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"That’s when I found out that there exist cables that have little screens on them that show the power consumption of the connected device."

Useful to get an idea how much your USB light draws and to check if your phone is really fast charging. I'd be careful with the numbers when the USB drives a more complicated load. Measuring the true power consumption of a microcontroller is surprisingly hard.

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How can measuring a DC load be complicated?

Sure the load can be very spiky and you have to sample fast enough to accurately integrate them. but that's doable given sufficient caps. Of course you'd end up with some filtered average instead of the instantaneous power, but that would be no good for a text display anyway.

When your range is from nano amps during sleep to spikes of several tens of amps you will not find a voltmeter with a range big enough to work with a single shunt. You need several shunts and switch between them very fast. That's what devices like the Joulscope do and why they are expensive.

fair enough, i had a use case in mind where you're just interested in a rough estimate of Watthours consumed.

Does it actually measure load, or just display the negotiated power?

It seems to measure load, it instantly responds to me hitting the boost button on my soldering iron.

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