How this works
Ah = (standby current × standby hours + alarm current × alarm hours) × safety factor
Add every device on the supply with its standby and alarm current from the datasheet, including the panel or power supply's own draw. The calculator totals them, multiplies by the required times, and applies the safety factor.
Fire alarm
NFPA 72 (2022), 10.6.7.2.1, requires the secondary supply to run the system for 24 hours in standby and then 5 minutes in alarm with every notification appliance operating. For an in-building emergency voice/alarm communications system the alarm period is 15 minutes at maximum connected load. Two-way emergency communications systems (24.10.2.1) need 24 hours and then 4 hours. The 2022 edition raised the battery correction factor to 1.25 (10.6.7.2.14) to allow for the capacity a battery loses as it ages. Earlier editions used 1.2, which older worksheets still show.
This is life safety. The calculator is a planning aid, not code verification. Use the panel manufacturer's own battery calculation for the submittal, and follow the edition and amendments your authority having jurisdiction has adopted. They have the final say.
Access control
UL 294 defines four standby power levels for access control equipment, at full load: Level I needs no secondary power, Level II 30 minutes, Level III 2 hours and Level IV 4 hours. UL 294 sets no safety factor. The calculator offers 1.25 as a default because batteries lose capacity with age, but it is a choice, not a requirement; power-supply makers use their own (Altronix's worksheet uses 1.8). In access control mode, the second column is "active" current: locks, strikes or sounders energised for the time you set.
Remember the code side of locks: doors in the means of egress generally must unlock on loss of power (NFPA 101, 7.2.1.6.2; IBC sensor-release requirements). A fail-safe maglock draws current the whole time the door is secured, so it belongs in the standby column.
What this does not account for
- Temperature. Cold batteries deliver less capacity.
- High discharge rates. A battery's rated amp-hours assume a slow discharge over the period on its datasheet; a large alarm load gets less out of it.
- Battery age beyond the safety factor. NFPA 72 (2022) requires batteries to be marked with a replacement date no more than four years after installation.
- The charger: a supply must also be able to recharge the battery within the time the code allows.
Common questions
- Is the answer the size of each battery?
- It is the capacity at the system voltage. A 24 V system usually uses two 12 V batteries in series; each needs the full amp-hour rating shown.
- Why 1.25 and not 1.2?
- The 2022 edition of NFPA 72 raised it. If your jurisdiction enforces an earlier edition, set 1.2; the calculator lets you.
- Where do the currents come from?
- From each device's datasheet or the panel's installation manual. Use the alarm current at the candela and volume settings you will actually use.
- What about cameras on a solar pole?
- That is a different calculation: several sunless days, cold derating and the panels to refill the battery. Use the off-grid solar calculator.
Sources
- NFPA 72, National Fire Alarm and Signaling Code, 2022 edition, 10.6.7.2.1, 10.6.7.2.14 and 24.10.2.1, as reproduced in the Talk-A-Phone AOR-10 battery worksheet (2025) and the Electronic Security Association and National Training Center summaries of the 2022 changes.
- UL 294, Access Control System Units, 7th edition, 41.3–41.6, standby power performance levels (UL preview, 2018 revision).
- NFPA 101, Life Safety Code, 7.2.1.6.2; International Building Code sensor-release provisions (1010.2.12 in the 2021 IBC).
Every source used on this site, and how each number is used.