Fire alarm standby power

Battery capacity calculator

Calculate a minimum capacity from your system loads, compare a battery and create a clear, branded calculation record.

Calculation tool

Calculation aid, not a compliance certificate. Confirm loads, durations and manufacturer limits with a competent designer. Method & limitations

The interactive calculator is not active on this page yet. No result has been calculated. For assistance, call 01604 459588.

System figures

Use confirmed battery-side loads, not mains current.

Standby current I₁

Total normal standby load.

Total alarm current I₂

Full alarm load, not the increase.

Standby duration T₁

Alarm duration T₂

Durations are starting values only. Enter those specified for your system.

Nominal battery capacity to compareOptional

Existing or proposed nameplate capacity. This is not a battery-health test.

Advanced settings & project details

Alarm de-rating factor D

Default 1.75. A change needs justification.

System voltage Optional

Record only; not a capacity multiplier.

Compatible capacity options (Ah) OptionalEnter only sizes confirmed for the panel, charger and enclosure. Example sizes are illustrative, not a compatibility list.

Details for your calculation record

Site or project Optional

Panel / supply Optional

Calculation basis / designer notes Optional

The calculator processes your entries in your browser. It does not save or send your figures.

Capacity result

Calculated minimum capacity

—Ah

Your calculation will appear here.

Ready when you are.

Enter the two current figures, check the durations and select Calculate capacity.

Ageing factor 1.25 · Default alarm factor 1.75

No current calculation is available. This page is not a calculation record.

Print a branded record or choose “Save as PDF” in your browser.

Capacity estimate only. Duration suitability, loads, chemistry, voltage, temperature, discharge behaviour, battery condition, enclosure fit, recharge capability and panel limits require separate competent review.

Technical review required before operational use.

Method, limitations & references

The calculation

This arithmetic aid applies the supplied valve-regulated lead-acid (VRLA) formula. Currents are converted to amperes and alarm minutes to hours. The ageing multiplier is fixed at 1.25; the default alarm de-rating factor is 1.75. A changed D requires a recorded technical justification.

The result is rounded upwards to 0.01 Ah for display. Nominal-capacity comparisons and selection from your entered list use the unrounded result. A capacity match does not confirm remaining battery capacity, compatibility or compliance.

Before using the result

Confirm the required durations and manufacturer limits with a competent designer. This tool does not determine standby duration from occupancy alone, approve an alternative factor or select a battery chemistry.

Reference edition: BS 5839-1:2025. The full licensed standard has not been independently reviewed for this implementation. Technical review is required before operational use. The model assumes a constant standby load followed by a constant alarm load; it is not a staged-load simulation or battery-health test.

Reference reading: BSI — BS 5839-1:2025 · IET — 2025 changes · Kentec Sigma ZXT manual. The Kentec manual references an older standard; it is corroborating guidance, not confirmation of the complete 2025 recommendations.