Fire alarm battery calculator
Work out the standby battery a fire alarm panel or power supply needs. Enter the quiescent and alarm currents, choose the standby period and alarm duration, and get the capacity in Ah plus the next available valve-regulated lead-acid battery size up. Built to follow BS 5839-1:2025 Annex E, including the de-rating factor D.
- The same calculator linked from Compliar's fire alarm servicing certificate
- Shows the working, so you can check it and note it on your certificate
- Runs in your browser — nothing you enter is stored
24 h — Category M/L, most buildings
For valve-regulated lead-acid batteries, to BS 5839-1:2025 Annex E. For other battery types, take the figures from the battery manufacturer.
How fire alarm battery capacity is worked out
If mains power fails, the standby batteries in a fire alarm panel or power supply unit have to do two jobs. First they keep the whole system running for the standby period, with every device drawing its normal quiescent current. Then they have to supply the full alarm load, with sounders and any other devices operating, for long enough to give a proper evacuation warning. The battery has to be big enough for both, with an allowance on top for ageing and temperature.
The method below follows BS 5839-1:2025 Annex E.1 for valve-regulated lead-acid batteries. For other battery types, consult the battery manufacturer (E.2).
- Cmin is the minimum battery capacity when new, at the 20 hour discharge rate and 20 °C, in Ah.
- I1 is the total standby (quiescent) load in amps: the panel, detectors, interfaces and all ancillary equipment.
- T1 is the total standby period in hours, set by clause 24.3.5: normally 24 hours for Category M/L, 6 hours with an automatically started standby generator serving the system, or up to 72 hours for Category P. See the standby FAQ below for the Category P rule.
- I2 is the total alarm load in amps, including all devices that operate in an alarm.
- T2 is the total alarm period in hours: at least 30 minutes (0.5 h) after the standby period.
- D is the de-rating factor. First calculate Cmin with D = 1. If Cmin / 20 is at least I2, keep D = 1. Otherwise, use the battery manufacturer's data or D = 1.75. This calculator uses 1.75 in that case.
- 1.25 is part of the standard's formula.
Worked example
A system with a quiescent current of 180 mA and an alarm current of 1.2 A, on a 24 hour standby with a 30 minute alarm:
6.15 / 20 = 0.3075 A, less than 1.2 A, so use D = 1.75.
Cmin = 1.25 × (24 h × 0.18 A + 1.75 × 1.2 A × 0.5 h) = 6.71 Ah.
The next available size is 7 Ah. With 72 hours of standby, the same system still needs D = 1.75: Cmin = 1.25 × (72 × 0.18 + 1.75 × 1.2 × 0.5) = 17.51 Ah, so choose 18 Ah.
Available battery sizes
Under Annex E.1.3, use the next highest available size if Cmin is not an exact available size. The calculator compares the unrounded capacity against these common 12 V valve-regulated sizes, from the small 1.2 Ah units used in compact power supplies up to 120 Ah:
Panels with a 24 V supply normally use two 12 V batteries wired in series. The capacity in Ah is the same for each battery, so you still size to the figure above.
What a calculator can't know
The sum is simple. Getting the inputs right is where batteries end up undersized.
Real current figures
Take the quiescent and alarm currents from the manufacturer's data for every device on the system, including loop devices, sounders, interfaces and ancillaries. An estimated quiescent current is the most common reason a battery turns out too small.
Charger limits
The panel or power supply has to recharge the batteries within the required time, and many have a maximum battery size they can charge. Check the manufacturer's data before specifying a larger battery.
Your specification
The standby and alarm periods come from the design, the system category and how quickly a fault can be put right. This tool is a guide for engineers, not a substitute for the design calculation.
Record the battery calculation on the certificate
Compliar's fire alarm periodic inspection and servicing certificate records the panel, PSU and battery readings, with a link to this calculator beside them so the measured values and the sizing are worked out together on the visit. The certificate goes to your customer as a branded PDF when you finish.
Common questions
How do I calculate the battery size for a fire alarm panel?
For valve-regulated lead-acid batteries, BS 5839-1:2025 Annex E.1 uses Cmin = 1.25 × (T1 × I1 + D × I2 × T2), with currents in amps and periods in hours. First calculate with D = 1. If that capacity divided by 20 is at least the alarm current, keep D = 1; otherwise use the manufacturer's de-rating data or D = 1.75, as this calculator does. Under E.1.3, choose the next highest available size if the capacity is not an exact available size. The calculator shows the working.
How many hours of standby does a fire alarm battery need?
BS 5839-1:2025 clause 24.3.5 sets 24 hours for Category M or L systems in most buildings, or 6 hours with an automatically started standby generator serving the system. For Category P, use 24 hours longer than the longest period the premises are likely to be unoccupied, or 72 hours in total, whichever is less. Follow each standby period with at least 30 minutes of alarm operation and check your system's specification.
How long must the batteries support the alarm?
At least 30 minutes at the total alarm load, following the standby period, under BS 5839-1:2025 clause 24.3.5. This calculator defaults to 30 minutes and also offers 60 minutes where a longer period is specified.
Can I use this instead of a design calculation?
No. It is a quick guide for engineers and installers. The design calculation, the manufacturer's current figures and the panel or power supply data take precedence, and the responsible person for the design signs off the final battery size.
Does Compliar do this automatically?
The fire alarm periodic inspection and servicing certificate in Compliar records the panel, power supply and battery readings, with a link to this calculator beside them, so the sizing is worked out on the visit from the measured values.
Battery sizing, built into your certificates
Compliar records the standby battery calculation on the fire alarm servicing certificate and sends the PDF to your customer. Book a demo to see it on your own jobs.