PoE Budget Calculator
Load, headroom, and verdict for PoE switches.
What it does
PoE switches carry a finite power budget shared across every camera, phone, and access point. Port counts mislead because the budget runs out before the ports do on fully loaded switches.
The tool above takes switch budget with device count and watts each. It returns total load with headroom and a within or over verdict, so shortfalls show before install day.
The defaults load 8 devices at 7 W on a 120 W budget: 56 W load, 64 W headroom, 46.7% used. Twelve 12 W devices instead draw 144 W and run 24 W over.
Classes follow IEEE 802.3af, at, and bt with roughly 15, 30, and 60 to 90 W per port at the source. Ethernet Alliance guidance matches the same three generations with class 8 near 71 W at the device.
How to use
- Enter switch PoE budget in watts.
- Enter powered device count.
- Enter watts per device.
- Read total load first.
- Read headroom beside it.
- Keep margin for inrush and growth.
Formula
- budget
- Switch total PoE watts available.
- load
- Sum of device draws in watts.
- headroom
- Budget minus load; negative means over.
Totals pass while single ports fail: per port class caps and cable loss sit outside this line. Budget in device watts against source watts, and keep 20 to 30% headroom for startup surges.
Worked example
Example: 8 devices at 7 W on 120 W
Defaults: budget 120 W, 8 devices, 7 W each.
- Load: 8 x 7 = 56 W.
- Headroom: 120 - 56 = 64 W.
- Used: 100 x 56 / 120 = 46.7%.
- Verdict: within budget.
- Tool shows 56 W load, 64 W headroom.
- Twelve 12 W devices draw 144 W: 24 W over.
Reference table
| Plan | Load | Result |
|---|---|---|
| 8 x 7 W | 56 W | 64 W headroom |
| 4 x 25 W | 100 W | 20 W headroom |
| 12 x 12 W | 144 W | 24 W over |
Limits
- Totals only; per port class separate.
- Cable loss excluded from load.
- Inrush exceeds steady state.
- Passive nonstandard PoE differs.
Common mistakes
- Counting ports instead of watts.
- Mixing device and source watts.
- Zero headroom at install.
- Ignoring per port class caps.
- Long thin cable runs unaccounted.
Try it yourself
Quick checks. Answers sit tucked under each question.
12 devices at 12 W on 120 W?
144 W load, over budget by 24 W.
4 devices at 25 W on 120 W?
100 W load with 20 W headroom.
Why keep headroom?
Inrush, growth, and warm switches eat margin.
Questions
What is PoE budget?
Total watts a switch can feed all powered devices together.
af vs at vs bt?
Roughly 15, 30, and 60 to 90 W per port at the source.
Device vs source watts?
Cable loss sits between; budget the source side.
How much headroom?
20 to 30% covers inrush and one more device.
Cameras typical draw?
Often 7 to 12 W, more with heaters and IR.
Budget passes but port fails?
Per port class caps bind first; move the device or port.