Port count is the number people shop on. Power budget is the number that decides whether the install works. Here is the arithmetic, with the margins that save a site visit.
Most PoE switch returns we see are not faults. They are a switch that was specified on port count alone, dropped into a site where the attached devices want more power than the supply can deliver. The symptom is rarely obvious: a camera at the end of the run reboots at night when the infrared cuts in, or an access point drops its second radio under load.
Start from the devices, not the switch
Write down every powered device, its PoE class and its real draw. A Wi-Fi 6 access point sits around 13–15W in practice but is rated 802.3at, so it can ask for up to 25.5W. A 4MP bullet camera idles near 5W and jumps to 9–12W when the infrared array fires. A door controller with a maglock is a different conversation entirely — the lock usually wants its own supply.
Add the real draws, then add the worst case. The gap between those two numbers is the one that bites, because infrared cuts in on every camera at the same moment: dusk.
Add cable loss
PoE is specified at the far end of 100 metres of Cat5e, and the cable eats roughly 10–15% over a full run. Short runs in a server room lose almost nothing; a camera at 90 metres across a compound loses real power. If a meaningful number of your runs are long, add 15% to the total rather than pretending the loss is theoretical.
Leave headroom for the install nobody told you about
Specify to about 70% of the switch budget. A 400W budget comfortably running 280W of hardware leaves room for the two cameras that get added after handover, and keeps the power supply out of its hottest operating band. Running a PoE switch pinned at 95% for years is how you turn a five-year switch into a three-year switch.
A worked example
Twenty-four ports: sixteen 4MP cameras and four Wi-Fi 6 access points. Cameras at 12W worst case is 192W. Access points at 25.5W worst case is 102W. That is 294W before cable loss; add 15% and you are at 338W. A 400W budget leaves 15% headroom — tight but workable. A 250W switch would have looked fine on the idle figures and failed at dusk.
The datasheet number you want is the total budget, not the per-port maximum. A switch advertising 30W per port on 24 ports almost never means 720W.
We will size it with you
Send us the site plan and the device count. Our technical team will come back with a bill of materials and the arithmetic behind it.
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