TL;DR: Fire alarm battery capacity is calculated as (standby amps x standby hours) + (alarm amps x alarm hours), then adjusted by the safety, aging, or derating factor that governs the project. Take every current value from the manufacturer’s published data for the equipment being installed and assign it to the standby or alarm column where that equipment draws it. Confirm the required standby and alarm durations before sizing anything, and make the final battery selection using the equipment manufacturer’s battery-sizing instructions and the code edition adopted for the project.
A fire alarm battery calculation looks simple on paper: determine the system load, apply the required standby and alarm durations, and select a battery arrangement that satisfies the resulting capacity requirement.
The harder part is making sure the inputs are right. Device counts, current draw, notification appliance settings, power supplies, operating conditions, required durations, and manufacturer battery-sizing instructions can all affect the result. A clean calculation starts with the requirements for the specific system and project, then works from published data for the equipment being installed.
What the standby requirement covers
A fire alarm system’s secondary power supply has to support the system for the operating conditions and durations required by the applicable code, listing, manufacturer instructions, and project documents.
For battery calculations, those operating conditions are typically evaluated separately. Standby load represents the current drawn while the system is operating normally on secondary power without an alarm condition. Alarm load represents the current required when the system is in alarm, including the equipment that operates only or draws additional current during alarm.
Keeping those loads separate matters because they may be applied for very different lengths of time. Standby operation may be measured in hours, while the required alarm period may be measured in minutes. Before doing the calculation, convert the durations to consistent units and make sure each current value is assigned to the operating condition in which the equipment draws it.
The required durations are not something to assume from a rule of thumb. They depend on the type of system and what governs the project. Confirm the standby and alarm requirements from the adopted code, manufacturer instructions, listing information, project specifications, and any requirements established by the authority having jurisdiction.
The battery also has to support the required alarm operation after the standby period, so the calculation needs to account for both operating conditions rather than treating alarm capacity as an alternative to standby capacity.
The battery calculation, stated plainly
The basic calculation separates the energy required during standby from the energy required during alarm.
Step one: calculate standby amp hours. Total the standby current in amps and multiply it by the required standby duration in hours.
Step two: calculate alarm amp hours. Total the alarm current in amps and multiply it by the required alarm duration expressed in hours. If the alarm duration is given in minutes, divide by 60 first. For example, 5 minutes is approximately 0.0833 hours.
Step three: combine the requirements and follow the manufacturer’s battery-sizing method. Add the standby and alarm amp hours, then apply any safety factor, derating factor, or other battery-sizing provision the manufacturer and the project require.
The basic relationship is:
(standby amps x standby hours) + (alarm amps x alarm hours) = calculated amp-hour requirement
From there, apply the battery-sizing requirements that govern the project and follow the equipment manufacturer’s published instructions. NFPA 72 (2022) introduced a minimum 1.25 correction factor for aging, while earlier requirements and some manufacturer worksheets use a 1.2 multiplier. Some manufacturer calculation tools or tables may also incorporate a sizing factor directly, so confirm the adopted code edition and the calculation method for the equipment rather than automatically applying a factor from a previous job.
Two details matter throughout the calculation.
The first is units. Current values may be published in amps, milliamps, or microamps, while alarm durations may be stated in minutes. Convert everything consistently before calculating. For example, 350 microamps equals 0.00035 amps, and 5 minutes equals approximately 0.0833 hours.
The second is sourcing the current values. Use the published documentation or manufacturer calculation tool for the specific equipment being installed rather than relying on a value from another model or a previous job. Manufacturers publish battery calculators and calculation tables specifically for their panels, power supplies, and related equipment.
If a system includes separate power supplies with their own secondary power, calculate those supplies according to the manufacturer’s instructions as well. Don’t automatically combine every load into the main control panel’s battery calculation when another listed supply is carrying that load.
A worked example on a small addressable system
The arithmetic in a battery calculation is usually the easy part. The bigger thing to watch is whether each load is sitting in the right column. The example below shows why that matters, especially once notification appliances are added to the alarm load.
The figures are illustrative. For an actual submittal, use the published current draw for the specific equipment being installed and follow the manufacturer’s battery-sizing instructions.
