Hmmm..... Since USB only guarantees 2.5W, I dunno how the iron is supposed to work from a regular USB port.
USB isnt necessarily 2.5W, it varies based on version and implementation.
The standards are:
USB1 - 100mA i think.
USB2 - 500mA
USB3 - 1500mA
Nope, that's not really correct.
USB1.x ports allow 500mA, however, you are supposed to negotiate anything above 100mA. In most cases nothing bad will happen and you can draw up to 500mA even without negotiation. In the worst case, e.g. when connected through bus-powered hub, the device will not work. Higher current draw will also likely trip the over-current protection on the port.
USB 2.x is the same as 1.x by default. There are ways to obtain higher current, but you must negotiate it first, otherwise the upstream hub/host will shut the port off due to overloading. USB 2.0 has "Battery charging specification" part that allows 1.5A draw for unconfigured devices (aka not talking to the host, only using some resistors to tell the host to about the charging spec compatibility) and up to 5A after negotiation.
USB 3.x allows also 1.5A but only when no communication is going on (USB 2.0 allows communication) and drops the 5A possibility of USB 2.x, even though the connectors must be able to handle 5A. On the other hand, low and high power devices get 150mA and 900mA instead of 100 and 500mA defaults.
There are also differences for the USB-C spec (the C type connector).