How to Monitor Your USB Device Power Usage and Output

By Jason Fitzpatrick on September 16th, 2015

Worried your USB charger is under performing or out of spec? Think you’re not getting the real charge time out of that new power bank you bought? If you’re willing to geek-out a little then you can test your USB devices to see if they’re up to snuff.

Why Do I Want To Do This?

In an age when everyone is at the mercy of their chargers, USB ports in the dashboards of their cars, and other sources of power for their numerous gizmos and gadgets, it’s easy to have a sneaking suspicion that the cheap charger you bought isn’t up for the task or that the USB port in your car console is really as under powered as it seems.

Rather than just tolerate a bad charging experience or wonder if that expensive USB power bank you got really can output all 20,000 mAh it promises to, you can investigate and test the quality of your charger, power bank, or any other power delivering or consuming device: check the output, check the input, check the voltage and amperage, all in one swoop.

So while you could just push that nagging suspicion to the back of your mind and not worry about the nuances of USB power and whether or not that $5 truck stop charger is worth its salt, but you don’t have to because testing your USB device’s power usage and output is really simple if you’re willing to buy a simple tool and spend a few minutes taking measurements. Let’s take a look at the tools and how to use them in application.

What Do I Need?

The device displays voltage, amps, power consumption (mAh)

The heart of our little experimentation is a USB tester. You’ll find piles of them on Amazon in a wide arrangement of cheap with no reviews, expensive with reviews, and very few cheap and with decent reviews.

While we’re sure most of them work well enough we only found one that was cheap, had good reviews, and had an array of features beyond simply displaying the voltage and amps: the PowerJive USB Power Meter ($9.99).

It’s cheap (nearly the cheapest available and the cheapest with any reviews, let alone good reviews) and it not only measures voltage and amps, but flashes a warning at you if the measurement is significantly out of spec, and monitors power consumption. Even better yet it retains data on up to 10 different devices (if you’re so inclined to track, compare, or otherwise care about the differences between chargers and devices).

Before we proceed, we’d like to emphasize that while you get a lot of value for $10 this device is most definitely not equivalent to a lab bench full of expensive gear and it really intended to be used as a simple and cheap check you can do to ensure your devices are functioning within the expected specifications (and not to diagnose complex power issues like dirty power, output frequency, or other advanced concerns). If you’re curious what that kind of testing looks like, we’d encourage you to check out this interesting and informative article by Ken Shirriff, A Dozen USB Chargers in the Lab, where he puts a bunch of OEM and generic chargers through some serious benchmarking.

What Can I Measure?

Armed with our cheap but reliable little USB tester what exactly can we monitor and test? Let’s run through some common scenarios and why/when you would test for certain factors.

USB Port Voltage

Generally speaking even cheap PC USB cards and device chargers are not so out of spec as to be dangerous. Nonetheless, the voltage is really the only thing you can measure off a USB port that poses any potential danger to your devices or is heavily indicative of a faulty charging device.

The USB meter allows you to easily check to ensure that a given port or charging device is not under-volted (more likely) or over-volted (much less likely). We tested everything from the USB ports in car consoles to official chargers from Apple, Samsung, and Motorola, all the way to cheap knock-off gas station chargers and found that output was consistently 4.92-5.15 volts. The tester is capable of reading in the range of 3.5-7V.

USB Port Amperage Output

Given that very few chargers are wildly out of spec in the voltage department, the amperage department is where most people find room to complain. Low amperage output means long charging times at best and failure to charge or function at all when it comes to very power hungry devices that need good connection.

In the photo above you can see the test we performed on the USB port located on the console of our car. We’d had a suspicion that the port was underperforming (it couldn’t keep an iPhone topped off or nearly topped off if the passenger was actively using it) and the test confirmed it. 0.48A output is on the low side. Enough juice to trickle charge and potentially keep an idle phone topped off but not enough to feed a hungry iPhone or iPad.

Now, one thing you should note here: the displayed amperage is not the total potential output of the port but the total power the device is pulling down. The 0.48 reading in the above image isn’t damning just by itself (as the device might be only pulling down that amount) but what was damning was when we plugged the exact same device immediately into a high-quality 12v car charger in the 12v socket right beside it and suddenly the amperage jumped to 0.92. Even with the device not maxing out the port it was plugged into it still used more power on the higher quality connection.

Power Consumption and Discharge

The final use for the meter is to test the power consumption of devices as well as the discharge of a power bank. Practically speaking testing the power consumption of a USB device, with most applications, is of pretty limited utility.

Outside of benchmarks or product testing it isn’t as important to know how much power, in mAh, a product is using as much as it is important to test how much power the charger and device together are pulling from the power outlet.

After all, you’re not paying for how many mAh worth of power end up in the device you’re paying for the overhead of the charging unit plus the actual energy pumped into the device.

To that end, the USB meter isn’t as useful a measure as a device that measures wall current like the Kill-a-Watt meter we showcase in our article The How-To Geek Guide to Measuring Your Energy Use. The cost of the actual power used by a cellphone per year is tiny (we’re talking a few bucks a year). The cost of a bunch of inefficient chargers with wasteful vampire loads? That’s a whole other story (and one you’ll need to reference the aforementioned article and use a traditional 120v power meter to sort out).

Where the USB power meter really shines, though, is in measuring the output of power banks. You bought a USB power bank, it’s rated for 20,000 mAh, and you want to make sure you’re getting your money’s worth. Hook up a USB device to the meter and then to the charging port on the power bank, and let the USB power meter tick away. Once the power bank is fully discharged plug the USB power meter into any USB port to power it back up and read the mAh reading and compare it to the rating of your power bank.


Can you get buy in life without a USB power meter? Sure. Can you put aside your insatiable curiosity over whether or not the USB port in your car is really 2.1A? Maybe. Without it can you win an argument with your significant other over which USB charger is superior? Absolutely not.

Have a pressing tech question about USB chargers, batteries, or portable tech in general? Shoot us an email at ask@howtogeek.com and we’ll do our best to answer it.

Jason Fitzpatrick is a warranty-voiding DIYer who spends his days cracking opening cases and wrestling with code so you don't have to. If it can be modded, optimized, repurposed, or torn apart for fun he's interested (and probably already at the workbench taking it apart). You can follow him on if you'd like.

  • Published 09/16/15
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