Assume a small addressable system requires 24 hours of standby followed by 5 minutes of alarm.
| Component | Qty | Standby A each | Standby A total | Alarm A each | Alarm A total |
|---|---|---|---|---|---|
| Addressable control panel | 1 | 0.165 | 0.165 | 0.290 | 0.290 |
| Addressable smoke detectors | 40 | 0.00035 | 0.014 | 0.00045 | 0.018 |
| Addressable pull stations | 6 | 0.00035 | 0.002 | 0.00045 | 0.003 |
| Monitor and relay modules | 8 | 0.00045 | 0.004 | 0.00060 | 0.005 |
| Remote annunciator | 1 | 0.040 | 0.040 | 0.060 | 0.060 |
| Communicator | 1 | 0.050 | 0.050 | 0.070 | 0.070 |
| Wall horn strobes | 24 | 0.000 | 0.000 | 0.098 | 2.352 |
| Totals | 0.275 | 2.798 |
Standby: 0.275 A x 24 h = 6.60 AH
Alarm: 2.798 A x 0.0833 h = 0.23 AH
Subtotal: 6.83 AH
The final sizing step depends on what governs the project. For example:
Applying a 1.2 multiplier: 6.83 AH x 1.2 = 8.20 AH
Applying a 1.25 aging correction factor: 6.83 AH x 1.25 = 8.54 AH
From there, select a manufacturer-approved battery configuration that satisfies that requirement and confirm the panel’s battery capacity, enclosure limitations, and charging requirements.
The important point isn’t the sample numbers. It’s keeping standby and alarm loads in the correct columns, then applying the requirements that govern the project and the published battery-sizing instructions for the equipment being installed.
Why notification appliances belong in the alarm column
Notification appliances can dominate the alarm load without contributing the same load during standby. In the example above, the horn strobes account for 2.352 of the 2.798 amps of alarm current, so putting that current against the wrong duration can materially distort the battery calculation.
The mistake can go either direction. Carry notification appliance alarm current into the standby calculation and the required capacity can be significantly overstated. Leave required notification appliance current out of the alarm calculation and the battery can be undersized.
Use the manufacturer’s published current values for the appliance and operating configuration being installed, including the applicable candela and other settings that affect current draw. Then place those values in the standby and alarm columns according to the manufacturer’s battery-calculation instructions.
A simple two-column worksheet makes the review easier: standby current and alarm current for every device and load, even when one of those values is zero.
Follow the applicable battery-sizing requirements
The calculated standby and alarm load is only part of the battery-sizing process. The correction or safety factor applied after that calculation can depend on the code edition and the equipment manufacturer’s published instructions.
Under NFPA 72 (2022), battery calculations include a minimum 1.25 correction factor for aging. Earlier requirements and manufacturer worksheets may use a 1.2 multiplier, and some manufacturer calculation tools or tables may incorporate their sizing factor directly.
That makes the adopted code edition important. Don’t automatically apply 1.2 because that’s what was used on the last job, and don’t add another factor if the manufacturer’s calculation method already incorporates it. Confirm the requirements that apply to the project and follow the equipment manufacturer’s published battery-sizing instructions.
Temperature and installation conditions can also affect battery performance and service life, so check the battery and equipment documentation when batteries will be installed outside the conditions covered by their published ratings.
The practical rule is simple: calculate the actual load first, then apply the battery-sizing requirements that govern that system and project.
When the battery does not fit the enclosure
The calculated battery requirement drives the selection. Don’t reduce the capacity just to use the batteries that fit the panel enclosure.
If the required battery configuration exceeds what the enclosure or equipment supports, the next step depends on the system and the manufacturer’s listed options. That may include:
- An approved external battery cabinet. Verify the permitted battery configuration, wiring requirements, conductor size, and any limitations between the batteries and the equipment.
- A remote power supply or NAC expander. Moving notification load to another listed supply can change the load at the main panel, but that supply then needs its own battery calculation.
- A different notification appliance configuration. If the design permits it, changes to appliance selection or settings can affect alarm current. Any change still has to meet the project’s notification requirements and approval process.
- Load distributed across multiple supplies. On larger systems, notification and auxiliary loads may be divided among listed power supplies, with each supply sized for the load it serves.
Also verify the equipment’s battery capacity and charging requirements before selecting a larger battery. A battery that satisfies the amp-hour calculation still has to be an acceptable configuration for the equipment supplying and charging it.
If the calculation and the enclosure don’t agree, change the system configuration or use an approved battery arrangement. Don’t change the calculation to make the hardware fit.
Before acceptance, make sure the calculation still matches the system
The battery calculation submitted for the project should match the system that was installed. If device counts, notification appliances, power supplies, or other loads changed during the job, revisit the calculation before acceptance rather than relying on the original submittal.
The same applies to the batteries themselves. Confirm that the installed battery configuration meets the calculated requirement and the equipment manufacturer’s requirements. Any project-specific labeling, documentation, testing, or acceptance requirements should also match the approved system and the requirements of the AHJ.
When you come to a Security Data Supply franchise for batteries, power supplies, or enclosures, have the system information with you:
- Panel manufacturer and model. Include the specific equipment information needed to identify the manufacturer’s battery-sizing requirements.
- Device counts and system loads. Bring the information used in the battery calculation, including any changes made after the original submittal.
- Notification appliance counts and settings. Have the appliance models and applicable settings used to determine alarm current.
- Required standby and alarm durations. Bring the requirements you’re designing to rather than relying on a rule of thumb.
