Power over Ethernet allows compatible network devices to receive power over their Ethernet connection. The important point is that not all PoE is the same.
Practical example: checking a PoE switch budget
A switch can have enough ports and still have too little PoE power.
Scenario: A switch must power six wireless access points and eight IP cameras.
Fourteen PoE-capable ports are required, but port count alone is not enough. The maximum power requirement of each connected device is checked against the PoE standard supported by each switch port and against the switch's total available PoE budget.
What can go wrong: A 24-port PoE switch can still be unsuitable if its total power budget cannot support all devices at their required power levels.
Common mistakes and selection checklist
Common mistakes
- Counting PoE ports without calculating the switch's total available PoE budget.
- Ignoring the PoE standard and maximum power required by each endpoint.
- Designing to the exact current load with no allowance for additional cameras, APs or higher-power replacements.
What happens if you get it wrong?
Some devices may fail to power, restart under load, negotiate a lower feature set or make future expansion impossible even though unused switch ports remain.
Selection checklist
- List every powered device and its required PoE standard.
- Calculate the expected total powered-device demand.
- Check per-port capability and the switch's total PoE budget.
- Allow headroom for growth and replacement devices.
- Check uplink capacity as well as power when many cameras or APs share the switch.
Active PoE vs passive PoE
Ubiquiti distinguishes active PoE from passive PoE. Active PoE uses standards-based negotiation such as 802.3af, 802.3at and 802.3bt. Passive PoE provides a fixed voltage without negotiation. This is why the voltage and PoE type must be checked before connecting an injector to a device.
| Type | IEEE | Maximum at PSE | Maximum delivered to PD |
|---|---|---|---|
| PoE | 802.3af | 15.4 W | 12.95 W |
| PoE+ | 802.3at | 30 W | 25.5 W |
| PoE++ Type 3 | 802.3bt | 60 W | 51 W |
| PoE++ Type 4 | 802.3bt | 100 W | 71 W |
PoE budget matters
When a switch has multiple PoE ports, the total power available is shared between connected devices. Ubiquiti states that the switch's PoE availability must be greater than the sum of the connected device power requirements to avoid instability or failure.
PoE injectors are useful, but check compatibility
A PoE injector can provide power when the switch does not provide PoE or when the switch's available PoE budget has been exhausted. Ubiquiti notes that its UniFi and UISP PoE adapters are passive and always output voltage, so they should not be connected to ordinary non-PoE equipment such as computers or printers.
Why this matters for your access points
The current Server Warehouse range includes Ubiquiti access points with different power requirements and PoE classes. For example, Ubiquiti documents U7 Lite at 13 W, U7 Pro at 21 W, U7 Pro Max at 25 W and U7 Pro XGS at 29 W. The correct switch or injector depends on the exact model.
Technical information in this article is based on current manufacturer documentation available at the time of writing. Product availability and specifications can change.
Confirm the exact product specification, supported configuration and compatibility before ordering. Platform generation, firmware, licences and optional components can change what a product supports.