I can't say how things work on mUPS in particular. I have read an explanation by @Normis that when device (a RB device with PoE in) has connected both PoE in and barrel-plug, it'll use which ever power source has higher voltage by some margin (makes sense as it only involves a pair of diodes to make this happen). If one intends to use both power supply options, it is best to have some systematic difference in voltages so that power source used won't change too much.
The scenario of changing power source is the following: let's say that PoE in has 0.5V lower than barrel. Then device will start using barrel power source. As power draw increases, the power adapter will drop voltage slightly and it might decrease to 0.5V below the PoE in. So device will switch over to using PoE in. This will drop voltage on PoE in and allow barrel to increase again, causing barrel voltage to rise significantly above PoE in voltage. And RB will change power source over again.
This changing doesn't harm much the device itself as voltage fluctuations won't be critically high. However it might upset both power adapters due to constant switching between full power draw and idle states.
One of intended installation places of a mUPS is also directly on tower / antenna pole, where AC power might not be available. Hence possibility to power device over ethernet cable using provided PoE injector. If, however, AC power is available near the mUPS, it is always advisable to use barrel-type power input due to lower power drop.
Your scenario is quite feasible if mUPS behaves the same way as other RB devices
. What I'd do is the following:
- feed barrel-type power input using provided 24V power adapter
- feed PoE-in using provided PoE injector and non-standard 18V, 2A adapter (this gives same wattage as provided power adapter).
Possibly use short ethernet cable between PoE injector and mUPS so that voltage drop (and power loss) is kept at minimum. This might not be feasible if you need long ethernet cable to connect to some switch ...
Accroding to mUPS brochure, mUPS regulates input voltage to 24V if input voltage is lower than that. Hence mUPS-powered devices won't notice the switch-over from barrel power source to PoE in.