How to calculate a PoE budget
Each powered device (PD) lists its maximum draw. The switch (PSE) has to supply that power plus what is lost as heat in the cable. Add up all ports and leave headroom so the switch never hits its total PoE limit, which would shut off ports.
Example
8 IP cameras at 10 W each on 50 m runs need 802.3af. Cable loss at 50 m is about 8%, so each port supplies ≈ 10.9 W. 8 × 10.86 × 1.2 = 104 W, so choose a switch with at least a 120 W PoE budget.
PoE standards
| Standard | Power at switch | Power at device | Typical devices |
|---|---|---|---|
| 802.3af (PoE) | 15.4 W | 12.95 W | IP phones, basic cameras |
| 802.3at (PoE+) | 30 W | 25.5 W | Wi-Fi 6 APs, PTZ cameras |
| 802.3bt Type 3 | 60 W | 51 W | Wi-Fi 7 APs, video displays |
| 802.3bt Type 4 | 90 W | 71.3 W | Laptops, large PTZ with heaters |
Frequently asked questions
How do I know if my switch has enough PoE power?
Compare the calculated budget with the switch's total PoE budget in its datasheet, not the per-port maximum.
Does cable length affect PoE?
Yes. Longer cables lose more power as heat. The standards budget for the worst case at 100 m, which is why switch output is higher than device input.
Can a PoE+ switch power a PoE device?
Yes. PoE standards are backward compatible; the switch only delivers what the device requests.