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AmpSize

Fire alarm battery calculator

Size the standby battery for a fire alarm panel.

NZS 4512

Reads NZS 4512:2010. Tables checked 26 Sept 2026.

Try a job 5

Each job fills in example values. Replace them with your values.

Inputs

Worked example with the default inputs. The live calculator replaces it when the page loads.

Non-alarm load
0.2 A
Maximum alarm load
2 A
Connected to a remote receiving centre
Yes
Alarm period, minutes
30
Ageing factor
1.25
Temperature factor
1
Cells in series
12

Required capacity at 20 °C

7.25Ah

Minimum capacity before factors
5.8Ah
Standby period
24h
Standby capacity
4.8Ah
Alarm capacity at the entered period
1Ah
Ageing factor
1.25x
Temperature factor
1x
Capacity from the load and the factors
7.25Ah
Governing figure
Capacity from the load and the factors
Charger current to restore in 24 h
0.5A
Float voltage
26.4V
Float voltage tolerance
± 0.36V
Monthly test minimum voltage
19.2V

This result is a design aid. Verify it against the current standard.

Calculation steps

  1. Standby period24h

    NZS 4512:2010 Cl 213.1

  2. Standby capacity4.8Ah

    NZS 4512:2010 Cl 213.1

  3. Alarm period entered, minutes30

    NZS 4512:2010 Cl 213.1

  4. Clause alarm period, minutes30

    NZS 4512:2010 Cl 213.1

  5. Governing alarm period, minutes30

    NZS 4512:2010 Cl 213.1

  6. Alarm capacity at the entered period1Ah

    NZS 4512:2010 Cl 213.1

  7. Alarm capacity at the governing period1Ah

    NZS 4512:2010 Cl 213.1

  8. Ageing factor1.25x
  9. Temperature factor1x
  10. Capacity from the load and the factors7.25Ah

    NZS 4512:2010 Cl 213.1

  11. Minimum capacity before factors5.8Ah

    NZS 4512:2010 Cl 213.1

  12. Required capacity at 20 °CResult7.25Ah

    NZS 4512:2010 Cl 213.1

  13. Governing figureCapacity from the load and the factors

    NZS 4512:2010 Cl 213.1

  14. Charger current to restore in 24 h0.5A

    NZS 4512:2010 Cl 212

  15. Float voltage26.4V

    NZS 4512:2010 Cl 212

  16. Float voltage tolerance0.36V

    NZS 4512:2010 Cl 212

  17. Current limit voltage per cell1.85V

    NZS 4512:2010 Cl 212

  18. Monthly test duration10s

    NZS 4512:2010 Cl 602.7

  19. Monthly test discharge rate, hours5

    NZS 4512:2010 Cl 602.7

  20. Minimum service life, years5

    NZS 4512:2010 Cl 213.5

  21. Monthly test minimum voltage19.2V

    NZS 4512:2010 Cl 602.7

About this calculator

How this is calculated

The tool sizes the standby battery for a fire alarm panel to NZS 4512:2010 Clauses 212 and 213. It sizes the capacity only. It does not pick the battery type.

  1. Standby capacity. It multiplies the non-alarm load by the standby period.
  2. Alarm capacity. It multiplies the alarm load by the alarm period. It compares your period with the period in Cl 213.1 and uses the longer.
  3. Factors. It adds the standby and alarm capacity. It multiplies by your ageing and temperature factors.
  4. Minimum. It also reads the minimum capacity before factors. The larger figure governs, and the tool names it.
  5. Charger. From Cl 212 it gives the charger current to restore the battery in 24 h. It also gives the float voltage and tolerance for your cell count.
  6. Monthly test. From Cl 602.7 it gives the minimum voltage the battery must hold in the monthly test.
StepTable or clauseWhat it decides
CapacityNZS 4512 Cl 213.1Standby and alarm capacity, with factors
Charger and floatCl 212Charger current and float voltage
Monthly testCl 602.7The minimum voltage in the test
Service lifeCl 213.5The minimum life of the battery

Worked example

A panel with a 0.5 A non-alarm load and a 3 A alarm load. The alarm period is 30 min. The system has 12 cells and is monitored. The tool gives 16.88 Ah at 20 °C. The charger current is 1.2 A. The float voltage is 26.4 V.

Limits

  • NZS 4512:2021 supersedes the 2010 edition this tool reads. Check the current edition.
  • The tool does not check the battery type or the service life.
  • It does not add up the load of each device. You enter the totals.
  • It gives the capacity at 20 °C. Enter a temperature factor for a cold or hot room.

See also: Fire alarm cabling, Sounder level and the DC battery sizing tool.

Questions

Each answer describes what this tool calculates. The result is a design aid. Verify against the current standard.

What does the fire alarm battery tool do?

The tool adds the ampere hours the panel draws during the standby period to the ampere hours it draws during the alarm period. It applies an ageing factor and a temperature factor, and reports the capacity at 20 degrees Celsius.

How long must a fire alarm battery last?

AmpSize uses 24 hours of standby where a monitoring link is present, and 72 hours where none is. It then adds 30 minutes of alarm load. All three figures come from NZS 4512:2010 Clause 213.1. The form option switches between the two standby periods.

Can the result fall below the clause figure?

No. Clause 213.1 and Clause 213.2 give minimum periods. AmpSize works out a minimum capacity before the factors, on the longer of the clause alarm period and the period you enter, and never reports less than that figure. A short alarm period raises a warning instead.

What charger and float voltage does it report?

AmpSize divides the capacity by 24 hours and adds the non-alarm current. That gives the charger current of Clause 212.1. It also multiplies 2.20 V and 0.03 V by your cell count, for the float band of Clause 212.5.

What is the monthly test criterion?

Clause 602.7 tests the battery for 10 seconds at the 5 hour discharge rate or higher. During that test the battery gets no help from another source. Its voltage must not fall below 80 percent of its nominal voltage.

Clauses and tables this tool reads

Every clause and table, with how each was verified

Tables last checked on 26 Sept 2026.

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