How Much Power Does PoE Deliver?

Power over Ethernet voltage is around 48 V DC, and the power it delivers depends on the standard. IEEE 802.3af (PoE) gives about 15 W per port, 802.3at (PoE+) about 30 W, and 802.3bt (PoE++) 60 W for Type 3 or 90 W for Type 4. Those are the figures at the source. The power available at the device is lower, because some is lost as heat in up to 100 m of cable: 12.95 W, 25.5 W, 51 W and about 71 W respectively.

The voltage is not one fixed number. Each standard allows the power source to output anywhere from about 44 V to 57 V, and the device must work from a slightly wider range to allow for the drop along the cable. In practice most switches output close to 50 to 54 V.

When sizing a deployment, plan against the figure at the device, not at the source, and leave headroom for the worst case each device can draw.

Within each standard, a powered device (PD) declares a power class during the PoE handshake. The class tells the power sourcing equipment (PSE) how much to reserve from its budget, so low-power devices do not tie up capacity they will never use.

Our PoE vs PoE+ vs PoE++ FAQ compares the three standards and the devices each suits.

The voltage window narrows as power rises, so higher-power ports keep current, and therefore cable heating, under control.

Power is voltage multiplied by current, so a higher voltage moves the same power with less current. Less current means less loss in thin Ethernet conductors, which are only about 0.5 mm across. The standards stop below 60 V DC so that PoE stays within safety extra-low voltage limits, and the basic Power over Ethernet design relies on that: it is safe to touch and can be installed like ordinary data cabling, without an electrician.

The cost of a lower voltage shows up in non-standard passive PoE, which often runs at 24 V. To deliver the same power it must draw twice the current, so it loses more over long runs and often specifies shorter maximum distances.

The gap between the two figures is the allowance for cable resistance. PoE+ assumes a worst-case 12.5 Ω loop through 100 m of Cat5e. At 600 mA, that cable drops about 7.5 V and turns about 4.5 W into heat, which is exactly the gap between 30 W at the source and 25.5 W at the device. Shorter or heavier-gauge cable loses less, but the standard guarantees only the lower figure.

That is why a power budget should add up device draw, not port ratings. A PoE switch with a 240 W budget across eight PoE+ ports cannot run all eight at a full 30 W. Our PoE power budgeting guide shows the sum for a multi-camera system, and our PoE computers list the per-port and total PoE power for each model.

Device datasheets give the real figure, and it is often well below the class maximum. As a rough guide for planning, before checking each datasheet:

We size PoE sources against each site’s device list and cable runs, and validate the power at every port before the system ships as standard. Send us your devices and distances and we will specify the PoE budget to match.