PoE budget calculator: switch power for access points, cameras and phones

List the powered devices on a switch by group, pick each group's PoE class, enter the switch's PoE budget and how much of it you want to keep free, and the calculator tells you whether the plan fits. It shows the watts each group reserves, the total, the usable part of the budget and a one-line verdict: fits, over the headroom, or over budget.

How it works

Each PoE class has a maximum the switch port (the PSE, power sourcing equipment) must be ready to deliver. The powered device (the PD) at the far end gets less, because up to 100 meters of cable turns some of the power into heat. A switch's PoE budget is counted at the port, so the calculator adds up port-side watts:

group watts  = device count * class watts at the port (PSE)
total        = sum of all groups
usable       = switch budget * (1 - headroom / 100)

verdict      = fits                 if total <= usable
               over the headroom    if usable < total <= switch budget
               over budget          if total > switch budget

The classes in the menu, with the device-side figure for comparison:

You can have up to eight device groups. A blank device name shows as "devices" in the output. Headroom must be at least 0 and below 100 %; anything else shows an error instead of a verdict.

Worked example

The calculator opens with a small office: six Wi-Fi access points at Class 4, four IP cameras at Class 0 / 3 and eight desk phones at Class 2, on a switch with a 370 W PoE budget and 20 % headroom.

access points   6 * 30   = 180.0 W
cameras         4 * 15.4 =  61.6 W
desk phones     8 * 7    =  56.0 W
total                    = 297.6 W on 18 ports
usable          370 * (1 - 20 / 100) = 296.0 W
297.6 > 296.0, and 297.6 <= 370

Everything would power up: 297.6 W is well inside 370 W, with 72.4 W left on the switch. It just breaks the 20 % margin by 1.6 W. Move one camera to another switch and the total falls to 282.2 W, which fits with 13.8 W to spare. At the device end, the same eighteen devices could draw at most about 257 W (6 x 25.5 + 4 x 12.95 + 8 x 6.49); the other 40 W or so is the allowance for cable loss that the switch has to hold in reserve.

Limits and gotchas

• Port watts and device watts are different numbers. A datasheet that says an access point "uses 25.5 W" is quoting the device side; the switch budgets 30 W for it. Add up device-side figures in a spreadsheet and a PoE+ plan comes out 15 % short.
• Switches allocate power in different ways. Some reserve the class maximum as soon as a device is classified, some allocate what the device asks for over LLDP, and some count only what it actually draws. The calculator uses the class maximum, the conservative case. A switch that allocates by real draw can power more devices from the same budget, but the margin then depends on load: cameras switching on infrared heaters at night, or access points running every radio at full power.
• Class 0 means "no class given". A device that does not announce a class is treated as 15.4 W, even if it uses 3 W. If the datasheet lists a lower class, pick that.
• The total budget is not the per-port limit. An 802.3bt device on an 802.3at port gets at most 30 W and may start with features switched off, or not at all. Check which ports support which standard as well as the total.
• The datasheet budget may assume a particular power supply, or two supplies installed. A switch running on one supply of a pair may offer far less PoE than the headline figure.
• PoE watts are part of the load on your UPS, on top of the switch's own consumption. Size the UPS and its shutdown thresholds for the full draw, as in UPS monitoring with NUT.
• Keep watching it after installation. Many managed PoE switches expose the budget and the measured consumption over SNMP through POWER-ETHERNET-MIB (RFC 3621) or a vendor MIB. Graph it next to the other power and sensor readings described in what you can monitor, and alert before the measured draw reaches the usable figure.

Cameras and access points usually belong on their own VLANs as well as their own power plan; the home VLAN layout shows one way to split them.

Questions

How many PoE+ access points can a 370 W switch power?

Nine, if you keep 20 % free: 9 x 30 W is 270 W of the 296 W usable, and a tenth would need 300 W. Without headroom, 12 fit in 370 W with 10 W left over.

What is the difference between PSE and PD watts?

PSE watts are what the switch port must be able to supply; PD watts are what the device can count on receiving after the cable. For PoE+ they are 30 W and 25.5 W. Budgets are counted at the PSE.

What headroom should I keep on a PoE switch?

20 % is a common rule of thumb. It covers devices added later, power-hungry modes such as camera heaters, and switches that allocate by class while devices draw more than expected.

What does over the headroom mean?

The total fits inside the switch budget but eats into the margin you asked to keep free. Everything would power up today, with less room for growth or for a busy night.