Battery calculation

A battery calculation sizes the secondary supply so it can carry the system through the required supervisory period and then the required alarm period, at full load, with a margin for temperature, ageing and tolerance. In Canada the two durations come from the building code, not from a ULC standard, and in Ontario they are 24 hours supervisory followed by 2 hours, 1 hour, 30 minutes or 5 minutes depending on the building. The calculation method itself sits inside CAN/ULC-S524, which is copyrighted and paywalled, so what follows explains the principle and names where the binding detail lives rather than reproducing it.

Last updated: September 2026

Where each half of this comes from

Two documents, two halves, and mixing them up is the root of most bad battery calculations.

How long is a building code question and it is free to read. Ontario requires supervisory operation for not less than 24 hours followed immediately by emergency power under full load for 2 hours, 1 hour, 30 minutes or 5 minutes depending on the building. Which row your building lands on is set out in standby power.

How to work it out is inside CAN/ULC-S524. That standard is purchased, copyrighted and paywalled. We have not read it and we do not reproduce it. If you need the method that binds you, that is the book, and which edition binds you is a separate question again, tracked in the CAN/ULC edition tracker.

What we can do is explain the principle honestly, and attribute everything.

The principle

You are answering one question: can this battery set carry this system for this long, twice over, in two very different load states?

Tabulate the load twice. The CFAA's own journal describes the two states clearly. Standby current is the operational load of the system when it is not in alarm. Alarm current is the load when it is in alarm, including notification appliances, relays and power supplies at full output. The journal's guidance to technicians is to measure and record both, rather than trusting a figure carried over from a previous job.

Every current draw belongs in the tabulation: the control unit itself, each addressable loop, each initiating and notification circuit, relays, modules, the annunciator, and anything the panel powers. Alarm current is normally an order of magnitude above supervisory current, which is why the short second period can dominate the answer.

Combine the two periods. In outline, the ampere hour demand is the supervisory current carried for the supervisory period plus the alarm current carried for the alarm period. The CFAA Journal sets it out as amp hours equal to standby current times 24 hours plus alarm current times the alarm duration, and works an example: 0.15 A standby for 24 hours plus 1.8 A alarm for 30 minutes gives 4.5 Ah, which after a factor of 1.25 leads to a 7 Ah battery.

Then derate. A battery does not deliver its nameplate capacity when it is cold, aged, or at the wrong end of manufacturing tolerance. The CFAA Journal states that a derating factor must be taken from the manufacturer's published installation instructions and typically falls in the 20 to 25 per cent range. It separately reports that ULC-S524:2024 introduces a 1.25 safety factor for battery sizing.

Two flags on that last sentence, and they matter. First, that is a trade journal reporting the content of a paywalled standard; we have not read it in the standard and neither should you take it from us. Second, it is not clear from the article whether the manufacturer derating and the 1.25 factor are the same adjustment described twice or two separate ones applied in sequence. That difference changes the battery you buy. Confirm it in the standard and in the manufacturer's installation instructions.

Battery derating was on the published revision topic list for the 2024 edition of the installation standard, so this is a moving target and older worked examples should be treated with suspicion.

What a calculation has to show

BCIT's published Division 28 standard states the deliverable well: submit design calculations substantiating that battery capacity exceeds supervisory and alarm power requirements, and show a comparison of notification appliance circuit alarm power requirements against the rated circuit power output. That second half is the one people skip, and it catches an overloaded circuit before commissioning does.

What goes wrong

One calculation for a building with several power supplies. Each transponder or remote booster with its own batteries needs its own calculation.

Alarm load taken from the drawing rather than the settings. Appliance current depends on the tap or candela setting actually specified.

Nothing revisited after an alteration. Adding devices changes both currents, and the calculation is part of what a verifier expects to see.

Fire Alarm Academy provides educational content only. It does not confer any certification, licence or credential, and it is not affiliated with, endorsed by, or accredited by the Canadian Fire Alarm Association, the Electrical Contractors Association of Ontario, ULC Standards, UL Standards and Engagement, CSA Group, the Electrical Safety Authority, any authority having jurisdiction, or any manufacturer. Always work from the edition of any standard adopted by your authority having jurisdiction.