- Calculated battery requirement. This gives the franchise team a starting point for identifying batteries, cabinets, power supplies, and manufacturer documentation that fit the equipment.
Security Data Supply franchises can help identify available batteries, power supplies, enclosures, and manufacturer documentation for the equipment you’re working with. The franchise team isn’t performing or approving the battery calculation. The contractor and project team remain responsible for the system design, calculation, code compliance, and required approvals.
Bring the equipment information and your calculation to your local franchise, and the team can help you find the products and documentation that match the job.
Need batteries or power equipment for an upcoming fire alarm job?
Bring your panel or power supply information and battery calculation to your local Security Data Supply franchise. The team can help you identify available batteries, enclosures, power supplies, and manufacturer documentation that match the equipment you’re working with.
Frequently asked questions
How do I calculate battery backup for a fire alarm panel?
Calculate the standby amp-hour requirement and alarm amp-hour requirement separately using the published current draw for the equipment being installed. Add those requirements together, then follow the panel or power supply manufacturer’s published battery-sizing procedure, including any required safety factor or derating.
How many amp hours do I need for 24 hours of standby plus 5 minutes of alarm?
It depends on the system load. The basic calculation is (standby amps x 24 hours) + (alarm amps x 0.0833 hours). Then follow the manufacturer’s battery-sizing procedure for the equipment being installed. In the illustrative example above, 0.275 amps of standby current and 2.798 amps of alarm current produce a calculated requirement of approximately 6.83 AH before the applicable sizing factor. Applying a 1.2 multiplier produces approximately 8.20 AH, while applying the NFPA 72 (2022) 1.25 aging correction factor produces approximately 8.54 AH. Use the requirement applicable to the project and the manufacturer’s battery-sizing instructions.
What is the correction or derating factor for fire alarm batteries?
There isn’t one factor that should automatically be applied to every project. NFPA 72 (2022) introduced a minimum 1.25 correction factor for aging, while earlier requirements and some manufacturer battery-sizing worksheets use a 1.2 multiplier. Manufacturer calculation tools may also incorporate a sizing factor into their tables or worksheets. Confirm the adopted code edition and follow the published battery-sizing requirements for the equipment being installed.
Do notification appliances count toward standby load or alarm load?
Use the manufacturer’s published standby and alarm current values for the notification appliances and configuration being installed. Notification appliances can make up a significant portion of the alarm load, so applying their current to the wrong operating duration can materially change the battery calculation.
Why might my installed battery requirement differ from the original calculation?
Changes to device counts, notification appliances, settings, power supplies, or other system loads can change the battery requirement. If the installed system differs from what was used in the original calculation, recalculate the affected supply using the final equipment and manufacturer-published current values.
What should I bring to Security Data Supply when I need batteries for a fire alarm job?
Bring the panel or power supply manufacturer and model, your battery calculation, the required standby and alarm durations, and enough system information to identify the battery configuration you’re looking for. A Security Data Supply franchise can help identify available batteries, enclosures, power supplies, and manufacturer documentation that match the equipment. The contractor and project team remain responsible for the calculation and required approvals.
About the author
Ron Broussard is Chief Executive Officer of Security Data Supply and has spent more than 30 years helping security, fire, and low-voltage businesses grow through strong distribution partnerships. He has held executive leadership roles across distribution and manufacturing throughout his career, giving him a broad perspective on the challenges contractors, integrators, and distributors face every day. At Security Data Supply, he guides the company’s franchise network, vendor relationships, and long-term growth strategy.
Methodology
This article explains a general approach to fire alarm battery calculations using published manufacturer current draw and battery-sizing information. The worked example uses illustrative current values solely to demonstrate the calculation and should not be used to size batteries for an actual system.
Actual battery requirements depend on the equipment being installed and the requirements applicable to the project. Contractors should use the panel, power supply, device, and notification appliance manufacturer’s published data and battery-sizing instructions, along with the adopted code, project documents, and AHJ requirements.
Sources and further reading
- NFPA: NFPA 72, National Fire Alarm and Signaling Code. Confirm the edition adopted by the authority having jurisdiction for your project. NFPA
- NFPA: Technical committee documentation supporting the change to a minimum 1.25 aging correction factor for battery calculations in the 2022 edition. NFPA 72 First Revision No. 5186 (A2021 cycle)
- Fire-Lite Alarms: Manufacturer instruction to multiply the sum by a derating factor of 1.2 to determine battery size. MS-9200UDLS Series Manual, P/N 52750, Section 7.4 Calculating the Battery Size
- Security Data Supply: franchise network and full product category lineup
- Fire alarm products at Security Data Supply
- Power supplies and batteries at Security Data Supply
- Fire Alarm Installation Checklist
- Initiating Devices vs. Notification Appliances
- What It Takes to Power an Access Control Door
- Dallas Fire Alarm Licensing Class