Power budget math for PoE: Total power needed when connecting many devices to a single port
Power budget math for PoE: Total power needed when connecting many devices to a single port
Yes, if your 8-port PoE switch has a total power limit of 60 watts and you are only using 4 of those ports, then each one gets about 15 watts. That is the math for how it works.
That's usually how it goes. The details on what a switch needs are pretty clear. You might see some fancy jargon that sounds confusing, like "802.3af" with a link to Wikipedia. But the official specs will tell you everything you need to know without needing to guess.
It really depends on which switch you use and how smart that model is. If you don't check the details, you might not understand what's going on and your info could be old. Some Cisco switches can ask for 4 watts, or even 7 or 15 watts. If all your devices only want 4 watts, you can probably plug in more ports without a problem. I'm not sure if this is special stuff just made by Cisco or if it follows the standard rules for PoE and what regular consumer switches do. I know that even normal consumer switches can tell the difference between devices that use extra power (Poe+) compared to regular ones.
When you talk about power budget with PoE, it's not really just a money or saving thing. It is about how much power is being shared between the devices on the switch and how many devices you can plug in at once. I don't know why they use the word "budget" instead of saying "capacity," but that has always been the name for this since I heard it years ago.
Hey @Ralston18, adding more details to my story could help. I have four routers. Each one uses up to 12.4 watts of power. If my PoE switch splits its total power limit of 60w across all eight ports, then each port only gets about 7.5w. That would mean my stuff won't work on power at all. But what if I just use four ports instead? Would that give me 15w per device, which is exactly enough for them to run?
I can't tell what kind of switch you have because you haven't told me its name or model number. Like, one brand makes switches with eight ports, but only four of them use Power over Ethernet (PoE). The other four don't support it at all.
This question asks about documentation rules. Usually, switches give power to the first devices that are plugged in. If every device needs 15 watts but you only have a 60 watt budget, then only four ports will work because the rest cannot get enough power. The other ports stay dark if everyone else takes their share. However, if there are four ports asking for 12 watts each and you add a fifth port that also wants 12 watts, that's okay too. There are way too many ways to mix them up though, because some devices only need 7 watts. Kanewolf says sometimes switches don't have POE on every single port. In other words, if your switch has limited power for some ports, it gets harder to figure things out than when all ports can give full power at once. If you want to know which ones work and how much they take, reading the small print is best. Usually, switches do not split power evenly among all ports. You need a special POE port just to get up to 15 watts from it.