Usually, included with the installation manual for the fire alarm system, the manufacturer shows a work sheet that can be filled out to calculate the needed standby power. This is a worksheet that has to be filled out for each panel and power supply that has standby batteries in the building's fire alarm system. If the manual isn't available, talk to the manufacture's technical support people to get a work sheet from them.
The following is only a summary of what the manufacture's battery calculation sheet does. Use the work sheet provided by the manufacture, it's easier.
The worksheet is based on the power needed by the system during a blackout.
In case there's a power outage, the fire alarm system still has to detect fire and warn people of fire. It would be nice if the fire alarm system could do this for days, weeks, or even years, until eventually the power can be restored, but the NFPA (National Fire Protection Association) is reasonable. In most cases, the NFPA is happy if the system can continue to work without power for 24 hours. This is the standby period.
If there is a fire detected at the end of that 24 hours, the system still has to sound the alarm. Usually sounding the alarm takes up a much higher level of current than when the alarm system is standing by, so sounding the alarm has to be calculated separately. This is the alarm period.
At this point, the electrical current used the whole system, including all input devices, output devices, control panels, and power supplies has to be known. The manufacturer of each device or panel will show the standby current and the alarm current in their installation instructions.
If the installations instructions aren't available, contact technical support.
To figure out how many amp-hours is needed for the standby batteries, the amp-hours needed for the standby period is calculated by adding up the total standby current used by each of the input devices and each of the output devices attached to a control panel or power supply, and the standby current for the control panel or power supply.
Multiply this total current used when standing by 24 to get the total amp-hours used in that 24 hour blackout period.
The alarm period may be 5 minutes long, or it may be 2 hours long, intermittently. Check with the local fire marshal or fire inspector to find out how long the alarms have to continue once the system is in alarm.
Do the same calculations again using the alarm currents rather than the standby currents.
Add the amp-hours used during the standby period to the amp-hours used during the alarm period. This gives the sub-total amp-hours.
To give a little headroom to the battery amp-hour rating, add at least 20% to the sub-total to get the total amp-hours needed for the standby batteries.
To make sure the calculations include all devices, when doing the battery calculations, use the manufacture's worksheets.