The best place for an access point is the ceiling. The worst place for a power socket is also the ceiling. PoE ends that argument.
The short answer
Power over Ethernet (PoE) sends DC power to a device through the same twisted-pair cable that carries its data [3]. The switch or injector is the power sourcing equipment (PSE), the camera, access point or phone is the powered device (PD) [3]. The IEEE standards 802.3af, 802.3at and 802.3bt define how much power a port may deliver. Our assessment: before you buy a switch, add up what your devices need and compare the total with the switch's PoE budget, not just with the per-port limit.
How it works
- Four types, two numbers each. The standards name Types 1 to 4. The PSE side and the PD side differ because the cable eats some power [2][3][4].
- Nine classes. The handshake assigns a class from 0 to 8; class 0 means the device announced none and is budgeted at 15.4 W, like class 3 [2][3]. The PSE guarantees at least the class power, and the PD may draw up to it [3]. Classes 1 to 4 correspond to 4 W, 7 W, 15.4 W and 30 W at the port, classes 5 to 8 to 45 W, 60 W, 75 W and 90 W [3].
- Detect, classify, power. The PSE first checks that a valid PD is attached, then classifies it, and only then switches on roughly 54 V DC [3]. Devices without PoE are therefore not supposed to be harmed by a PoE port.
- Pairs. Types 1 and 2 use two of the four pairs. Types 3 and 4 can use all four, and classes 5 and above require four pairs [2]. The Ethernet Alliance notes that Type 3 and 4 equipment also supports older PDs and can halve cable losses by using four pairs for them [2].
- Reach. As with data, the cable can be up to 100 metres, and a fair amount of power is lost on the way [3]. Because of that loss, size the PD column for what a device may draw, and the PSE column for what your switch must reserve.
- Standards live inside Ethernet. IEEE 802.3-2022 includes the provision of power over selected twisted-pair PHY types, and the 802.3bt amendment extends it to four pairs [1].
Power by type [2][3][4]:
| Type | Standard | Common name | Power at the PSE port | Power at the PD |
|---|---|---|---|---|
| 1 | 802.3af | PoE | 15.4 W | 12.95 W |
| 2 | 802.3at | PoE+ | 30 W | 25.5 W |
| 3 | 802.3bt | PoE++ / 4PPoE | 60 W | 51 W |
| 4 | 802.3bt | PoE++ / 4PPoE | 90 W | 71.3 W |
Some documents round 12.95 W to 13 W [2]. The names "PoE+" and "PoE++" are marketing terms; vendors also use "tier" instead of "type" [4][5].
A homelab example
We have not recorded PoE use at Bytearium (inventory as of 14 to 16 Oct 2026), so this is a worked example with made-up devices. Four cameras at class 3 (15.4 W each at the port) plus one access point at class 4 (30 W) need 4×15.4 + 30 = 91.6 W of switch budget. A switch that offers 90 W in total cannot power all five at their full class, however many ports it has. Look for the "PoE budget" line in the data sheet, and for the per-port standard next to it.
Checklist
- Read the class or the wattage in the device manual for every PD.
- Add the port-side watts (the PSE column) and compare them with the switch's total PoE budget.
- Match the standard: a Type 3 or 4 device needs a switch that can deliver four-pair power [2].
- Use proper cable, in line with the switch maker's cable advice, and keep runs within 100 metres [3].
- Do not plug a device that was not built for PoE into a passive injector without reading its manual.
- Plan the switch's own power supply and UPS runtime for the full PoE load.
- Label the PoE ports and note which device sits on which port.