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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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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. –  AndrejaKo Nov 22 '12 at 10:01
    

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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.

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

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