I would like to find a device that:

  1. Plugs into the wall outlet.
  2. Plugs into a USB connection to a computer.
  3. Measures the voltage in the wall outlet.
  4. Reports the voltage to the computer, presumably as a HID class device. (No display needed.)
  5. Is powered by USB, so I can get readings when the power is failed.
  6. Is cheaper than buying a UPS. (Some UPSes can do everything listed so far, except that they probably provide their own power.)
  7. Is significantly smaller than a UPS.
  8. Can measure to fraction of a volt precision, and finer is better.

Just to be clear, I do NOT want to measure the voltage of the USB bus.

Any ideas?


A key feature you need for this that many of the cheapest solutions don't offer is full isolation between the mains input and the USB port. I did a search on the term "USB multimeter with isolation" and came up with the following product that I've never used but I think might fit the bill:

Hantek 365B

It runs at $US105 from Amazon and from the user guide measures up to 600 VAC with 100 mV resolution. It uses the USB CDC class rather than HID and if you wanted to roll your own software instead of using the included software I found the following project on GitHub that you could base it on:

Linux Software for Hantek 365B Datalogger

Like most multimeters it has banana plug inputs so you'd need to buy shrouded banana plugs with an appropriate voltage rating and securely terminate them onto a mains lead.

  • (1) I was hoping for something simpler (and cheaper) than a multimeter (2) The term "isolation" doesn't seem to appear in their manual. (and isn't it achieved easily with a small transformer on the input? (3) They speak of "Chassis grand" and "Earth (grand) Terminal" (instead of "ground"), and their USB driver is a "Deriver". --- can you reference some without isolation? or that are more specialized?
    – David G.
    Jun 21 '20 at 11:40
  • @David the isolation is mentioned on the product page but you'd probably want to double-check with them - like most cheap Chinese products the documentation isn't the best. The problem with a transformer is the tolerance, your figure is < 1% and a transformer would probably be more like 10% so you'd calibration and probably temperature compensation to get it that accurate. If you went down that path there's quite a few DIY solutions using an Arduino and the like but I can't really think of anything off-the-shelf that isn't a lot more expensive.
    – PeterJ
    Jun 21 '20 at 12:15
  • The last UPS I bought (an APC Corp. BN650M1) has 1 volt precision and runs about $80 ($79.19 on Amazon, $76.99 at Best Buy). I'm just hoping for something smaller, cheaper, and maybe 0.1V precision.
    – David G.
    Jun 21 '20 at 14:41
  • @David Something reminded me of this question and I hadn't thought of much else meanwhile. I can totally get where you're coming from and I imagine something like this could be made in high volume for something like $20 but I don't think there's much of a market. Most labs would use a multimeter and there are power analyzers that utilities use that cost $1k+ but do more that just voltage. BTW what's the need for that resolution? In most electrical installations you'd probably get a volt or so of drop when loads switch on and 5% variation from a utility is normal.
    – PeterJ
    Sep 5 '20 at 12:07
  • Yeah, the lack of [perceived] market kills a lot of good possibilities. I was mostly just hoping to do better than my three existing UPSes (which, as I write this, read 116, 117, and 116).
    – David G.
    Sep 5 '20 at 17:13

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