How it works
A switch can have enough total power but still fail to power a device on an individual port. This calculator checks three separate constraints: total source allocation against the budget after reserve, the total device count against available PoE ports, and each active group’s per-device allocation against the entered per-port limit. The result assumes one device per port and the same source limit on every selected port. For switches with mixed port capabilities, evaluate the groups against their actual assigned ports separately.
The formulaRequired source watts = sum of (device count × source watts per device)
Budget after reserve = total PoE budget × (1 − reserve % ÷ 100)
Minimum total budget = required watts ÷ (1 − reserve % ÷ 100)
Put it into practice
EXAMPLEFour devices allocated 15 W each and two allocated 25 W each need 110 W and six ports. A 150 W switch with 20% held in reserve has 120 W available, leaving 10 W of planned headroom. With eight 30 W ports, the entered checks pass.
EXAMPLEOne device allocated 45 W on a 30 W port exceeds the per-port limit even if the switch has a 200 W total budget. Increasing the total budget alone cannot fix that port limitation.
Good to know
Why use source watts instead of device consumption?
Power is lost in the cable, and a switch may reserve a power class or negotiated allocation rather than the current measured draw. Device-side watts and source-side allocations are not interchangeable. Consult the device and switch documentation; this tool does not guess cable losses or the negotiated class.
Does passing mean any PoE device will work?
No. Check supported PoE standards/types, power classes, cabling, temperature limits and any vendor requirements. Passive PoE and proprietary powering methods need separate compatibility checks. This arithmetic does not verify voltage, pinout or electrical compatibility.
Is a 20% reserve mandatory?
No. It is an editable planning assumption. Here, 20% means leaving 20% of the switch budget unused, not adding 20% to device demand. A 100 W load therefore needs a 125 W total budget at that setting.
What about heaters, infrared illumination and startup?
Use allocations that cover maximum required operation, including startup and enabled accessories. Typical idle consumption can be much lower. The total PoE budget should be the manufacturer’s device-power budget, not the AC input power-supply label.