PoE budget: how to size a PoE switch

Two limits: power per port and the total budget

Every PoE switch data sheet gives two figures, and you need both:

• Power per port: the highest PoE class a port can deliver, for example 30 W on a PoE+ (802.3at) port or 60 W or 90 W on an 802.3bt port. If a device needs more than the port can give, it will not run at full function.
• Total PoE budget: the power the switch can share across all its PoE ports at once. Cisco puts it plainly for the Catalyst 1200: PoE+ models give "Maximum power of 30.0W to any Gigabit Ethernet port until the PoE budget for the switch is reached".

So a 24-port PoE+ switch with a 195 W budget can power any one port at 30 W, but not all 24 at once. The budget, not the port count, usually decides which model you need.

PoE classes: what the switch reserves and what the device gets

The IEEE standards define eight power classes. The switch (the power sourcing equipment, PSE) reserves more than the device (the powered device, PD) can use, because some power is lost in the cable:

Figures from the Ethernet Alliance's Overview of 802.3bt Power over Ethernet (opens in a new tab). It also notes that "For assigned Class 5 and higher, power delivery requires 4 pairs to be used", and that Class 0 is an alias for Class 3. Vendor names sit on top: Cisco's UPOE and UPOE+ are its 60 W and 90 W options, and Fortinet's -FPOE FortiSwitch models power every copper port. The managed PoE switches page explains the vendor terms.

Step 1: list every powered device and its class

Write down each device that will take power from the switch, with the PoE class or maximum power from its own data sheet:

• Access points: the data sheet gives the class needed for full function, and often a lower class with some radios turned off. Wi-Fi 6E and Wi-Fi 7 models frequently need 802.3bt.
• Desk phones and conference phones: usually a low class, but colour-screen and video models draw more. VoIP handsets are covered on voip-phone-system.uk (opens in a new tab).
• Cameras, door entry, sensors and thin clients: check each one; heated outdoor cameras and pan-tilt-zoom models need more than fixed indoor ones.

Some access point figures, as their vendors publish them:

Where a data sheet gives only a maximum wattage rather than a class, size on the class that covers it: a device that draws up to 19.5 W needs a Class 4 (PoE+) port.

Step 2: add up what the switch must reserve

Most switches reserve power by class when a device connects, so the safe sum uses the switch-side (PSE) figure for each device's class: 30 W for every Class 4 device, 45 W for Class 5, 60 W for Class 6, and so on. Some switches allocate on the power a device actually draws, or adjust it once the device reports its needs over LLDP (the Ethernet Alliance overview describes LLDP being used "to fine tune their power demand"), which can leave more room. That behaviour differs by vendor and model, so we plan on the class and treat anything better as extra headroom.

Step 3: add headroom, then pick the switch

Allow for the devices you will add, for access points being swapped for hungrier models at the next refresh, and for one or two ports used at a higher class than planned. We usually plan 20 to 25 per cent headroom on the PoE budget and keep a few spare powered ports.

Then shortlist switches where both limits fit: enough ports of the right class (802.3bt ports for Wi-Fi 7 access points that need them) and a total budget above your sum plus headroom. The managed PoE switches page compares budgets by range.

Worked example 1: a 30-device office on PoE+

An office with six Wi-Fi 7 access points, twenty desk phones and four cameras, all on one 48-port switch:

Switches to compare against that total (budgets from each vendor's own data sheet or part-number description):

The 375 W and 370 W models look close, but they are below the 381.6 W total before any headroom. A 740 W full-PoE model is the comfortable choice; two smaller switches would also work if the devices are split across them.

Worked example 2: Wi-Fi 7 access points on 802.3bt

A floor with eight Wi-Fi 7 access points that need 802.3bt Class 5 for full function:

Here the per-port class matters as much as the budget: a PoE+ switch could run these access points only with reduced radios, if at all. Switches with 802.3bt (or Cisco UPOE) ports and enough budget:

Wi-Fi 7 access points also have 2.5G, 5G or 10G uplink ports, so check the port speeds on each model's page as well. See Wi-Fi 7 access points for what each one needs.

Power supplies: capacity or a spare

On modular switches the budget depends on the power supplies fitted. Cisco's Catalyst 9200 data sheet gives two figures per model:

If your sum relies on the second supply's capacity, losing one supply can reduce the power available to PoE ports, so decide whether the second supply is there for capacity or as a spare, and size the budget on the answer. We set this out in the quote.

Other things that change the sum

• PoE on the firewall: some desktop firewalls have a few PoE ports of their own, such as the FortiGate 50G-SFP-POE and the Cisco Secure Firewall 240P (four PoE+ ports, up to 25.5 W each and 120 W in total, in Cisco's data sheet). Handy for one access point and a phone in a small office.
• Cabling: Class 5 and above use all four pairs, so check older or patched cabling before relying on 802.3bt.
• Stacks: on stacking switches each member keeps its own PoE budget unless the vendor's power-sharing feature is in use; we check this per model.
• Port priority: managed switches let you choose which ports keep power first if the budget runs short, so access points and phones can be protected over less critical devices.

How to order PoE switches

How ordering works. Press Add next to a part number and it goes into your basket; where a subscription or support contract goes with the hardware, you choose it in the same step. Then View basket and Request a quote. We check every line, quantity and term against your sites and send back a personalised quote by email. Nothing is charged online. We can also design, supply, stage, install and configure the equipment, with the scope confirmed at quote (see network installation).

Contact us with your list of powered devices and we will do the sum and suggest the switch, free of charge, before you buy.