Here is a quick guide on how to calculate how big the batteries in your FACP need to be.
Here's what to do:
Step 1. Check your panel manufacturer's website for a tool or a spreadsheet that allows you to enter values and get a precise calculation based on the manufacturer's specs without having to dig through lots of paperwork to find all this data.
If not then...
Step 2. Look for your panel's amp draw specification for when the system is normal. Let's call this X.
Step 3. Look for the amp draw specifications for each of your initiating devices and multiply that by the quantity of each device. Add that product to X.
Step 4. Look for your panel's amp draw specification for when the system is in alarm.Let's call this Y.
Step 5. Repeat step 3 but for your signals instead of initiating devices. Add that product to Y.
Step 6. Repeat step 3 with any other devices you may have in your system (annunciators, cards, modules, etc). Add that to the appropriate variable based on whether they are on all the time or they are on in alarm only.
Step 7. Determine what standby time requirements you need, in hours. In most cases, at least 24 hours of standby power and at least 5 minutes of alarm power are required by code. So, in our example, let N = 24 and A = 0.084 (5 minutes, in hours)
Step 8. Calculate the result with this formula: H = 1.2(Nx + Ay)
1.2 = Derating factor, changes the result so that it is slightly higher than the bare minimum.
Step 9: Round the Ah result (that's H) up to the nearest battery size using this list of available sizes:
2 Ah, 4.5 Ah, 7 Ah, 12 Ah, 18 Ah, 26 Ah, 50 Ah
Step 10: You're all done!
Now for an example: 24 hours of standby, 5 minutes of alarm, system draws 1/4 amp on standby and 2 amps in alarm:
Ah = 1.2(24 * 0.25 + 0.084 * 2)
= 7.5 so you'd need at least a 12 Ah battery.