The question we get most often at the counter is "I need an eight-port switch." That is the wrong question, or rather, it is only half of the question. The ports tell you how many cables you can plug in, not how much power you can send through them. The number that determines whether the system works or shuts down halfway is called the PoE budget; it is measured in watts and is written on the switch's datasheet.
What PoE budget means
A PoE switch powers devices through the same network cable, but it has an overall power ceiling to distribute among all ports. An 8-port switch with a 120 W budget does not give 120 W to every port: it gives 120 W in total. If each camera requests 12 W, eight cameras total 96 W and we are within the limit; if two of those eight are motorized and have heaters, the math fails and the last powered ports turn off—usually at night, when the infrared lights turn on and consumption increases.
The standards tell you the maximum per port: 802.3af reaches 15.4 W delivered (12.95 W to the device), 802.3at, or PoE+, reaches 30 W (25.5 W to the device), and 802.3bt goes up to 60 W and beyond. The total budget of the switch is a different matter and should be checked first.
How much what you plug in actually consumes
These are the values we use for our quotes, taken from real consumption rather than theoretical maximums. A fixed indoor camera is between 4 and 8 W. An outdoor bullet or turret camera with infrared or white light is between 8 and 12 W, with the peak occurring at night. A motorized camera with pan-tilt and a heater requires 30 to 60 W and must be placed on a dedicated Hi-PoE port. A modern Wi-Fi access point is between 15 and 25 W. An IP video intercom outdoor station is around 10 W.
Add two margins to these figures. The first is cable loss: on a long run with medium-quality cable, you lose 10-15 percent between the switch and the device. The second is the growth margin: leave at least 20 percent free, because that extra camera always comes up, and it always comes up when the job site is closed.
The math, in practice
Six outdoor cameras with infrared: 6 × 12 = 72 W. Add the losses (about 10 W) and you are at 82 W. With a 120 W switch, you have 38 W free, meaning space for two more cameras or an access point. If, however, those six cameras become twelve, 12 × 12 = 144 W plus losses: the 120 W is no longer enough, and you need a 250 W device. With twenty-four capture points, you move up to the 370 W range.
The opposite rule applies when the budget is abundant: a very generous switch on a small system does no harm, but it is money sitting idle. The break-even point is covering the actual consumption plus one-fifth.
Not all ports are created equal
Many devices reserve one or two ports for high power, for motorized units and radio points. If you have a PTZ, check that there is at least one Hi-PoE port and that it is among those you have left over after assigning the others. And check the port priority: when the budget runs out, a managed switch shuts down the ports with the lowest priority, and you want to decide which ones, not leave it to chance.
What to look for on the datasheet
First the total budget in watts, then how many ports are PoE (often not all of them are: uplinks almost never are), then the supported standard, then whether the switch is managed, because management is what allows you to see how much power each port is drawing instead of guessing.
In our catalog, you can find the complete range in managed switches and unmanaged switches, with budgets from 60 to 370 W and multigigabit versions. If you prefer that we do the math, write the list of devices to be powered to info@elettricity.com or call 351 969 5157: we will tell you which device can support the system, today and two years from now.