as i said though (a few times now), with ubiquiti kit you have to make sure you match the correct POE variant.
some devices are 24V poe and some are 802.3af, use the wrong one and you may blow the device.
the ER-X has POE 24v passthru, so the ER-X is powered via an POE injector and the AP is powered via POE from the ER-X.
this is fine as i only have one device needing POE power (the AP).
as a compariosn, the Unifi US-8 60W switch has 802.3af poe only, so couldn't power my AP AC-Lite via POE as it needs 24V POE.
the US-8 150W, however, would be able to power the AC-Lite via POE as it support 24v and 802.3af POE.