Do you mean "How do you calculate how big a battery needs to be (in Amp / Hours) to keep the panel running during a power outage as long as required?" These calculations have to be performed for each power supply or panel that has battery backup.
Should there be a power blackout, the calculations are designed to show what size of battery is needed to supply enough energy (in the form of electrical current) to make sure the fire alarm system able to detect fire and, once a fire is detected, warn people of the danger.
The calculations show:
"The Number of Amps" of electrical current the battery can supply when the fire alarm system is standing by for "the Time of the Blackout" (usually 24 hours). This is the number of Amp/Hours needed for standby.
The current is the total current used by any device that uses power during a blackout. Each smoke detector, each door holder that gets its power from the panel, the panel itself, the annunciator, etc. has to be included in the calculations for the battery that supplies power for that device.
To figure out the standby Amp/Hours needed, add up the total current used during standby (in Amps) and multiply that by 24 (Hours).
"The Number of Amps" of electrical current the battery can supply when the fire alarm system is "Sounding the Alarm" (either 5 minutes or 15 minutes). This is the number of Amp/Hours needed for Alarm.
Many of the devices use a different amount of current during standby condition or alarm condition. Make sure to include the different current amounts in standby or in alarm calculations. Also, make sure to include all the horns and strobes, added together, they use a lot of current.
To figure out the alarm Amp/Hours needed, add up the total current used during an alarm (in Amps) and multiply that by either 0.83 for 5 minutes (Hours) or 0.25 for 15 minutes (Hours).
"Extra Headroom" Amp/Hours, just in case. Because the fire alarm system will be modified and added to over the 15- to 20-year lifetime of the fire alarm system, and because of natural degrading of the batteries during their lifetime, this headroom is added to the total of the standby Amp/hours and the alarm Amp/hours is some extra headroom Amp/Hours. The headroom is usually 20%.
Calculations Rounded Up
When adding these three sets of Amp/Hours, a total Amp/Hour rating for the battery is obtained. This exact size of battery doesn't exist; instead, batteries come in standard sizes. Use your calculations as the minimum battery size needed, and then round up to the next commonly available battery size. Remember, no one cares if the battery is bigger, everyone cares if the battery is too small.
Battery Calculation Worksheet
Included in the installation manual that comes with each power supply or control panel is a worksheet to help with the calculations. Use that worksheet. If the manual isn't available, Google the manual on the web, or contact technical support for the manual. That's what Google or technical support is there for.
And by all means, read the manual, that's why the manual is included with the power supply or control panel. The manual has all kinds of information. I read manuals, you should too.