What Is PoE (Power over Ethernet)? Standards and Wattage Explained

Every PoE link has two roles. The power sourcing equipment (PSE) is the source of power, typically a PoE-capable network switch or a standalone injector. The powered device (PD) is the endpoint that runs on that power, such as an access point or camera. Before supplying power, the PSE performs a detection handshake: it applies a low probing voltage and looks for a specific signature resistance in the PD. Only when a valid PD signature is found does the PSE ramp up to the operating voltage (nominally in the range of about 44 to 57 volts DC). It then classifies the PD to learn how much power the device needs, so it can allocate the port's budget appropriately.

Power travels over the same Cat5e or better twisted-pair cabling as the data, so no new wiring is required. Because some power is lost as heat over the cable run, the standards define power at two points: what the PSE delivers at the port, and the lower amount guaranteed to reach the PD at the far end of a 100-meter run. This is why a port rated to source about 30 watts guarantees only roughly 25.5 watts at the device — the gap is expected cable loss, and it is why published PD power figures are always lower than PSE port figures.

PoE is defined in successive IEEE amendments, each raising the available power. The original 802.3af (2003), often called standard PoE or Type 1, sources up to about 15.4 watts per port and guarantees roughly 12.95 watts at the device — enough for basic phones, entry-level access points, and many fixed cameras. 802.3at (2009), known as PoE+ or Type 2, roughly doubles this to about 30 watts at the port and roughly 25.5 watts at the device, covering pan-tilt-zoom cameras, dual-radio access points, and more demanding endpoints.

802.3bt (2018) added two higher tiers. Type 3 sources up to about 60 watts per port (roughly 51 watts at the device) and Type 4 up to about 90 watts per port (roughly 71 watts at the device); some vendors round the Type 4 figure to around 100 watts, but the IEEE ceiling at the port is about 90 watts. These higher tiers use all four twisted pairs in the cable rather than two, and are marketed under names like PoE++ or 4PPoE. They power devices such as Wi-Fi 6/6E/7 access points, video conferencing units, LED lighting, thin clients, and small displays. Higher standards are backward compatible: a Type 4 switch port will correctly power an 802.3af device by negotiating down to what that device requests. Be cautious of pre-standard or 'passive' PoE, which applies voltage without negotiation and can damage standards-based gear — always confirm devices are IEEE-compliant.

A PoE switch is a network switch whose ports supply power directly, so a single device powers and connects many endpoints at once. This is the cleaner choice when you have multiple powered devices, want centralized management, or plan to grow — access-point-heavy offices, camera systems, and phone deployments are typical. Switches are specified both by per-port capability (for example, 802.3at on every port) and by total power budget across all ports, which matters when many high-draw devices run at once.

A PoE injector adds power to a single existing data link. It sits between a non-PoE switch port and one powered device, injecting voltage onto the cable. Injectors are useful for adding one or two powered devices without replacing an existing switch, powering a single remote access point, or supplying a device that needs more wattage than your current switch delivers. A related device, a PoE splitter, does the reverse — it separates power and data back out at the far end to feed a non-PoE device, though the device must tolerate the voltage the splitter provides. For anything beyond a handful of endpoints, a PoE switch is usually more economical and easier to manage than many injectors.

Start with your powered devices and match the standard to their demand. Confirm each endpoint's PoE class and required wattage from its datasheet, then choose PSE ports that meet or exceed it — a Wi-Fi 6E access point or PTZ camera may need PoE+ or 802.3bt, while a basic phone is fine on 802.3af. Then size the total power budget, not just per-port ratings: a switch may advertise a high per-port maximum yet lack the aggregate wattage to run every port at full draw simultaneously. Add headroom for growth and for cable loss over long runs.

Also weigh cabling, management, and resilience. PoE runs are limited to roughly 100 meters like ordinary Ethernet, and higher-power tiers benefit from Cat6 or better cabling to limit heat and voltage drop. Decide whether you need managed switches (for VLANs, monitoring, per-port power control, and scheduled power cycling of stuck devices) versus simpler unmanaged units. For access points, cameras, and phones that must survive a power outage, plan to back the PoE switch or injector with a UPS, since the endpoints have no power of their own. Finally, verify every device is IEEE-standard compliant to avoid the compatibility and damage risks of passive PoE.