Power over Ethernet is the reason a modern camera needs one cable instead of two. It is also the single most common place a camera system is quietly under-specified.
How it works, briefly
The switch puts DC power onto the same twisted pair that carries data. The camera takes what it needs. There is a negotiation first, so a device that does not want power does not get any, which is why plugging a laptop into a PoE port does not damage it.
The classes worth knowing
There are three generations in common use. 802.3af, the original, supplies up to 15.4 watts at the switch port. 802.3at, usually sold as PoE+, supplies up to 30 watts. 802.3bt, sometimes called PoE++, goes to roughly 60 or 90 watts depending on type.
Note that those are numbers at the port, not at the device. Some of it is lost in the cable, and the loss grows with distance. A device at the end of a long run gets meaningfully less than one three meters away, which is why the specification is written with that headroom built in.
The budget mistake
A switch has a total power budget as well as a per-port maximum. A sixteen port switch does not necessarily power sixteen cameras at full draw. If the budget is exceeded the switch starts dropping devices, and it will usually drop them at the worst moment.
That moment is night. A camera with infrared illuminators draws far more power in the dark than in daylight, and a heater in an exterior housing draws more again in a Denver winter. A system sized against daytime draw is a system that browns out on the first cold night, which is exactly when it was supposed to be working.
What to ask about on a quote
The switch model and its total PoE budget. The class each camera needs. The expected draw with illuminators and any heaters running, not the idle figure. And whether there is headroom left for the cameras that get added later, because there are always cameras that get added later.
Injectors, midspans and the cheap switch problem
If a switch does not supply power, an injector can add it to a single run and a midspan can do it for several. Both are legitimate, and both are also how a system accumulates small boxes on shelves that nobody documented. If a design leans on them heavily, it is usually cheaper and tidier to buy the right switch.
The other trap is a switch advertised as PoE at a price that seems generous. Read the total budget rather than the port count, check which standard it supports, and check whether every port supplies power or only some of them. Passive PoE, which puts voltage on the line without the negotiation step, is a separate thing again and is not interchangeable with the standards above.
What to leave room for
Specify the switch with spare ports and spare watts. Cameras get added, and the second most expensive time to discover the budget is full is when somebody is already on site to add one.
Sizing the power budget against night draw rather than daytime draw is standard practice at Elite Security and is one of the quiet differences between two quotes that look alike. More in the blog, or start with low-voltage cabling.