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Battery Backup Calculator

Size a backup battery for an alarm panel, NVR, or PoE switch using the standby-current-plus-alarm-draw method used in fire and security system design — not just a flat runtime guess.

Your system

Battery capacity needed

—Ah
minimum rated capacity to specify
Standby stage—
Alarm stage—
Subtotal (before aging factor)—
Nearest standard battery: —

How this calculator works

Backup batteries for alarm and fire systems aren't sized on a flat "how many hours will this run" guess — codes like NFPA 72 use a two-stage method: enough capacity to hold standby load for a required number of hours (commonly 24 hours for commercial fire, sometimes 60 or 90 hours depending on jurisdiction), plus enough extra capacity to run the system at full alarm draw for a few minutes after that standby period ends.

Formula: Required Ah = [(standby hours × standby current) + (alarm minutes ÷ 60 × alarm current)] × (1 + aging factor).

Why the aging factor?

A sealed lead-acid battery loses usable capacity as it ages — a battery rated for 7Ah when new might only deliver 5.5–6Ah after a couple of years in a warm equipment cabinet. Building in a 20% aging factor (the commonly used code-compliance margin) means the system still meets its standby/alarm requirement even as the battery ages, rather than passing inspection on day one and failing 18 months later.

Typical standby/alarm requirements by system type

System typeTypical standbyTypical alarm time
Residential burglar alarm4–24 hours4 minutes
Commercial fire alarm (non-voice)24 hours5 minutes
Commercial fire alarm (voice evac)24 hours15 minutes+
NVR / PoE switch backupSite-dependent — often 1–4 hoursN/A (no separate alarm stage)
Always check your local code and AHJ requirements. This calculator uses the standard NFPA-style calculation method as a planning tool — the actual required standby/alarm times for a fire or life-safety system are set by code and your local authority having jurisdiction, not by this page.

Sizing backup for an NVR or PoE switch

For non-life-safety equipment like an NVR or PoE switch, there's usually no separate "alarm stage" — just set the alarm current and minutes to 0 and use the standby fields for your desired runtime during a power outage. For a more general watts/amps-based battery sizing (solar, RV, or any DC load), use the Amp Hour Calculator instead.

Need more capacity or a higher voltage than one battery provides? See our guide on wiring batteries in series vs. parallel: How to Choose the Right Battery Size for a Security Panel.

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