Free engineer tool
Fire Alarm Battery Calculator
Size fire alarm standby batteries in seconds — enter the loads and durations, get the required capacity and a sensible standard battery size.
Free to start — no card needed. Sign in to open the tool.
Standby battery sizing is one of those calculations every fire alarm engineer does often and nobody enjoys doing on the back of a job sheet. The battery has to carry the system through a mains failure for the required standby period and still have enough left to drive the alarm load at the end of it — and the sums are easy to get slightly wrong under time pressure.
This free calculator does the arithmetic for you. You enter the quiescent (standby) current, the alarm current, and the standby and alarm durations you are designing to, and it returns the required battery capacity in amp-hours together with a recommended standard battery size to specify.
What it does
Standby and alarm loads
Enter the quiescent current the system draws in normal standby and the higher current it draws in full alarm. The calculator keeps the two loads separate so each is applied over its own duration.
Standby and alarm durations
Set the standby period (how long the battery must support the system on mains failure) and the alarm duration at the end of it. Change the figures to match the design basis for the installation.
Required capacity in amp-hours
The tool combines the standby load over the standby period with the alarm load over the alarm period to produce the required battery capacity, so you can see the number the design has to meet.
Recommended standard size
Rather than leaving you on an awkward figure, it suggests a standard battery size at or above the requirement — the size you would actually order and fit.
How it works
- 1
Enter the standby load and period
Put in the system's quiescent current and the standby duration you are designing to for the installation.
- 2
Enter the alarm load and duration
Add the alarm current and how long the system must sustain full alarm at the end of the standby period.
- 3
Read the required capacity
The calculator returns the required amp-hour capacity and a recommended standard battery size to specify.
- 4
Check it against the system and standard
Confirm the loads against the panel and device data and the durations against the applicable standard and fire strategy before you commit to a battery.
Why get battery sizing right
An undersized standby battery is a real safety and compliance problem: if the battery cannot carry the system through the required standby period and still raise the alarm, the protection the system is supposed to provide is not there when the mains fails. It is also a common inspection finding.
Doing the calculation properly — separating standby and alarm loads, applying each over the correct duration, and allowing for the practical size you can actually fit — turns a fiddly manual sum into a quick, repeatable check. The calculator is part of the wider Incognito engineer toolkit alongside the zone chart designer, fault database and reporting tools.
Frequently asked questions
Is the fire alarm battery calculator free?
Yes. It is one of the free engineer tools in Incognito Fire & Security. Create a free account to open it — no card is required to start.
What do I need to enter?
The system's quiescent (standby) current and alarm current, plus the standby period and alarm duration you are designing to. Take the currents from the panel and device data for the specific installation.
What does it give me back?
The required battery capacity in amp-hours and a recommended standard battery size at or above that requirement, so you can specify a battery you can actually order and fit.
Does it replace checking against the standard?
No. The calculator handles the arithmetic; the design basis — the standby and alarm durations, deratings and any project-specific requirements — must be set and checked by a competent person against the applicable standard and the fire strategy for the building.
Ready to try it?
Create a free account and open fire alarm battery calculator in your browser.