How to choose a PoE switch: the total power budget is the number that catches people out

Introduction

A PoE switch datasheet carries two wattage figures, and most people read only one of them.

"Eight PoE+ ports, 30W per port" sounds like eight devices drawing 30W each. In practice, further down the same datasheet there is usually a line reading total power budget: 120W. Eight times thirty is 240 — this switch can deliver 120.

What happens then: the first few devices come up fine, later ones do not start, or every camera begins rebooting in turn. The switch log will show power budget exceeded — but if you sized the installation from "30W per port", you will suspect the cabling and the cameras first.

This article sets out what actually needs calculating.

1. The three generations of PoE

There are two wattage numbers: what the switch sends and what the device receives. The gap is cable loss. Datasheets usually quote the sending side, device manuals usually quote the receiving side — align them before comparing.

Fixed cameras mostly sit in af or at (before picking models, see cybersecurity certification for IP cameras — that is where tender requirements bite); outdoor domes with heaters, PTZ units, and access points that power other devices move up a tier. IR illuminators and heaters are the heavy consumers, and they only run at full load at night and in winter — a figure measured during a daytime handover will read low.

2. Sizing the total budget safely

Do not use "per-port maximum × number of ports"; that number means nothing. Add up the devices you are actually connecting, then leave headroom.

A typical small site:

Buy a switch with a 180W budget for this site and the arithmetic works — the installation will not. On the winter night when every heater is on and every camera has switched to infrared, there is no headroom at all.

Target 70–80% utilisation. A 178W requirement wants a 240W switch or larger. Also note that the switch's own consumption does not come out of the PoE budget, but the power supply has to carry both.

3. Class is something the device declares

PoE negotiates: a device tells the switch which Class it belongs to, and the switch reserves the corresponding wattage. Two practical consequences follow.

First, the switch reserves the Class ceiling, not actual consumption. A camera that really draws 6W but declares Class 4 will have 30W reserved for it. Eight such devices consume a 240W budget while actually using 48W between them. Some switches can allocate by measured consumption instead — that setting is worth finding.

Second, devices that do not negotiate properly cause trouble. Cheap non-standard equipment may draw power without negotiating, or use a proprietary scheme. Connected to a standards-compliant switch there are two outcomes: no power, or power that the switch cannot account for in its budget. The second is worse, because it silently invalidates your total and leaves nothing to find.

4. Cable and distance

The 100-metre limit in PoE is a networking limit, not an electrical one. Power does however lose energy over that distance, which is exactly why the sending and receiving wattages differ.

Common problems on site:

• CCA (copper-clad aluminium) cable — cheaper than solid copper and far more resistive; the voltage drop starves devices at the far end. Whatever you save on cable comes back as a second site visit
• Existing legacy cable — building cabling may have only four conductors. Early PoE runs on that; the high-power modes of 802.3bt need all four pairs
• Joints and patch outlets — every connection adds resistance, and the difference between a run with three joints and a straight pull is far more visible under PoE than with data alone

Some switches offer a long-range mode that trades bandwidth (dropping to 10Mbps, for instance) for power delivery beyond 100 metres. Cameras rarely need gigabit, so this is genuinely useful in corners that cannot reach the comms room — provided the far-end device accepts the reduced speed.

5. A checklist before you buy

1. List the actual devices and add them up — never "per-port maximum × port count"
2. Use peak, not average — IR illuminators and heaters run fully only at night and in winter
3. Leave 20–30% headroom so that adding devices later does not mean replacing the switch
4. Check which generation each device needs — one device requiring bt means the whole switch must support it
5. Is the cable solid copper? How many conductors does the existing cable have? Ask both before quoting, not on installation day
6. Do you need management? PoE status, per-port power cycling and overload logs exist only on managed switches. Past ten devices on a site, that pays for itself at the first fault

On the sixth point: per-port remote power cycling is the most useful feature of a managed PoE switch. When a camera hangs, cutting and restoring power to that one port from the office — versus sending someone up a ladder — is an entirely different maintenance cost.

Three questions we ask first

1. How many devices, and what are they? This sets the total budget, and it is the only number that genuinely needs calculating
2. Is the cabling existing or new? Conductor count and material in existing cable limit which generation of PoE you can use
3. Where are the devices mounted? Outdoors, in unmanned rooms, or high up — each of those turns remote power cycling from a nice-to-have into a requirement

Models are listed in the switch catalogue. For where the recordings go, how to choose a NAS covers bays and network ports.