Measuring watts with a multimeter (VOM)

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Measuring watts with a multimeter (VOM)

Post by lagagnon » Sun Apr 18, 2010 6:21 pm

If you already own a multimeter (volt-ohm meter or VOM) that is capable of measuring amperes (on a scale of up to at least 2 Amperes) then it is unnecessary for you to purchase a "Kill A Watt" meter to measure watts used by various appliances.

You can use an easily constructed extension cable "harness" to put your multimeter's amp meter function in series and then use Ohm's Law to calculate watts.

Ohm's Law tells us that P = I x E, where P is power in Watts, I is current in Amperes and E is voltage (or ElectroMotive Force) in Volts. We know that standard voltage for North American electrical distribution systems is 117 VAC. In Europe it is usually 230VAC. So if we plug in an appliance and our ammeter (ampere meter) tells us the appliance is drawing 0.85 Amps then the power it is consuming (assume North America) is 0.85amps x 117volts = 99.5 watts.

You first of all must set your multimeter to read amperes. This usually requires selecting the appropriate amps scale on the rotary dial and often also requires you to connect one of the measuring leads to the ampere measuring socket on the multimeter.

You then need to construct a extension cable harness and place the ammeter in series with the harness:

wall plug --)--------------| |----------------------------( socket
| |
^ ^
attach multimeter
cable ends here

The trick is to make the harness such that no exposed wire ends exist that could cause electrocution. Electrical tape usually suffices.

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Re: Measuring watts with a multimeter (VOM)

Post by electrodacus » Sun Apr 18, 2010 7:38 pm

Sorry but is not true :) You will measure Apparent Power [VA] not Real Power [W]. Real Power will always be smaller or equal with Apparent Power. But Apparent Power can be used as a reference if you do not want to spend the money for a "Kill a Watt".
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Post by KayDat » Mon Apr 19, 2010 10:19 pm

Also very dangerous. Only certified sparkys are legally allowed to work on anything mains powered.
Also, you'd have to turn everything off everytime you wanted to wanted to use the multimeter for something else. A mains watt meter is a fairly permanent fixture, but you'd use a multimeter for a bunch of other uses.

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Post by MikeC » Tue Apr 20, 2010 7:41 am

lagagnon's instructions should not be followed. As the other critics have pointed out, not only will it give you incomplete info, it is dangerous. This thread is now locked.
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