As a lucky roadtester of the METCAL GT120, I noticed that the USB interface got mostly ignored in all the reviews (myself included). With the port at the back of the unit, the discoverability is not great and that is probably the main reason for it to be forgotten. Access on the side or even the top of the housing might improve that. With my latest tool ( a cheap DL24 150 W electronics load ), I decided to give both items a quick test.
While connecting the USB cable to the METCAL, The first thing I noticed is, that the USB port at the back of the unit is upside-down. This is rather annoying as you now need to turn the USB cable 4 times 
to make it slide in. USB power does still work when the unit is in sleep mode, which is nice when you are charging a mobile device. Here is a picture of the test setup.
Without any knowledge of the internal power supply, I decided to test current draw only up to 1.0 A, to not damage any internal components. Here are the results in 100mA steps.
| Voltage [V] | Current [A] | Power [W] |
| 4.93 | 0.0 | 0.00 |
| 4.90 | 0.1 | 0.49 |
| 4.86 | 0.2 | 0.97 |
| 4.82 | 0.3 | 1.45 |
| 4.79 | 0.4 | 1.92 |
| 4.75 | 0.5 | 2.38 |
| 4.72 | 0.6 | 2.83 |
| 4.68 | 0.7 | 3.28 |
| 4.64 | 0.8 | 3.71 |
| 4.60 | 0.9 | 4.14 |
| 4.57 | 1.0 | 4.57 |
As measured, the voltage stays inside USB specifications up to 1 A and I have never seen the iron draw more than 100 W when heating up, there should be enough overhead to maintain the power draw on the USB port. A quick visual test confirms a stable 1.0 A draw when the unit is waking up from sleep mode and heating the tip to operational temperature. It also charges my iPhone SE, which is sometimes picky about being charged by generic chargers.
In conclusion, the USB port on the METCAL GT series soldering iron is a useful addition to power or charge devices on the workbench. Leaving a USB cable plugged in, is going to help remembering to using this feature.
