PoE Power Budget Planner
Check whether a switch can really power everything you plug into it.
What you provide
Printed on the datasheet as the total PoE power budget, not the power supply rating.
Most IP phones, older access points, basic cameras.
Wi-Fi 5 and 6 access points, pan-tilt-zoom cameras.
Result
PASS126.8 W spare out of 370 W
The budget covers these devices with 127 W to spare. Worth noting that switches negotiate down under LLDP where devices support it, so the real draw usually sits closer to the 206 W device figure than to the allocated total.
- Within budget
- Tight
- Over
Switch budget: 370 W
- Devices
- 12
- Switch budget
- 370 W
- Allocated at the portsWhat the budget is actually charged
- 243.2 W
- Drawn at the devicesThe lower figure a power meter would show
- 205.6 W
- Headroom
- 126.8 W
- Budget used
- 66%
What this cannot tell you
- Arithmetic on the class allocations you enter. It does not query your switch and cannot see what is really drawing power.
- Class allocation is a worst case that assumes a full-length cable run. Devices supporting LLDP often negotiate down to their real draw, so a switch may comfortably power more than this suggests.
Take this with you
How this calculation works
A PoE budget is spent at the port, not at the device. The standard reserves the difference between the two figures for cable loss, because the switch has no way to know how long the run is and 100 metres of thin copper really does lose that much. A camera drawing 12 W is therefore a 15.4 W line item against the budget. This adds up the port allocations for every device class you enter, compares that against the switch budget, and shows the lower device-level draw alongside it so you can see the gap.
What the results mean
- Allocated at the ports
- What the switch charges against its budget. This is the number that decides whether ports stay up, and it is always higher than what a power meter would show.
- Drawn at the devices
- What the connected equipment actually consumes. Useful for sizing a UPS, and misleading if used to plan the switch budget.
- Headroom
- Watts left unspent. Below about 15% of the budget, you cannot add another PoE+ device without a rethink.
Common problems and fixes
- Some devices lose power after a building power cut
- This is the classic overbooked-budget failure. Everything restarts at once and every device requests its full class allocation before negotiating down, so a switch that was stable at steady state exceeds its budget during startup and sheds ports in priority order. Set port priorities deliberately, or move devices onto a second switch.
- A device negotiates a lower class than it should
- Check the cable before the device. Copper-clad aluminium and damaged pairs increase resistance, the voltage at the far end sags, and the powered device either falls back to a lower class or refuses to come up at all. Longer runs make it worse. Try the same device on a short known-good patch lead at the switch to separate the two.
Frequently asked questions
Why is 802.3af listed as 15.4 W when devices only draw 12.95 W?
The 2.45 W difference is the loss budget for the cable. The standard guarantees the powered device gets its 12.95 W even at the end of a full 100 metre run of specification-compliant copper, so the switch has to send more than the device receives. Your budget is charged the sending figure.
Can I use a PoE injector instead of a bigger switch?
Yes, and it is often the cheapest fix for one or two power-hungry devices. An injector sits between the switch and the device and supplies power from its own adapter, taking that device off the switch budget entirely. The trade-off is another mains socket and another thing to fail quietly.
Does a passive PoE adapter count?
Passive PoE does not negotiate at all. It puts voltage on the line permanently and expects the far end to cope, which means plugging the wrong device into it can destroy that device. It is not part of the 802.3 standards and should not be mixed with equipment that expects proper negotiation.
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