A battery calculation is only as reliable as its inputs. The formula can look compact while hiding several different design decisions: the normal load, required standby time, maximum alarm load, alarm duration and the factors used for ageing or higher discharge current.
The Fire Regs Pro standby battery calculator mirrors the method currently implemented in the iOS project. It is useful for learning the relationship between the inputs and for a preliminary check—not for replacing manufacturer data or a competent design review.
Break the duty into two parts
The calculation first considers the quiescent duty:
standby current × standby hours
It then considers the alarm duty:
alarm current × alarm hours × alarm-current derating
Those ampere-hour demands are added, then multiplied by the selected ageing or temperature allowance.
Keeping the two duties separate makes mistakes easier to spot. An unusually large alarm contribution may point to minutes entered as hours, or milliamps entered as amps. An unexpectedly small standby contribution may mean that a peripheral load was omitted.
Use maximum alarm load, not a convenient sample
The alarm current should reflect the system condition the supply must sustain. Depending on the design, that may include a larger set of sounders, visual alarms, relays or other outputs than a technician sees active during a limited test.
Obtain current information from the equipment data and the system design. Avoid guessing a “typical” device value when the installed mode, volume setting or protocol state changes the load.
Treat derating as a design input
The ageing factor and alarm-current factor are not decorative safety margins. Their appropriate values depend on the battery method and manufacturer information. The calculator defaults are examples from the current app implementation; they are not universal instructions for every battery technology or environment.
Sanity-check the result
After calculation:
- confirm all current units and time units;
- compare the duty with the power-supply and charger limits;
- select the next suitable available capacity rather than rounding down;
- account for temperature and manufacturer constraints;
- record the assumptions with the design or commissioning evidence.
Use the Power & Batteries quick reference to revisit the variable meanings. For real work, always verify the current published standard, the project specification and the panel and battery manufacturer data.
Frequently asked questions
Does the calculated capacity tell me the exact battery to install?
No. It is a minimum new-capacity estimate based on the entered duty and factors. Select a suitable available size and verify battery, panel and manufacturer requirements.