Earth Notes: On DC and USB Power Meters: ReviewUpdated 2020-07-19.
To measure the power used by a mains-powered gadget or appliance, use a plug-in meter.
For a non-mains gadget that can be powered from a power adapter, such as USB, as long as the adapter is efficient, power the gadget from the adapter and plug the adapter into a plug-in meter.
For something that can be powered from 12V nominal, directly or (say) via an efficient 12V car USB adapter, I can with care measure power draw by plugging it into my off-grid system, then subtracting whatever power everything else is drawing at the time, eg with the server quiet and dump loads turned off.
But it is better to get closer to the device and measure down-stream of any adapter or converter where possible.
A multimeter simply isn't meaty enough to measure large current draws without dropping enough voltage to disturb the device under test (DUT).
Back in 2007 I used my E-flite meter to measure the power draw of my laptop-as-server to its 12V input. (As well as measure my off-grid battery's voltage!)
I measured the consumption at 5V of the laptop-server's successor, the SheevaPlug, with the E-flite also.
A more recent acquisition, in 2018, was the Muker. It has a USB-A plug input, and two female USB-A sockets.
I haven't used it as much as I'd like, but I did for example establish that a USB-A to micro-USB cable, even a good one from RS, didn't seem meaty enough to properly power my Raspberry Pi 3B+.
Anticipating a rushed upgrade to a Pi4, which is USB-C powered, I wanted to measure and tune its power consumption. Given the implicit recommendation of a site that I trust, I bought the Satechi in 2020-06.
Satechi nice features out of the box:
- It's tiny, and can probably plug into many places without physically blocking adjacent ports for example.
- The Satechi can handle power flow in both directions and show the direction with an arrow on the display, which is clever.
- The display is not very bright.
- The plug does not seem to grip tightly in the places I've tried so far, and feels like it could fall out. Not something I expect with USB.
- The minimum current it shows is 10mA or 20mA even with no load.
- The interesting cumulative mAh display may not be much use given that the voltage can vary, nominally up to 20V with Power Delivery.