I might be able to borrow a USB meter, though maybe not till after lockdown. I wouldn't mind having a play with my own set up, rather than guessing and getting it wrong when I need it.
One has to wonder whether a USB meter won't reach you from China before the lockdown ends.
Mine was delivered here under three quid and looks like this, though they're all pretty much the same, so you may as well buy the cheapest one with all the facilities you want:
It tells you, remarkably accurately for such a cheap device, the instantaneous voltage, the time elapsed, the instaneous current, the accumulated charge. The only other control is a reset button. so that you can use it to determine how much in total over a period you charged a device only intermittently attached, like a phone. One male and one female USB connection. I'm on my second one in quite a few years, so it is pretty reliable as well as being accurate; mine lives on my treadmill, which is not conducive to the survival off electronic devices.
