How is alarm panel battery capacity calculated?

Battery Ah = (standby current × required standby hours) + (alarm current × required alarm time, typically 15–30 minutes), multiplied by a 25% aging factor. A panel drawing 350mA in standby for 24 hours needs roughly a 12Ah battery.

How many hours of battery backup does an alarm system need?

Common requirements are 4 hours for monitored commercial systems and 24 hours for residential or unmonitored systems, plus alarm-condition time. Exact requirements come from the applicable standard (ULC-S304 in Canada, UL 985/1023 in the US) and your monitoring contract.

Why does my alarm battery die after 3-4 years?

Sealed lead-acid batteries on continuous float charge age chemically regardless of use, losing capacity every year. Panels only test under load briefly, so a failing battery often isn't caught until an outage. Replace alarm batteries proactively every 3–5 years.

Alarm Panel Battery Backup Calculator

Size the standby battery for an intrusion alarm system: add the panel, keypads, detectors and sirens, choose your required standby time, and get the battery Ah — with the alarm-condition load included.

System devices

Device	Qty	Standby mA	Alarm mA

Required standby time

Check ULC-S304 / UL requirements and your monitoring contract.

Alarm time after standby (minutes)

Battery required

—

minimum battery capacity → —

Total standby current—

Total alarm current—

Standby consumption—

Alarm consumption—

Suggested battery—

Includes a 25% aging factor. Most residential panels fit up to a 7–8 Ah battery; larger capacities need bigger cans or auxiliary supplies.

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Typical intrusion device current draw

Device	Standby	In alarm
Alarm panel (main board)	100–150 mA	150–200 mA
LCD/touch keypad	60–120 mA	120–160 mA
PIR motion detector	10–25 mA	25 mA
Glassbreak detector	20–30 mA	35 mA
Door/window contact (wired)	~0 mA	~0 mA
Indoor siren	0 mA	300–500 mA
Outdoor siren/bell	0–20 mA	750–2000 mA
Zone expander / bus module	30–65 mA	65 mA
Cellular/IP communicator	60–100 mA	250 mA (transmitting)

The siren is the battery killer

Standby math is easy to pass and alarm math is where designs fail: an outdoor siren pulling 1.5 A will flatten a marginal battery in minutes at the exact moment you need the communicator transmitting. That's why the standard formula reserves capacity for a full alarm event after the standby period has already drained the battery.

Frequently asked questions

How is battery capacity calculated?

Ah = (standby current × standby hours) + (alarm current × alarm time), × 1.25 aging factor. This calculator does exactly that.

How many hours of standby are required?

Commonly 4 h for monitored commercial, 24 h for residential/unmonitored — but the binding number comes from the applicable standard (ULC-S304 in Canada, UL in the US) and your monitoring contract.

Why do alarm batteries fail after 3–4 years?

SLA batteries age chemically on float charge regardless of use. Replace them proactively every 3–5 years — don't wait for the low-battery trouble signal during an outage.

Alarm system in Toronto or the GTA?

AMT SYSTEMS LTD. installs and monitors ULC-listed intrusion alarm systems with properly engineered battery backup.

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