PoE budget: how to check the switch, ports and devices

For a PoE device, three things must match: the supported power type, the individual port's capabilities and the switch's overall PoE budget. The presence of a RJ45 connector does not prove that the port supplies power, and a sufficient overall budget does not mean every port can power every device.

Before choosing a switch or injector, make a list of the exact cameras, access points or other end devices. For each, record the requirements from its data sheet. Then browse routers and network devices against this list. This way, you seek a specific combination, not simply a box with enough Ethernet ports.

A data port and PoE port require different checks

Establish the Ethernet connection and power over it separately in the description. If a port is stated only as a network connection, do not assume it also has PoE. Check exactly which ports on the chosen model are described as supplying power.

PoE, PoE+ and PoE++ designate different power-over-Ethernet classes. Use the exact designation from both sides' documentation: the device supplying power and the device receiving it. A general “PoE compatible” label without further data may not be enough to confirm the specific combination.

For an initial comparison, you do not need to memorise a power table. It is more useful to establish which mode your end device requires and which mode the selected port supports. If either side is unclear, leave compatibility unconfirmed until you find the exact specification.

Passive PoE requires an explicit check

Ubiquiti describes its passive PoE adapters that supply voltage continuously without negotiation. This example is a reason to consider passive power separately during selection. The word PoE does not imply that every adapter can be used with every Ethernet device.

If the existing network has an injector, record its exact reference together with that of the powered device. Do not rely on external resemblance, an identical connector or the fact that the adapter has already worked with another model. Compatibility of the old combination does not establish compatibility of the new one.

This is a pre-purchase check, not an invitation to test an unknown combination by connecting it. Where documentation does not clarify the power supply type, resolve the question using the device models. Substituting any other PoE adapter leaves the essential unknown unresolved.

How to add up the total PoE budget

The switch's total PoE budget must exceed the combined requirements of powered devices. This is a separate condition from the number of ports. A device with enough physical sockets may lack the total resources required for the planned combination.

Take a hypothetical example that does not describe specific products: two devices requiring 12 W each and a third requiring 18 W. The total is 42 W. With these entered values, seek a budget above 42 W and check each port's limits too. The numbers only illustrate the calculation, not standardised wattages for PoE classes.

For a real project, replace example values with documented requirements for the exact models. Avoid mixing differently described measures: if one sheet gives maximum demand and another an unspecified typical value, clarify before adding. Arithmetically correct totals do not fix incomparable input numbers.

Also check the most demanding device

After the overall total, go through the rows individually. Every device must have a suitable port with the power type required and a sufficient individual limit. Do not mentally distribute the total budget arbitrarily between ports: the switch documentation determines their capabilities.

If planning expansion, add future devices as separate, clearly labelled rows. Where models have not yet been chosen, do not present unconfirmed wattages as an exact calculation. This distinguishes the verified current configuration from a future version that still requires data.

In the hypothetical example with a total of 42 W, let the selected switch have an overall budget of 60 W, while the port considered permits only 15 W. The overall check would pass, but the device requiring 18 W would not meet that port's limit. This is a purely arithmetic illustration with invented values, rather than a characteristic of a real model. It shows why the two checks cannot be replaced by a single general line saying “sufficient power”.

For the same reason, when changing an end device, update its row, the total and the corresponding port. You do not need to start the entire list again, but the old result no longer verifies the changed combination. Record which model has been replaced and where the new data come from.

Power supply does not determine the network role

For a Wi-Fi device, checking does not stop at PoE. Access Point mode extends a wired network with Wi-Fi, while Range Extender mode relays an existing wireless network. This distinction, described in the TP-Link mode guide, is useful when formulating the task.

For example, if an Ethernet connection reaches the desired location, record it as a condition when seeking an access point. If the task is relaying an existing wireless network, record that instead. Do not assume every model offers both modes simply because it belongs to the general Wi-Fi device category.

Router and Access Point are also different roles, even when one device offers both. Check the functions available in the selected mode and specific firmware. PoE support answers the power supply question; it does not verify the network function required.

Prepare a list that can be checked

For each end device, record its model, quantity, PoE type, power required and planned network role. Beside them, add the switch or injector model and port being compared. Store the datasheets with the list so that the source of every value remains clear.

Organise the other components separately. The copper network systems section is a suitable starting point for the cabling, and communication cabinets and accessories for its placement. Choosing from these categories does not replace checking the electrical requirements of PoE devices.

The final comparison has four separate outcomes: confirmed power type, suitable individual ports, sufficient total budget and required network role. If a field is empty, formulate that exact point as a question. This is more useful than the general "will they work together?" because it shows what has already been established and which information remains missing.