I did an analysis on the consumptions of my racket population (router, switch etc) and have two questions on my switch:
On the Health tab it states PSU1 = 0mA @ 26.47V and PSU2 =112mA @ 52.67V, which results to 5.9W according to some online calculator. But when I measure the consumption with a shelly plug it says 25W. Interestingly connecting my Unifi AC adds 8W to that measurement, which is the same as statet on the PoE tab of the Switch. So where does the difference come from??
Are there any ways to optimize the power consumption of the Switch? Would it make it better or worse to switch from SwOS to ROS?
8 devices are connected, one with PoE
Health tab shows measurements on the DC output of the power supply.
The “shelly plug” is on the AC input of the CRS328 built-in power supply.
No power supply is 100% efficient, there are always some losses.
Cheap watt-meters are not very accurate either - especially with low power and non-linear loads like switching power supplies.
No need for an online calculator for a simple DC power calculation, just multiply voltage by current.
With AC it is more complicated, as there may be phase shifts and harmonics. Sometimes different electricity meters (old mechanical one vs new electronic one) measure quite different results.
If you need just one PoE device, it may be better to use separate PoE injector and a switch without PoE (like CRS326) which has a smaller power supply, wasting less power at light load.
It’s even worse: the rated efficiency (e.g. 92%) is only achieved when power supply works close to or at rated capacity. When under light load, efficiency goes down and at (near) idle state it still consumes quite considerable input power.
I always interpreted the figures on the health tab to be only the power consumption of the POE devices fed by the switch.
Total consumption from the wall is then the POE load + the consumption of the switch itself + power supply loss.
