I want to measure how many kilowatts or kilowatt hours are going through some of electric lines in my house. For reasons too numerous to list here I can't use a standard kill-o-watt (basically some of these appliances are hardwired and don't have plugs).

Can I use data that you get from an AC clamp meter to calculate kilowatts going through a line?

There are a whole bunch of calculations here: http://www.rapidtables.com/convert/electric/Amp_to_kW.htm

but I am not sure which one to use and if I can get all the information I need from an AC clamp meter. If I can't use an clamp meter is there anyway to measure killowatts flowing through a line?

To be clear this is what I am calling an Clamp Meter: http://www.homedepot.com/p/Commercial-Electric-Digital-Clamp-Meter-MS2002/202353289?keyword=ms2002

Clearly I am not an electrician so any help would be appreciated!

  • Buy yourself a kill-a-watt is very useful
    – user70085
    Oct 13, 2018 at 0:47

2 Answers 2


You can use a clamp meter to measure current through a single conductor.

Current is the number of electrons flowing through the wire per unit time, and is measured in amperes. If you know the voltage, you can use that to calculate power (watts):

Power [watts] = Current [amps] X Voltage [volts]

If this is a standard residential power system you can assume 110V or 220V for the voltage, or you can measure it with a voltmeter/multimeter. If you want kW instead of watts, you divide by 1000. (The formula on your website includes a "Power Factor" which is based on the appliance being used. In most cases it's close to 1, so you can just leave it out unless you need to be super-precise or happen to know that you have a low power factor.)

The major limitation of a clamp meter in measuring household power is that it can only be clamped around a single conducting wire, not the entire romex cable. If you clamp around the entire cable you will get a reading of 0, since the two conductors have opposite currents and they cancel each other out. This limitation means the meter is probably only practical inside the circuit breaker box or an electrical box, where space and safety concerns may make it a bad idea.

  • You can buy something that temporarily splits an electrical device's current into its two components so you can clamp just the conducting wire. They cost about $15 circa 2019. (Look for an AC Line Splitter.) Functionally it's a short extension cord with the exterior casing pulled off. This is an idea, not advice: One could fashion one with a short, appropriately-gauged extension cord and a paring knife. A Youtuber made one by assembling the components of a short extension cord (spoiler: ...and splitting the cord down the middle with a paring knife). youtube.com/watch?v=x_RbRM1QySQ Apr 12, 2019 at 18:14

You can use an Efergy Elite meter to measure the KWH per circuit at your circuit breaker box. I've used one for several years and have made several energy saving air conditioner changes based on what I learned by tracking the KWH used. It can measure both 120 and 240 volt KWH usage. It also stores one week of daily KWH usage. I put each day's usage in a spreadsheet for further calculations.

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