Is a switch's PoE budget its ports times 30 watts?

Almost never. The budget is set by the power supply, so a 48-port switch offering 30 W a port commonly has 370 or 740 W in total rather than 1,440.

Why does the switch supply more than the device uses?

Cable loses power as heat, and the loss grows with distance. 802.3at supplies 30 W at the switch for about 25.5 W at the device. Budget against the switch figure.

What happens when the budget runs out?

Ports stop delivering power, usually the lowest priority ones. It often happens when everything powers up together after an outage rather than when you add the device.

How much headroom should I leave?

Around 20% is a reasonable default. It covers a replacement device drawing more than its predecessor and the surge of everything starting at once.

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PoE Power Budget Calculator

A switch has a total power budget that is usually far less than its ports could each deliver. Adding up the devices before you buy is cheaper than finding out afterwards.

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Switch PoE budget (watts)

Headroom to keep

Devices

Classes, budgets, and where the watts actually go

The budget is not the port maximum times the port count. A 48-port switch advertising 30 watts a port very rarely has 1,440 watts to give. It has a power supply, and that supply is the real limit — often 370 or 740 watts. Running out means ports stop delivering power, usually the ones with the lowest priority, and usually at the worst time.

The standards

802.3af (PoE) — up to 15.4 W at the switch, about 12.95 W at the device.
802.3at (PoE+) — up to 30 W at the switch, about 25.5 W at the device.
802.3bt Type 3 (PoE++) — up to 60 W at the switch, about 51 W at the device.
802.3bt Type 4 — up to 90 W at the switch, about 71 W at the device.

The two numbers differ because cable loses power as heat. The switch has to supply more than the device consumes, and the gap widens with distance. Budget against what the switch supplies, which is what this calculator does — a device drawing 25.5 W costs you 30 W of budget, not 25.5.

Reserve headroom. A budget consumed to 100% has no room for a replacement camera that draws a little more than the one it replaced, and no margin for the surge when everything powers up together after an outage.

Related tools

Camera Network Planner covers the bandwidth and storage those cameras also need. VLAN + Subnet Designer for the addressing they sit on.