or dual-path signalling unit typically draws 0.02–0.08 A (check the manufacturer's data).

Alarm current (I₂)

Total current drawn with the system in full alarm.

Standby period (T₁)

24 hours Category L and M systems 72 hours Category P maximum, e.g. premises empty over a weekend

Result

Enter your system Fill in the capacity and currents to see the result.

Minimum capacity required (C_min)

—Ah

Installed capacity: —
Margin: —

Working

Standby T₁ × I₁	—	—
Alarm D × I₂ × T₂	—	—
Subtotal		—
C_min	× 1.25 ageing	—

How it's calculated

BS 5839-1:2025 Annex E gives the minimum battery capacity needed to run the system on standby for the required period, then in full alarm for 30 minutes, with allowances for battery ageing and for the lower efficiency of a battery at high discharge rates:

C_min = 1.25 × ((T₁ × I₁) + D × (I₂ × T₂))

1.25 — ageing factor (allows for capacity lost as the battery ages)
T₁ — standby period, in hours (see below)
I₁ — standby (quiescent) current, in amps
D — 1.75 derating factor (battery inefficiency under high alarm load)
I₂ — alarm current, in amps
T₂ — alarm duration: 0.5 hours (30 minutes)

The battery passes when its rated capacity is at least C_min. If it fails, the calculator suggests the next common battery size above C_min.

Standby period

Category L and M: 24 hours, or 6 hours where an automatically started standby generator is fitted.
Category P: 24 hours more than the longest period the premises are left unoccupied, up to 72 hours — or 24 hours where power supply faults will be noticed within 6 hours, or are signalled to an ARC that alerts a keyholder.
Either way, the battery must then run the system in alarm for 30 minutes.

The calculator offers the two most common periods, 24 and 72 hours. For any other period, choosing the next longer option gives a safe (larger) answer.

Network modules and remote access

Always-on network equipment — IP communicators, dual-path signalling units and remote access modules — draws current continuously, so include it in the standby figure. Typical draw is 0.02–0.08 A; check the manufacturer's specification for the unit fitted.

Typical figures

Standby current: 0.05–0.2 A
Alarm current: 0.3–2.0 A
Common battery sizes: 7 Ah (small panels), 12 Ah (medium), 17 Ah (large), 24 Ah (very large systems)