Tuesday, July 12, 2016


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This is a graph of my freezer's energy use last night. To isolate the freezer from the refrigerator since they look similar in power draw, I unplugged the fridge from about 22:00 to 3:00 so it is not running during the time on the graph. I zoomed on the freezer cycling, not including the fridge startup to get the max zoom on the vertical axis, the energy use of the freezer.

My freezer is an old smaller Woods chest freezer sitting on the cold basement floor. It must be from the 1990s. Sometimes I notice condensation of the top of the door so I don't think it is very well insulated. But it is quiet and does it's job.

I can see on the chart that the freezer follows a regular cycle like the refrigerator, drawing less power about 100watts but for longer, about an hour every couple of hours. This is going to vary a bit, mostly with the room temperature in the basement. These days that is about 23C. In the winter much lower. I don't open the freezer door very often, perhaps once every couple of days.

The fridge and the freezer are like dripping taps. They don't use much power but they are always on. What energy they use goes in cycles on and off. At any moment they might be on or off. Digital? More like Binary?

The shape of the freezer energy signature is interesting as it ramps down slightly as it runs. The spike on the first peak doesn't repeat on the other two. This is probably the freezer compressor motor starting up. Most household motors start with a heavy burst of current known as the "locked rotor current" but that only lasts for a second or less and then the motor current drops dramatically.

I think a limitation of the Blueline equipment is that the samples are taken every minute or so. Blueline may be limited by the hydro meter sample frequency since it gets its data from the hydro meter on the outside of my house. Some guy named Nyquist figured out that the sampling rate had to be at least twice the data rate but thats another story... you can only see slow data with a slow system.

Here is the longer view this morning looking at the past 12 hours.

The freezer you can now see relative to some other appliances. D is the dishwasher. W is the water heater which comes on with the dishwasher and also about 4:30am. You can see that the water heater runs much longer when triggered by the dishwasher. There was no hot water used after the dishwasher. F is the refrigerator being plugged back in. S is me out in the shop with some of the big overhead lights on, doing some painting. T is the toaster oven warming up a bagel.

Here I am in CorelDraw taking a more detailed look at the freezer data and figuring out a yearly consumption estimate from last night's data.

You can use your own favorite graphics program to help with the visualization and calculation. You can be as detailed as you want to be or just eyeball the data to get a sense of what is going on. After I do a couple of calculations, I think I see more in the data. As I said previously, I am disappointed that the Blueline software doesn't do more of this calculation for me but I don't mind doing this for the knowledge I am getting about my actual use real time thought whole house monitoring.

I realize now that I did the calculation incorrectly for the duty cyle for the refrigerator yesterday so I'll go back to correct that.

Looking first at the vertical energy axis I have placed three numbers in watts reading off the rather bad axis which has no tick marks, I see that my baseline last night was a little lower than I expected, about 90 watts. This is probably because I didn't leave any lights on by mistake. The baseline is the computers, the network (which sends Blueline my data from the actual power company meter), a few CO2 detectors and some miscellaneous chargers and power bars.

Next I estimated the peak ppower draw of the freezer at 215 watts, (215-90)= 125 watts not including the baseline. When the freezer has run for a while just before it turns off, the power draw falls to 195 watts or (195-90)= 105 watts. Since the droop is approximately triangular, I will use the rough midpoint of 115 watts for the calculation. About half the time, the freezer uses more, half the time less, so it should be about right.

Now I calculate the time duty cycle, in other words, what percentage of the time the freezer runs. On the chart, 1.21cm = 1 hour. The numbers at the top (0.91, 0.86, 0.85 are samples of run times shown in cm. At the bottom are the lengths of the cycles (2.03, 1.99). I can't find the length of the third cycle since the rising edge of the next ON is off the right of the chart so I'll use only the first two samples. Using the ratio of the lenght to reepresent time, I can say that for the first cycle, the freezer was on (0.91/2.03X100)= 44.8% of the time, drawing 105 watts. During the second cycle, the freezer was on 43.2% of the time. I'll use the midpoint of 44%.

Based on only these two samples, for a whole day, the freezer would use (0.105x0.44x24)= 1.1088kWh or (1.1088x365)= 404.7kWh per year or virtually the same as my refrigerator (412.2kWh)!

Thanks for your interest,
George Plhak
Lion's Head, Ontario, Canada

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