PoE Budget Calculator

What "PoE budget" actually means

The number on the datasheet is a shared pool, not a per-port figure. A 48-port switch advertising 370 W cannot deliver 30 W on every port — that would need 1,440 W. The pool is the constraint, and the moment you cross it the switch starts denying power to ports, usually by priority, usually to whichever device you least wanted to lose.

Two different numbers: at the device and at the switch

PoE loses power as heat in the copper. The standards account for it explicitly: 802.3af allocates 15.4 W at the switch to guarantee 12.95 W at the device over a worst-case 100 m Cat5e run — about 16% lost in the cable.

So the figure that matters against your budget is the one at the switch, which is always higher than the sum of the device draws. This calculator applies loss based on your actual run length and cable type, anchored to that spec. Short runs of Cat6 lose very little; a 90 m Cat5e run to a camera on the far side of a warehouse loses real power.

The PoE standards, briefly

• 802.3af (PoE) — 15.4 W at the switch, 12.95 W at the device. Fixed cameras, readers, desk phones.
• 802.3at (PoE+) — 30 W / 25.5 W. PTZ cameras, most access points, door controllers.
• 802.3bt Type 3 (PoE++) — 60 W / 51 W. Wi-Fi 6E and 7 access points, heated outdoor PTZ domes.
• 802.3bt Type 4 — 90 W / 71.3 W. Displays, laptops, high-power specialist gear.

A switch that only does 802.3af will not power a PoE+ device, budget or no budget. Check the standard per port, not just the total.

Size for 70–80%, not 95%

Running a PoE switch near its ceiling is how installations become fragile. Three reasons to leave margin:

• Growth. Someone will add a camera. They always do.
• Redundancy. If you have dual power supplies for a reason, the surviving PSU has to carry the whole load alone.
• Cold weather. Heated outdoor domes draw substantially more in a Central Oregon January than they do on the bench in July. That is exactly when you least want a camera to drop.

Where this gets designed

PoE budget is one line in a bigger conversation about switch selection, VLANs, uplinks and UPS sizing. That is network design, usually alongside cameras and access control on the same fabric.

Working out storage as well as power? The camera storage calculator sizes the drives and the uplink.