The short answer: add the cameras, the PoE switch overhead and the NVR, then divide the usable energy by that total. For most four-camera sites the switch and NVR draw more than the cameras. Backing up the cameras without the switch protects nothing, because every camera behind a dead switch goes dark at once.
The load nobody counts: the PoE switch
A camera's datasheet says 6 W. The switch's datasheet says “120 W PoE budget”. Neither number is the load. The real figure is the cameras, plus the switch's own electronics (3–5 W), plus conversion loss in the PoE stage (about 10 %). Four cameras at 6 W lands near 32 W, not 24 W.
That 30 % error is the difference between four hours and three — and it is why we ask for the switch model before we quote.
Worked examples
| Site | Load breakdown | Total | Runtime on 100 Wh usable |
|---|---|---|---|
| 4 cameras, no NVR | 4 × 6 W + switch 8 W | 32 W | 3.1 h |
| 4 cameras + NVR | 32 W + NVR 30 W | 62 W | 1.6 h |
| 8 cameras + NVR | 8 × 8 W + switch 12 W + NVR 40 W | 116 W | 52 min |
| 16 cameras + NVR | 16 × 8 W + 2 switches 24 W + NVR 60 W | 212 W | 28 min |
Three configurations, three different products
One camera, one cable
Where a single PoE camera sits far from the switch, a unit with an integrated injector removes the need for a separate PoE brick. Our PE-DU18-P is built for exactly that: 18 W, PoE out, 10 ms transfer.
Four to eight cameras, one cabinet
The switch, NVR and cameras share a cabinet. Size for the whole cabinet load, not per camera, and put the NVR on the same backed-up circuit. This is a 100–300 W requirement with a runtime target — a project build rather than a stock item.
Multi-site rollout
Standardise on one cabinet design and one runtime target, then let the camera count vary. Sizing per site sounds diligent and produces six different SKUs, six spares policies and a commissioning headache. Standardise the platform, vary the pack.
What to check before you order
- Transfer time. Under 20 ms keeps a camera stream alive; 10 ms is under one AC cycle. Above 50 ms the NVR usually re-negotiates the stream and you lose 10–30 seconds.
- PoE standard. 802.3af (15.4 W) and 802.3at (30 W) need different injector behaviour. A 12 V output is not a substitute.
- Cabinet temperature. A sealed outdoor cabinet runs 10–15 °C above ambient. Above 40 °C internal, derate the pack by 20 %.
- Cold start. If the site is already dark when the unit is installed, it must start on battery. Not every unit does.
If you send the camera count, the switch model and the hours you need to cover, the answer takes about a working day — and it comes with the runtime arithmetic shown, so you can check it. Send the details, or run the numbers in the calculator first.