Earth Notes: On DC and USB Power Meters: Review

Updated 2023-02-19.

Found out how I measure off-grid and portable device consumption to understand and manage and conserve! #powerMeter #frugal

I have 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.

live power stats for off-grid system — LIVE 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!

LIVE 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 is not meaty enough to measure large current draws without dropping enough voltage to disturb the device under test (DUT).

E-flite

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.

Muker

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, did not seem meaty enough to properly power my Raspberry Pi 3B+.

Satechi

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 USB C ST TCPM power meter — 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:

It is 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.

Satechi quibbles:

The display is not very bright.
The plug does not seem to grip tightly in the places I have 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.