I am trying to develop a non-contact whole-house power usage monitor.

I have reviewed jasonT's project, which uses AC clamps to measure current usage.

However, from reading this post on how meters measure power, and this post on apparent vs. real power, it seems I need to be able to measure the instantaneous voltage as well as the instantaneous current, in order to calculate an accurate "real" power usage. From the first referenced post:

Real power is calculated by multiplying the voltage and current at any instant (one pair of samples). Apparent power can be calculated by measuring the amplitudes of the voltage and current over the course of one or more cycles and multiplying the two results together.

(I'm wondering if they got it backward! Real power = requires measurements over multiple cycles, but Apparent power = one instantaneous VA measurement?)

My question is this: how can I measure the instantaneous voltage in a non-contact manner? I understand that current can be measured inductively with a clamp, which is just a simple transformer which outputs a voltage proportional to the current flowing through the conductor.

If I'm not mistaken, it seems that using a capacitive measurement technique would be correct (like in an electrician's voltage detector probe), but I can't fully wrap my head around what such a circuit would look like. Would it be just a simple wire placed near the mains conductor, at a fixed distance, with only one end connected to an amplifier (for example, similar to this circuit)? If so, how would such a circuit be tuned to the correct proportional output?

If anyone could point me to any resources or suggestions I would be grateful.

  • \$\begingroup\$ Why do you need it to be a non-contact measurement? Any outlet in the house gives you a location to make contact (at least for one phase). \$\endgroup\$
    – The Photon
    Nov 3, 2015 at 2:39
  • \$\begingroup\$ Though of course you should be aware of the safety implications if your device does contact the mains. \$\endgroup\$
    – The Photon
    Nov 3, 2015 at 2:40
  • 2
    \$\begingroup\$ Use a low voltage transformer, (with fully isolated primary and secondary windings, and one with a safety agency rating). Connect the primary winding across the AC voltage line (or just plug it into a wall receptacle), and measure the low voltage across the secondary windings. Multiply the voltage reading by the transformer winding ratio to get the original AC voltage value. \$\endgroup\$
    – Nedd
    Nov 3, 2015 at 2:50
  • \$\begingroup\$ I want non-contact for the safety reasons, as you say above. I would prefer not to have a transformer directly connected to the mains. Also I would like to learn more about how non-contact voltage detection works. Thanks. \$\endgroup\$ Nov 3, 2015 at 3:08
  • 1
    \$\begingroup\$ This paper may contain some useful information. I've been hunting for a similar solution myself. Non-intrusive voltage measurement of ac power lines for smart grid system based on electric field energy harvesting. (IEEE, 2017) \$\endgroup\$ Nov 18, 2018 at 4:34

1 Answer 1


You don't want contact with the mains. OK, so is your project going to be battery powered?

If you buy a low voltage, AC output wall-wart, with UL, CE, DIN and every other safety marking you can think of, you can regard its output as at least as safe as any other appliance in your house. Then, in the low voltage output, you have a reasonable facsimile of the mains voltage waveform. The accuracy will degrade if you also rectify that output to power your project. If the last 1% accuracy is important, then why not buy two, one for the reference, and one to power the project, or a DC output one to power the project without further messing about. Your current measurement is already transformer coupled, why not have the voltage measurement transformer coupled as well?

To confirm real and apparent power calculations ...

for real power, repeatedly compute the instantaneous power as the product of the instantaneous voltage and the instantaneous current many times a cycle, then average the power

for apparent power, compute the mean voltage by averaging, and seperately the mean current by averaging, then take the product of these

... and here an average means either a) average over a long time, the more cycles the more accurate or b) sum synchronously over an integer number of whole cycles

  • \$\begingroup\$ Hi, thanks for your input. I realize I could use a transformer to couple to the mains. But I want to learn about how to measure mains voltage in a non-contact way. This is an educational, as well as functional, project. So learning new things is part of the project. Thanks \$\endgroup\$ Nov 4, 2015 at 19:55

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