From what I understand, a multimeter can be used to measure the resistance in in a circuit. Multiplied by the voltage, measured in the same circuit, I would get the total power consumption. Is this correct?

I want to use an inexpensive multimeter to measure the average power consumption in an appliance (computer). I was thinking that it should be possible to achieve this by connecting the "measuring hands" of the meter to the connectors in the power socket, and then over a couple of minutes measure first the average voltage and then the average resistance. Then, multiplying these values would give me an approximation of how many Watts a device is using in a given time period.

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    \$\begingroup\$ Measuring the resistance of a computer power supply input side won't help at all, since the actual impedance depends on the load and is determined by the operation of the unit itself. Cheap multimeters can only measure the actual resistance of a passive element. As soon as you add semiconductors into the mix (and rectification is done using transistors in switch-mode power supplies), the resistance value shown by the meter is almost meaningless. Also you can't use the multimeter to measure resistance of a powered circuit. Cheap meters will die from that and better meters will show warning. \$\endgroup\$
    – AndrejaKo
    Nov 22 '12 at 10:01

A computer or other appliance power supply is not a resistive load, it is a reactive load. It has a phase relationship to the incoming voltage, which is itself an alternating (AC) voltage. AC voltages inherently show an "average" of essentially zero. What is measured for power computation is an "effective" or "Root Mean Square" (RMS) voltage across, and current through, the appliance power feed.

Therefore measuring the resistance across its power supply leads will not provide meaningful results.

At a simplistic level, Voltage measurement could be done with a RMS voltmeter across the supply leads. See this EE.SE answer for more details.

Current measurement would need an RMS current meter either inserted into the power line in series, or using a clamp-type non-invasive current sensor.

Low cost AC power line meters use a basic rectifier circuit and internal computation to indicate power consumption. These are designed for specific power line types (e.g. 110V 60 Hz, or 230V 50 Hz), and will deviate from precision if used on a different line frequency, if they work at all.

The above does not take into account Power Factor calculations, another element impacting actual power consumption calculations.

The proposed multimeter approach will yield nothing except possibly a damaged multimeter and the risk of electrocution if you are not qualified to work with mains voltages.

There are commercially available power meter devices that plug into your wall socket, with the appliance plugged into the device, and log or display power consumption. That would be the recommended way to go.

  • \$\begingroup\$ Whoa, so much info. Way over my head. Thanks, I'll ask around in a local store. \$\endgroup\$ Nov 22 '12 at 10:34
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    \$\begingroup\$ Apologies for making the answer complex. Simple answer: No, your multimeter won't do, get an AC power monitor. \$\endgroup\$ Nov 22 '12 at 10:38
  • \$\begingroup\$ Electrons don't read your qualifications badge, unfortunately. :) \$\endgroup\$
    – Kaz
    Nov 22 '12 at 21:45

I'm surprised nobody on this page mentioned Kill A Watt yet. You plug it into the wall, you plug your computer into the Kill A Watt, then the Kill A Watt displays volts, amps, and wattage within 0.2 percent accuracy.

  • 1
    \$\begingroup\$ I do love my Kill A Watt, but technically the question was how to measure power with a multimeter, not what can be purchased that will measure power (though I would agree that a KAW wouldn't exactly break the bank...). \$\endgroup\$
    – I. Wolfe
    Jun 8 '15 at 21:57
  • \$\begingroup\$ Right, however, when the top answer is "You can't," the next best answer is this alternative. You're technically correct though! \$\endgroup\$
    – MattSayar
    Jun 9 '15 at 1:24

Since it drives reactive power, you can't directly measure with a simple ampermeter.

But you still have a simple solution. Use the energy meter of your house. You use the digital ones, right? Does it have a blinking led showing the power rate? You can read how it is dependent to power, somewhere near the led. For example, let's say it is 1000 blink/kWh. So it is 1000/(60*60)=0.278 blink per kilowatt.second. It is also 3600/1000=3.6 kilowatt.second per blink. Run only the computer in the house and get the time between two blinking with a chronometer. Say it is X seconds per blink. It is the problem that you have 3.6 kW.sec energy that is consumed within X seconds. Simply calculate the power by calculating 3.6/X (kW).

For the mechanical ones, you can figure it out by reading the turning rate.


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