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By Ali Zafar
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
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.
Worked example
200mA standby draw, 24-hour required standby time, 600mA alarm draw for 5 minutes, 20% aging factor, 12V system.
| Step | Result |
|---|---|
| Standby stage | (200mA ÷ 1000) × 24h = 4.80 Ah |
| Alarm stage | (600mA ÷ 1000) × (5min ÷ 60) = 0.05 Ah |
| Subtotal (before aging factor) | 4.80 + 0.05 = 4.85 Ah |
| Apply aging factor (20%) | 4.85 Ah × 1.20 = 5.82 Ah |
| Minimum battery to specify | 7 Ah @ 12V (nearest standard size) |
A panel drawing 200mA at standby needs a 7Ah battery, not a 4.8Ah one — the alarm-stage draw and the 20% aging allowance are what code-compliant sizing actually requires on top of the raw standby figure.
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 type | Typical standby | Typical alarm time |
|---|---|---|
| Residential burglar alarm | 4–24 hours | 4 minutes |
| Commercial fire alarm (non-voice) | 24 hours | 5 minutes |
| Commercial fire alarm (voice evac) | 24 hours | 15 minutes+ |
| NVR / PoE switch backup | Site-dependent — often 1–4 hours | N/A (no separate alarm stage) |
Common mistakes
- Using the alarm current for the whole standby period. Standby current runs for hours; alarm current runs only for the alarm minutes. Entering alarm current as standby oversizes the battery badly.
- Entering only the panel's draw. Standby current should be the total drawn from that battery: panel, power supply, sounders, relays, and any NVR or switch on the same supply.
- Mixing units. The current fields are in mA, not amps. 0.2 A is 200 mA.
- Rounding down to the calculated Ah. The result is a minimum. Specify the next standard size up, as the calculator's recommendation does.
- Using this as a runtime estimate. This sizes a battery for required standby and alarm durations. To estimate how long an existing battery will run on a load, use the Battery Amp Hour Calculator.
- Assuming a 24V system needs double the Ah. Two 12V batteries in series give 24V at the same Ah; wiring them in parallel adds Ah instead.
Assumptions & limitations
- Currents are entered in mA at the battery's system voltage, and the result is in Ah at that same voltage. The voltage selector labels the recommendation (for example "7 Ah @ 12V"); it does not change the Ah figure. If equipment is rated in watts, divide by the battery voltage to get amps first. Converter losses are not modelled.
- Each stage uses one constant current: standby for the standby hours, then a single alarm period. Varying loads and repeated alarm events are not modelled.
- The aging factor is a single percentage margin (default 20%). The calculator does not model temperature, battery chemistry, discharge-rate effects, end-of-discharge voltage or the charger.
- The recommended size is rounded up from a fixed list of common capacities (4, 5, 7, 9, 12, 18, 26, 33, 35, 55, 75 and 100 Ah). Above 100 Ah it rounds up in 25 Ah steps and suggests banking batteries. Available sizes vary by supplier.
- This is a planning tool, not a code-compliance determination. Required standby and alarm times come from your local code and AHJ.
Frequently asked questions
What standby time should I actually use?
That's set by your local fire code and AHJ, not by this calculator. 24 hours is the most common baseline for commercial fire systems, but always confirm the actual requirement for your jurisdiction and system type before finalizing a design.
Do I need the alarm stage for a non-life-safety system like an NVR?
No. Set alarm current and minutes to 0 and use only the standby fields for your desired backup runtime during an outage. The two-stage method exists for fire and burglar alarm systems that need to run a siren or notification device after the standby period.
Why is the aging factor included by default?
A sealed lead-acid battery loses usable capacity as it ages, sometimes 15 to 20 percent within a couple of years in a warm cabinet. Building that margin in up front means the system still meets its requirement after the battery has aged, not just on the day it's installed.
Does the system voltage change the required Ah?
No. The calculation uses the currents you enter, so the Ah result is the same for 12V and 24V. The voltage selector only labels the recommendation. A 24V system built from two 12V batteries in series uses the same Ah rating per battery.
How is this different from the Battery Amp Hour Calculator?
This calculator works backwards from required standby and alarm durations to a battery size. The Battery Amp Hour Calculator works forwards: given a battery and a load, it estimates how long the battery will run.
Where do I find the standby and alarm current?
Usually in the installation manual or datasheet for the panel and each device on the battery. Add up every device powered from the same battery. If a figure isn't published, measure it rather than guess.
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.