Power over Ethernet or PoE technology describes a system to safely transfer electrical power, along with data, to remote devices over standard category 5 cable in an Ethernet network. It does not require modification of existing Ethernet cabling infrastructure.

The IEEE standardized version of PoE supplies up to 15.4 W of DC power (minimum 44 VDC and 350 mA) to each device. Only 12.95 W[4] is assured to be available at the powered device as some power is dissipated in the cable.

An updated standard in progress referred to as PoE Plus or 802.3at introduces increases available power. Numerous non-standard schemes had been used prior to PoE standardization to provide power over Ethernet cabling. Some are still in active use.

Long term DC power cabling standard; displaces other connectors
This technology is especially useful for powering IP telephones, wireless LAN access points, cameras with pan tilt and zoom (PTZ), remote network switches, embedded computers, small ethernet switches, thin clients and LCDs. It has been proposed as a long term replacement for the MIDI cabling standard for music devices.[citation needed]

All these require more power than USB offers and very often must be powered over longer runs of cable than USB permits. In addition, PoE uses only one type of connector, an 8P8C (RJ45), whereas there are four different USB connectors.

PoE is presently deployed in applications where USB is unsuitable and where AC power would be inconvenient, expensive (mains wiring must often be done by qualified and/or licensed electricians for legal or insurance reasons) or infeasible to supply. However even where USB or AC power could be used, PoE has several advantages over either including:

Cheaper cabling - even category 5 cable is cheaper than USB repeaters and the task of meeting building code requirements to run AC power cable is eliminated
A Gigabit of data per second to every device, which exceeds 2009 USB and the AC powerline networking capabilities
Global organizations can deploy PoE everywhere without concern for any local variance in AC power standards, outlets/plugs or reliability
Direct injection from standard 48 V DC battery power arrays; The ability to keep critical infrastructure running more easily in outages and make power rationing decisions centrally for all the PoE devices

Power management features; 802.az integration; complements PoE
Most advocates expect PoE to become a long term global DC power cabling standard and to replace so-called "wall wart" transformers which cannot be easily centrally managed, are often cheaply made and quite inefficient, and which are vulnerable to damage in surges and brownouts. The combination of G.9960 networking on existing AC power lines to an outlet where a PoE router was plugged in would be capable of moving a gigabit to every device with minimal wiring and participating fully in both AC and DC device power demand management.

When integrated with the 802.3az standard, the energy management capabilities of the combined standard are expected to be formidable. However, that integration has not yet occurred.

There are several PoE implementations, including ad-hoc techniques, but using the IEEE standard for supplying power over Ethernet is very strongly recommended. Some features, such as ability to operate with injected power as low as 10 Volt DC, set low DC-DC controller switching frequencies to control EMI or improve no-load efficiency", higher frequencies "to reduce the size of components used in the power converter output stage," surge detection and protection, do not affect standards compliance directly but configurations that depend on them may not be able to switch to other designs easily.