Category: Experiments

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

Menus

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.

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.

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.

Manual

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

MS6115 Manual

Conclusion

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.

I’m using a really old fridge/freezer. Really old. I wanted to investigate if it would be worthwhile replacing it with a more energy efficient model. After the hot water heater this is one of the bigger power users in the apartment. I have been measuring the power use over the past few days using an MS6115 plug-in power meter. I will elaborate more on it in a later post.

I worked out the average daily power use for this fridge to be about 1.4 kWh. This does not take into consideration how many times the door was opened, if new items were stored or what the ambient temperature is, but it gives me a good idea to work from.

I received this fridge for free so it cost me nothing apart from transport. If I take a look at a similar fridge a few minutes of research comes up with the Samsung SR227MW. This fridge uses 280 kWh per year, or about 0.8 kWh per day. This is approximately an 80% improvement in efficiency. For this improvement I need to pay $566 and get a 2 year warranty.

Is it worthwhile replacing my fridge with the more efficient new one? I have the following information:

Current fridge usage	1.4	kWh per day
Electricity price	23.8	c per kWh
New fridge usage	0.8	kWh per day
Cost of new fridge	566	Dollars

With this information I can workout the payback period. The formula to determine this is reasonably simple.

This gives me a total of 3970 days or 10.9 years before the savings of the new fridge have paid for itself. If I consider that my current fridge is possibly older than I am and is still working, compared to a new unit that only has a 2 year warranty this does not seem like a good idea.

What people often don’t realise is that all objects have another energy aspect attributed to them. This is called embedded energy. This refers to the energy needed to manufacture the item in question. The diesel to mine the steel, the coal to melt the steel etc. I will discuss this in a future post.

With all this taken into account replacing my working fridge just does not make sense.