How to use a plug-in power meter

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In my post on Dollars and Fridges I referred to the use of my MS6115 plug-in power meter to measure the power consumption of my fridge. I wanted to discuss this tool as well as it’s advantages and shortcomings.

The MS6115 is a $22 device that is similar to the US Kill-a-Watt. The basic operation is really simple. You plug in the MS6115 into a power outlet and then plug the device you want to investigate into the MS6115. It then tells you the power usage of the connected device. There are a few more advanced features where you can include your electricity price as well as dates and times but the menu’s are difficult to navigate and I prefer to only use the basic features. Let’s have a look at the menu’s


When you first turn the device on it shows all the display elements at once.


The different features we have are WATT, kWh, SET, AMP, VOLTac, COST/kWh, some warnings, various date and time related features, POWERFACTOR and Hz (frequency). You cycle through the menus by pressing the FUNC button below the screen.


The first page shows your AC line voltage and frequency. This is interesting but does not tell us much regarding the power usage. For more information have a look at Table of mains voltages and frequencies.


The next page shows us the amperage and power factor. Keep going.


Here things get interesting. The WATT page shows us the power usage of the connected device.


Finally we get to the energy used menu, the kWh display.


The last menu before we start at voltage again shows us the total price. This will only show the correct values if you entered the electricity price during the initial setup.

How to perform a measurement

The types of devices we want to measure fall into two rough categories. The one type of device is under our control like a floorlamp or a TV set. The other type of device can turn on and off by itself like a fridge or a electric hot water heater.

For the first type of device it is sufficient to just write down the power usage on the WATT screen. Since we know, or can estimate, the time the device is on we can multiply the power usage with the time in use to obtain the energy usage. So for example:

If we have a floorlamp and we measure its power usage as 120 watt and we know we only use it for about 2 hours per day, we can work out that the floorlamp uses 87.6kWh per year.

87.6\ kWh = \frac{120W}{1000W}\times{2\ hours}\times{365\ days}

For the second type of device I do the following. Firstly plug in the device and then take note of the time, or even better set an alarm for the next day at the same time. Then you let the device run. The next day just navigate to the kWh menu and write down how much energy was used for the past day. So for example:

I measured my fridge and it showed that it used 1.4 kWh  per day. I just multiply it out with the days in the year to determine that it used roughly 511 kWh per year.

511\ kWh = 1.4\ kWh\times{365\ days}

Note that the second method does not account for human behaviour or seasonal changes. For example if you open the fridge more often or if it needs to work harder during the summer. It does however give you a good approximation of the average usage.


If you are interested in any of the other features have a look at the manual.

MS6115 Manual


The MS6115 is a very cost effective way of getting a handle on the flows of energy in your house. It is simple to use and has very few limitations. It can measure the power usage for any device that has a plug.

7 thoughts on “How to use a plug-in power meter”

  1. Hi,
    Got a couple of these for teaching and was wondering if you worked out the unit of measurements on the kWh screen?!?!?!

    1. Hi Garry,

      I’m not sure I understand the question? If you are on the kWh screen the unit will show the total energy used since it was turned on. The unit will be kWh.

      1. The reading under the ‘Total Cost’ label is formatted like 80 (in larger text) 56(in smaller text). Would it be read as 80.56 kWh?

        1. It sounds like you have gone one screen too far. If you look at this image, you should have the kWh at the top (Pink Arrow) and then the large number (Blue Arrow) will show you the total kWh used.

          1. Thanks for the photo, I understand your question a bit better now. If you want the total energy usage you need to press the FUNC button to get to the kWh screen. The screen you are on will only show you the total cost (if you have programmed in a price). The numbers on the lower right hand side show the total time that the unit has been turned on. I’m not sure exactly of the units but I guess it is 1 hour, 36 minutes and 49 seconds.

          2. OK, now we are getting somewhere…. It’s not time, the unit has been plugged in for 6 days….

            I’ll check the rest, thanks for the info.

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