PoE Power Budget Calculator
Calculate whether a PoE switch can power its devices, accounting for cable length losses, the difference between what the switch sources and the device receives, and headroom for failover.
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The device receives less than the switch sends
Power over Ethernet standards specify two numbers for a reason. 802.3af sources 15.4 watts at the switch and guarantees 12.95 at the device; 802.3at sources 30 and guarantees 25.5; 802.3bt Type 4 sources 90 and guarantees 71.3. The difference is resistive loss in the cable, and the standards assume the worst case of 100 metres of category 5e. Sizing a budget on the device requirement rather than the sourced power under provisions the switch, and the failure mode is ports shutting down under load rather than a clear error.
The switch budget is shared, not per port
A 24 port switch advertised as PoE+ rarely supplies 30 watts to all 24 ports simultaneously, because that would need 720 watts of power supply. A typical model has a 370 watt budget, which is enough for twelve devices at full PoE+ or all twenty four at af levels. When the budget is exceeded the switch removes power from ports by priority, and if priority was never configured, it removes them in an order nobody chose.
Leave room for failover
Where two switches provide redundancy, each must be able to carry the full load alone, so each needs the total budget rather than half of it. This is routinely missed, and the result is a redundant pair that works until the day it is needed, at which point the surviving switch sheds half the access points. Planning at eighty percent of budget on each switch, with the ability to take the whole load, is what makes the redundancy real.
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Frequently Asked Questions
What is the difference between 802.3af, at and bt?
af sources 15.4 W and delivers 12.95 W, at sources 30 W and delivers 25.5 W, and bt has two types sourcing 60 W or 90 W and delivering 51 W or 71.3 W. The gap in each is the assumed worst case cable loss over 100 metres.
Why does my switch cut power to some devices?
The total budget is exceeded, so it sheds ports. Most switches shed by configured priority and, where none is set, by port number or in an unspecified order. Setting priority explicitly ensures the access points survive and the desk phones go first rather than the reverse.
Does cable length matter?
Yes. Resistive loss rises with length and with thinner conductors, so a device at 90 metres on cheap cable receives noticeably less than one at 10 metres on solid copper category 6. Copper clad aluminium cable is far worse and is a common cause of PoE devices that boot and then reset.
How much headroom should I leave?
Twenty percent under normal operation, and where two switches provide redundancy, each must be able to carry the entire load alone rather than half of it. A redundant pair sized at half each is not redundant.
Can I use PoE injectors instead?
For a handful of devices, yes, and they avoid the shared budget question entirely. They also add a power supply per device to fail, sit outside any UPS the switch is on, and remove the central visibility a managed switch provides.
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How to Use
Add your powered devices and switch budget to check the power allocation and cable losses.
Disclaimer: This tool is provided "as is" without warranty of any kind. Results are for educational and utility purposes.
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