Documenting my attempt to cut my energy usage in half.

Monday, August 13, 2012

Refrigerator Analysis Part #2


While collecting data on my current refrigerator to determine if it makes sense for me to replace it, I placed a data collector inside the refrigerator to monitor how the temperature changes throughout the day and also to see when and how long the refrigerator door is open. 

The data logger was an Arduino Uno with an Adafruit Data Logging Shield, a DS18B20 temperature sensor and a photo resistor to detect when the door was open/shut.

Below is a 24 hour graph detailing the temperature swings and the frequency at which they occur.  I was quite surprised by how often the refrigerator cycles on and off.  I assume that the temperature spike at 11pm was due to the freezer going through its defrost cycle.

Refrigerator Temps

Also included in the graph (in red) is when the refrigerator door was opened and for how many seconds within that one minute period that the door was open.  It’s interesting to see the effect that opening the door has on the temperature.

Has anyone else ever put a temperature logger in their refrigerator?  And if so, is my temperature cycle normal?

Refrigerator Analysis


Our current refrigerator is a GE Profile 24.7 ft^3, model number TBX25PAB that was manufactured in March of 2001.  As part of our Half Project, I was interested is seeing how our refrigerator (which is not Energy Star rated) compared to a newer model refrigerator and to see if it made sense to upgrade.

The power consumption of the current refrigerator was measured with a Kill-A-Watt meter over the course of several weeks at 2.4 kWh/day, or 876 kWh/year.  For our area, Dominion Power charges us approximately $0.11/kWh, so our cost to run the refrigerator is $96/year which is about 3% of our yearly energy consumption.

The Energy Star website has a refrigerator/freezer calculator that you can use to calculate the operating costs of your current appliance.  I used this to compare the actual usage data that I collected with the estimated usage from Energy Star.  The chart below shows that the estimated usage is slightly higher than my actual usage, but it is within 10% which is acceptable for an estimate.

Model Data kWh $/kWh

Per year cost


Actual 876 0.11 $96


Estimated 961 0.11 $106


GE makes a newer version of the Profile series which is a direct replacement for what I currently have.  It is a little pricey at $1800 but let’s look at its performance.  First of all I’ll ignore the fact that the Sears website has conflicting data with the Energy Star label over yearly kWh usage and I’ll just use the data from the Energy Star label. 

Model Data kWh $/kWh

Per year cost


Estimated 560 0.11 $61

Using the Energy Star estimates for both the old and new refrigerator I can expect to get the following savings.

Energy Saving/year 401 kWh
$’s Saved/year $44.11
Initial Cost $1800
1st year Return < 2.5%
CO2 Reduction 441 lbs

With a payback period of ~40 years, upgrading to the newer model GE Profile just doesn’t make sense right now.  It’s time to research other models of refrigerators.