How to size a UPS and battery bank for cameras, NVRs, PoE switches and access control — the arithmetic, the device loads worth trusting, and the two mistakes that put an undersized battery on a job that needed code-compliant standby.
Every device that must stay alive during an outage. Not the nameplate maximum — the steady-state draw. A PTZ at rest and a PTZ slewing with its heater on are different numbers, and the heater is the one that catches people out.
UPS units are rated in volt-amperes, not watts. Switching supplies in NVRs, PoE switches and cameras run around a 0.8 power factor, so divide watts by 0.8 — then add headroom, because a UPS run at 100% of rating has no margin for the day someone adds four cameras.
Runtime comes from stored energy, not from the UPS rating. Watts multiplied by hours gives watt-hours; divide by system voltage for amp-hours — then correct for inverter losses and depth of discharge.
The two divisors at the end are the ones most calculators bury. 0.85 is inverter and charge/discharge efficiency. 0.5 is depth of discharge — routinely pulling a sealed lead-acid or AGM battery below half its capacity shortens its service life sharply, so you size as though only half the bank is usable. Skip that divisor and you will specify a battery that hits its runtime on day one and misses it by year two. This is a service-life design choice for security batteries — it is not part of NFPA 72 fire alarm battery sizing, which is covered separately below.
| Device | Qty | Watts ea. | Total |
|---|---|---|---|
| Outdoor fixed camera | 18 | 18 W | 324 W |
| Outdoor PTZ | 2 | 60 W | 120 W |
| Indoor fixed camera | 4 | 12 W | 48 W |
| NVR, 32-channel | 1 | 90 W | 90 W |
| PoE switch base draw | 2 | 15 W | 30 W |
| Card readers | 6 | 5 W | 30 W |
| Continuous load | 642 W |
642 W ÷ 0.8 = 803 VA minimum. With 25% headroom, specify a 1,000–1,100 VA unit. For four hours at 48 V: (642 × 4) ÷ 48 ÷ 0.85 ÷ 0.5 = 126 Ah. Two 12 V 100 Ah batteries in a 24 V bank will not do it; plan the bank around the amp-hour figure and the voltage you are actually building.
| Device | Typical | Max | PoE class |
|---|---|---|---|
| Indoor fixed camera | 12 W | 15.4 W | 802.3af |
| Outdoor fixed camera | 18 W | 25 W | 802.3at |
| Indoor PTZ | 25 W | 30 W | 802.3at |
| Outdoor PTZ (heater/blower) | 60 W | 90 W | 802.3bt Type 3 |
| Thermal camera | 20 W | 25 W | 802.3at |
| LPR camera | 18 W | 25 W | 802.3at |
| Card reader, standard | 5 W | 8 W | 802.3af |
| Card reader, biometric | 12 W | 15 W | 802.3af |
| Card reader, mobile credential | 6 W | 10 W | 802.3af |
| Access point, Wi-Fi 5 | 15 W | 15.4 W | 802.3af |
| Access point, Wi-Fi 6 | 22 W | 30 W | 802.3at |
| Access point, Wi-Fi 6E | 30 W | 60 W | 802.3bt Type 3 |
| IP motion sensor | 3 W | 5 W | 802.3af |
Treat these as planning figures and confirm against the datasheet before you order. Outdoor housings with heaters are the widest variance in the table — a camera that draws 18 W at room temperature can more than double at −20°C.
A PoE switch draws its own base power plus everything it delivers downstream. A 16-port switch with a 15 W base and 240 W of connected cameras is a 255 W load on the UPS, not 15 W and not 240 W.
The reverse error is just as common: counting each camera individually and the switch's full PoE budget, which double-counts the same watts and specifies a UPS twice the size the job needs. Count the load once, at the point where it is actually drawn from the battery.
On a fire alarm or mass-notification system, backup duration is not a design preference. It is a code-mandated secondary power requirement — and which figure applies depends on what the system does, not on how long you would like it to run.
NFPA 72 generally requires secondary power sufficient for 24 hours of standby operation followed by 5 minutes of alarm operation, unless another system-specific requirement applies. That last clause carries real weight — voice, mass notification, CO and several special applications each have their own figures.
Where the system includes in-building fire emergency voice/alarm communications, secondary power must sustain 24 hours quiescent plus 15 minutes during a fire or other emergency condition, at maximum connected load.
Occupancy codes determine when you need voice evacuation. NFPA 72 determines how the installed voice system is designed and powered. The IBC, NFPA 101 or a local amendment decides whether EVACS is required at all; once the fire alarm system incorporates that function, the 15-minute secondary-power calculation follows the function — not the occupancy class.
In-building mass notification systems generally require 24 hours of standby operation followed by 15 minutes of emergency operation at maximum connected load. Do not generalize that figure across every MNS configuration — wide-area MNS, emergency command centers, high-power speaker arrays and textual notification each carry separate secondary-power requirements.
NFPA 72 requires a battery-capacity correction/safety factor in addition to the calculated standby and alarm load. The required factor depends on the adopted edition — earlier editions specified a minimum 20% margin; the 2022 edition uses a 1.25 correction factor. Verify the governing edition before sizing rather than carrying a remembered percentage between projects.
The 50% depth-of-discharge practice discussed earlier on this page is a service-life design choice for security-system batteries. It is not part of NFPA 72 fire alarm battery sizing, and applying it here roughly doubles the bank for no code reason.
20 Ah × 1.25 = 25 Ah correct (2022 edition)
20 Ah ÷ 0.5 × 1.25 = 50 Ah wrong — DoD does not belong here
From the corrected figure, select an available battery capacity that satisfies the equipment manufacturer's listing and installation instructions.
Always verify the locally adopted edition of NFPA 72, applicable amendments, and AHJ requirements. Published requirements can differ from the edition legally enforced on a project — NFPA issuing a newer edition does not make it governing where you are working.
These govern different things and do not impose the same battery calculation as one another. UL 681 covers burglar and holdup installations and is not the standard for a monitored fire alarm installation.
The practical takeaway: the security system's runtime is a design choice; the life-safety system's runtime is a requirement. Size them separately, and never let a shared UPS quietly put a code-required load behind a battery specified for cameras.
For a graceful NVR shutdown or a few minutes of ride-through, yes. For hours of recording through an outage, no — a desktop UPS has a small internal battery and no provision for an external bank. Once you need multi-hour autonomy you are specifying a unit with external battery connections, and the battery bank becomes a bigger line item than the UPS.
Higher system voltage means lower current for the same power, which means smaller conductors and less loss. Small loads under roughly 150 W run fine on a single 12 V battery; past that, 24 V or 48 V banks are easier to build and kinder to the wiring.
20–25% over the calculated VA is the working convention. It covers the device that gets added six months later and keeps the UPS off its own ceiling, where efficiency drops and runtime estimates stop holding.
Almost always one of three things: depth of discharge was ignored, the load included nameplate maxima instead of steady-state draw, or the batteries have aged. Sealed lead-acid loses usable capacity over its service life, so a bank sized with no margin meets spec when new and misses it later.
Design Right AI computes PoE budgets, power draw and battery load from the devices already in your design — so the backup calculation comes out of the actual bill of materials instead of a spreadsheet you rebuild per job, and lands in the proposal with it.