How it works
The formula, without the black box.
planned watts = cameras × maximum watts each × (1 + headroom)
This is an electrical budget, not an energy-bill estimate. The switch’s own AC consumption and power-supply efficiency are separate from the PoE power it can deliver.
Inputs that deserve a second look
- Maximum draw
- Use the exact camera datasheet and the most demanding enabled configuration.
- Headroom
- Capacity held for startup variation, environmental derating, and modest future change.
- Total PoE budget
- The switch datasheet’s deliverable PoE wattage, not its AC input rating.
- Per-port limit
- The maximum the relevant port/standard can deliver to one device.
Common planning mistakes
- Multiplying typical daytime power while ignoring night IR or heaters.
- Treating total budget as proof that each port supports the camera.
- Assuming every switch port supports the same PoE standard.
- Ignoring cable, temperature, injector, midspan, and UPS constraints.
Questions installers ask
Is 25% headroom required by a standard?
No. It is a configurable planning choice, not a universal requirement. Use the margin appropriate to the equipment, environment, and expansion plan.
Why can the total pass while per-port fails?
A switch may have ample aggregate watts but cap each port below one camera’s maximum requirement.
Does this choose 802.3af, at, or bt?
No. Compare both required wattage and the camera/switch PoE class and standard in their datasheets.
Sources and boundaries
These links support the method or terminology. They do not turn a planning estimate into a site guarantee.
IEEE 802.3 Power over Ethernet overview ↗
Primary standards reference; equipment implementation still governs.