In short. The PoE budget is the total power a switch can deliver across all ports at once, and it is always lower than the sum of the port maximums. Size it by device class: 12.95 W for an 802.3af camera, 25.5 W for an 802.3at access point, 51 and 71.3 W for 802.3bt devices. Add cable losses — up to 12 % over a hundred metres — and 20 % headroom. Twenty cameras plus eight Wi-Fi access points call for a switch of roughly 600 W.
Power is counted at the device: 12.95 W for af, 25.5 W for at, 51 and 71.3 W for bt Type 3 and Type 4. Twisted-pair loss is up to 12 % over 100 metres. A 20 % margin is added for devices you will connect later.
Why port power and switch budget are different numbers
A 48-port switch supporting 802.3at could in theory deliver 48 × 30 = 1440 W, but the actual power supply is rated at 370 or 740 W. That is the PoE budget: how many devices you can power at the same time. Once the requests exceed it, the switch starts shutting ports down by priority — usually from the highest numbers.
The standard specifies power at the port output, not at the device: 15.4 W for 802.3af, 30 W for 802.3at, 60 and 90 W for 802.3bt Type 3 and Type 4. What reaches the device is less — 12.95, 25.5, 51 and 71.3 W respectively. Budgeting from device consumption plus losses is the accurate way; otherwise the margin ends up twice what is needed.
What eats the power
Cable length dominates: twisted-pair resistance over a hundred metres costs about 12 % of the power. On short runs within a floor the loss stays at 3 to 5 %. Next comes cable quality: copper-clad aluminium instead of copper adds another 10 to 15 % and heats up, which is why only solid copper belongs under PoE.
Third is duty cycle. A camera with infrared illumination draws twice as much at night as during the day, and a Wi-Fi 6 access point with two radios hits its nameplate maximum under load. That is exactly why the calculation carries 20 % headroom rather than 10.
Choosing the switch
An office of twenty to thirty devices is covered by a 24-port unit with a 195 or 370 W budget. Video surveillance with infrared cameras calls for 370 W over 24 ports, 802.3bt for 740 W over 48. Models with prices: HPE Aruba Instant On switches and the full managed switch section. Access point and port planning for an office is covered in the Aruba Instant On guide, and server sizing for cameras in the video surveillance guide.