PoE Power Reality Check: 802.3bt, Real Watts at the Device, and Why People Get Confused

Power over Ethernet is both amazing and routinely misunderstood in sales conversations. Most confusion comes from mixing up:

• PSE port power (what the switch can supply)
• PD input power (what the device is guaranteed to receive)
• cable length, losses, and “extended power” options

1) The numbers that matter (Type 3 / Type 4)

IEEE 802.3bt is the “high power” PoE standard. A commonly cited figure is 71W available at the powered device for Type 4 in the baseline model, even though the PSE may supply up to ~90–100W depending on class and implementation. (Microchip white paper PDF: https://ww1.microchip.com/downloads/en/DeviceDoc/Next-Generation-PoE-IEEE-802.3bt-White-Paper-DS00002992A.pdf)

Analog Devices’ overview also references PD power levels and details the standard’s behavior around MPS and standby. (ADI PDF: https://www.analog.com/media/en/technical-documentation/technical-articles/m131_en_-ieee_802.3.pdf)

A quick sanity reference (non-authoritative but readable): Type 3 is often summarized as ~60W at PSE / ~51W at PD, and Type 4 as ~100W at PSE / ~71W at PD. (FS.com explainer: https://www.fs.com/sg/blog/understanding-poe-standards-and-wattage-326.html)

2) Why spec sheets mislead

The classic trap is marketing that says:

“100W PoE!”

But the question you should ask is: “How many watts are guaranteed at the device, at my cable length, in my thermal environment?”

If the device needs 75W and you design as if “100W at the port” means “100W at the device,” you can end up with brownouts, thermal throttling, or unexplained resets.

3) Cable length and “extended power”

Some references discuss “extended power capability” where the PD can get more power if channel conditions are known. (DigiKey summary referencing the Microchip paper: https://www.digikey.ca/en/articles/design-in-high-power-poe-ieee-8023bt-solutions)

The practical takeaway:

• Long runs + warm bundles + worst-case conditions reduce margin
• Design for margin, not marketing maximums

4) The safe way to quote PoE power in proposals

Use a 3-line standard:

1. PSE port capability (what the switch can supply)
2. Guaranteed PD input under the relevant standard/type
3. Your design target (what you will actually allow per device)

5) The buyer-facing soundbite

“PoE is fantastic, but it’s not magic. Quote the watts at the device, not the watts on the box.”

If you’re selling a power+controls system, this one framing reduces a ton of friction.