ok so battery calcs. same math, every time. NFPA 72 gives you the formula, every panel brand accepts the formula, and yet somehow doing this calc on a Potter job feels completely different from doing it on a Fire-Lite job or a Silent Knight job. because it is.
the worksheets look different. the numbers live in different places in the docs. and the derating is presented differently even when it's mathematically the same thing. last spring i had a small office retrofit where the engineer swapped panel specs mid-project. ran the whole battery calc three times. same building, same device count, three different worksheets. here's what that looked like.
The formula, real quick
standby current times standby hours, plus alarm current times alarm hours. divide by 0.8. that's your minimum battery Ah. if you want the full NFPA 72 walkthrough with code citations, that's here.
the 0.8 derating accounts for battery aging and end-of-life capacity loss. all three manufacturers use it. the disagreement is in how the worksheet presents it, and more importantly, in WHERE you find your standby and alarm current numbers.
Finding the numbers, by brand
Fire-Lite. Their current figures live in two places: the panel datasheet (for the panel's quiescent draw) and the individual device datasheets (for everything on the field wiring). Fire-Lite also has a free online calculator called Lite-Calcs that does the summing for you, but the worksheets in the back of the panel programming guide give you the manual version. Look for Appendix A or B depending on the panel model. The worksheet shows panel current and then has rows for NAC and auxiliary current that you fill in line by line. Derating: they write "multiply by 1.25," which is just dividing by 0.8 with the algebra rearranged. Same result.
Potter. Potter's approach is similar but the manual formatting is a little more spread out. The panel's own current draw is in the spec sheet, under Electrical Specifications, usually listed as "quiescent current." Mid-size panels typically run somewhere in the 60 to 90 mA range, but check the actual datasheet for your model. Device currents come from cut sheets. The worksheet has you list each device category with a count and per-device current, multiply, sum. Standard stuff. The thing that trips people up: Potter sometimes presents a separate "maximum standby current with optional modules installed" figure in the spec table. If you've got an annunciator or wireless module, that's the number you use, not the base panel figure. Read the footnotes.
Silent Knight. SK splits the calculation into distinct line items rather than a single summed column. Their worksheets typically separate panel current, auxiliary device current, and NAC current at full alarm as three separate entries. It's actually more explicit in some ways. You just need three source documents instead of one: the panel datasheet, the accessory datasheets, and the NAC circuit ratings. Their derating is also 0.8, and they write it as dividing by 0.8 right on the form, which is cleaner than the "multiply by 1.25" framing some people find confusing.
Same job, three panels
Small office building. Three-story, about 40 devices on the initiating side, two NAC circuits. The GC got three quotes with three different panel specs. Same device count, same wiring assumptions, same AHJ. I ran the battery calc for each one.
Standby hours: 24. Alarm time: 5 minutes or 0.083 hours. The building had central monitoring, so the AHJ accepted 24-hour standby. Check your own AHJ on this, it varies.
| Fire-Lite | Potter | Silent Knight | |
|---|---|---|---|
| Panel quiescent (A) | 0.085 | 0.092 | 0.078 |
| Devices in standby (A) | 0.094 | 0.088 | 0.094 |
| Total standby (A) | 0.179 | 0.180 | 0.172 |
| Alarm current, all loads (A) | 1.82 | 1.75 | 1.80 |
| 24h standby Ah | 4.30 | 4.32 | 4.13 |
| 5-min alarm Ah | 0.151 | 0.145 | 0.149 |
| Raw required Ah | 4.45 | 4.47 | 4.28 |
| After 0.8 derating | 5.56 | 5.58 | 5.35 |
| Battery selected | 7 Ah | 7 Ah | 7 Ah |
all three landed on 7 Ah. which is kind of the point. the brands are not producing wildly different answers. the answer was always going to be 7 Ah for a system this size. what differs is how long it takes you to get there, and how many tabs you have open while doing it.
One gotcha per brand
Fire-Lite: Lite-Calcs is convenient but it's a separate login and it doesn't always reflect the most recent panel firmware. If you're working with an older panel variant, pull the worksheet from the actual installation manual and run it by hand to confirm. The manual numbers are the authoritative ones.
Potter: Watch the footnotes in their electrical specifications table. Standby current with certain optional modules can be meaningfully higher than the base panel figure. Missed this on a job with a wireless gateway module and the number changed enough to matter. Now i read the footnotes first.
Silent Knight: Their NAC current figures in alarm are listed for regulated 24V. If your voltage drop analysis shows significant sag on a long NAC run, devices at the far end of the circuit may draw more than the spec table assumes. Worth running the voltage drop calc before you finalize the battery number, not after.
anyway
all three worksheets get you to the same place. the underlying NFPA 72 math is identical. it's really a question of how much documentation hunting you're doing and which form your AHJ expects to see in the package.
full disclosure: i built a tool called FireDeck that runs the battery calc from panel and device data and generates the completed manufacturer worksheet automatically. obviously biased. the calc above is the calc either way. if you want to try it free, it's at firedeck.app.