Free tool
PoE Power Budget Calculator
Check how many cameras or access points a PoE switch can really carry, allowing for cable losses, PoE standards and sensible headroom.
The budget is shared, and the cable takes a cut
A switch advertising thirty watts per port does not give thirty watts on every port at once. The total PoE budget is shared across all of them, so an eight port switch with a 120 watt budget averages fifteen watts per port, which is fine for fixed cameras and not fine for PTZ units with heaters.
Cable length matters because copper has resistance. Over a full hundred metre run you lose a noticeable share of the power as heat before it reaches the device, which is why the standard specifies more power at the switch than the device is allowed to consume. Thin or copper clad aluminium cable makes this considerably worse, and is a common cause of cameras that reboot at night when the infrared illuminator switches on.
Size for the worst moment, not the average. Cameras draw their peak after dark with IR at full power, and if every camera on the switch does that at once and the budget is marginal, ports start shutting down in sequence. Leaving around twenty percent spare avoids that entirely.