Earth Notes: On DC and USB Power Meters: Review

Updated 2020-07-19 15:24 GMT.
Found out how I measure off-grid and portable device consumption to understand and manage and conserve! #powerMeter #frugal
I've taken a number of approaches to measure the power consumption of smaller gadgets, such as those that run off-grid at 12V or 5V, including my low-power servers.

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.

Off-grid power graph. Adding a load to be measured and then subtracting (quiet) server power draw works reasonably well, though using sped-up logger samples, rather than reading off the graph!

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!)

E-flite: I've had the E-flite meter since at least 2007. It has flying leads and is good for measuring power draw from nominal 12V and 5V supplies. Current resolution is 100mA.

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.

Muker UT-KWS-10VA USB-A Multi Meter / Voltage Tester / Battery Tester / Ampere Meter 0A--3.3A, 3.5V--9V. Bought 2018-10, £6 including VAT.

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 photos including unboxing and packshots.)

Satechi ST-TCPM USB-C power meter. Handling up to 65W (enough for a 2018 MacBook Air and later), with current in steps of 10mA, and running up to 20V or 10A when passing through Power Delivery. Bought 2020-06, £22 including VAT.

Satechi nice features out of the box:

Satechi quibbles: