Raspberry Pi PoE HAT
With the Raspberry Pi PoE HAT, you can use a simple Ethernet cable to power your Raspberry Pi 3 B+ through compatible Power over Ethernet networks.
Including €0.02 for ecotax
How to use the Raspberry Pi Power over Ethernet HAT
The Raspberry Pi PoE HAT is very simple to use. First, attach the four standoffs included with the HAT, then connect it to your Raspberry Pi. You can now power your Pi through the RJ-45 network connector. This Raspberry Pi HAT embeds a small fan that you can control via an I2C link.
Technical specifications of the Raspberry Pi PoE HAT