Two Chest Freezers Are Better Than One
2008-11-01 by RobIn March of 2008, I retired my 10 year old 22 ft^3 chest freezer and replaced it with these two new smaller 5.5 ft^3t chest freezers.
Why Two?
When I retired my old freezer, I was aware that I only needed about 10 ft^3 of freezer storage. I could have purchased a single 10 ft^3 chest freezer for less than the two 5.5 ft^3 chest freezers that I purchased. Furthermore, a single 10 ft^3 chest freezer would be more efficient due to its higher ratio of volume to surface area. So did I make the right choice? In the remainder of this article I hope to convince you that I did. Why are two freezers better than one? There are four reasons I can think of:
1. Maneuverability
The only place I can easily fit a large freezer is in our garage which is outside the building envelope (ie it’s unheated and uninsulated). The smaller freezers, however can easily be maneuvered into the house. We heat our home with electric space heaters (see Convert from Gas to Electric Space Heating to learn why). Our heating season is about 6 months of the year. Freezers, like all electrical appliances, produce heat. It takes the same amount of electricity to produce a given amount of heat whether that heat is generated by an electric space heater, or a freezer, or any other electrical appliance. Therefore, moving the freezers inside effectively reduces the operating cost to zero for 6 months of the year when the heat produced by the freezers offsets the heat produced by our space heaters. If I had purchased a single larger freezer, I would have had to keep it in the unheated garage and pay for its operation a full 12 months of the year.
2. Versatility
The amount of freezer space we need varies throughout the year. During the summer months, one 5.5 ft^3 freezer is plenty. We need the extra 5.5 ft^3 just before winter when we preserve various foods we’ve harvested or purchased. With two freezers, we can simply unplug one during the summer months. The cost of operating the two freezers during the 6 month heating season is zero for reasons described above, and during the other six months, only one of the freezers needs to be operated. Thus the annual operating cost of the two small freezers is only around 25% of a larger freezer of equivalent total volume.
3. Ease of cleaning
It is much easier to clean out or defrost freezers when you have two of them. In the summer when one freezer is empty, contents can be transferred from one to the other making it easy to clean and/or defrost the freezers. It is also much easier to move a small freezer to clean around it.
4. Redundant backup
If either freezer should fail in the summer, we can move the contents into the other. If either freezer should fail in the winter when both freezers are operating, we can move the most important contents into the working freezer.
Cost Comparison
Using a kill-a-watt meter, I measured the performance of the old and new freezers. I also estimated the performance of the 10 ft^3 freezer I was also considering.
22 ft^3 freezer
- 63.3 kWh over 528 hours = 119.9 Watts continuous
- 5.45 Watts per ft^3
- 1050 kWh per year running continuously
- $0 to purchase (since I already had it)
- $73.50 per year running continuously ($0.07 per kWh)
10 ft^3 freezer
- 30 Watts continuous (estimated)
- 3 Watts per ft^3
- 262.8 kWh per year running continuously
- $418 to purchase
- $18.40 per year running continuously ($0.07 per kWh)
two 5.5 ft^3 freezers
- 18.2 kWh over 526 hours = 34.6 Watts continuous
- 3.15 Watts per ft^3
- 303 kWh per year if run continuously
- $496 to purchase
- $21.22 per year if run continuously
- $5.30 actual cost per year (1 freezer for 6 months since operation offsets heating at all other times)
Payback Calculations
I won’t go into details, but it is not a difficult matter to do an NPV (net present value) analysis given the above information. Assuming a 3.5% interest rate (typical savings account), the cumulative cost (purchase price plus operating costs) in today’s dollars of the three options are as follows:
| Year | 22 ft^2 | 10 ft^3 | two 5.5 ft^3 |
| 1 | $ 71.01 | $ 421.64 | $ 484.35 |
| 2 | $ 139.63 | $ 438.82 | $ 489.30 |
| 3 | $ 205.92 | $ 455.41 | $ 494.08 |
| 4 | $ 269.97 | $ 471.45 | $ 498.69 |
| 5 | $ 331.86 | $ 486.94 | $ 503.16 |
| 6 | $ 391.65 | $ 501.91 | $ 507.47 |
| 7 | $ 449.42 | $ 516.37 | $ 511.63 |
| 8 | $ 505.24 | $ 530.35 | $ 515.66 |
| 9 | $ 559.16 | $ 543.85 | $ 519.55 |
| 10 | $ 611.27 | $ 556.89 | $ 523.31 |
Aside
All electrical appliances produce heat. If you are heating your home with electricity, it doesn’t matter whether the heat is generated by an electric space heater, or a freezer, or a computer, or any other electrical appliance. You will consume the same amount of electricity to produce the same amount of heat. So if you’re going to use the electricity for heating anyway, you might as well do something more useful with it first. Operating a deep freeze is just one possibility. You might also consider growing some of your own food indoors using grow lamps, or doing more cooking and baking in the winter months.
why no additional insulation?
Hi John,
Adding more insulation to the walls of a deep freeze isn’t as easy as it sounds since the refrigeration coils are embedded inside the walls. Adding insulation in the wrong place will reduce efficiency rather than improve it. I considered adding insulation to the lids and UNDER the freezers but frankly the $5.30 per year it currently costs in energy to operate the freezers doesn’t make them a high priority for further energy reduction strategies.
I like the concept… We put an analog switch on our fridge on an unheated mudroom and it didn’t use any electricity for most of the winter. It meant our food wouldn’t go bad due to a power outage. On the other hand, it was more like walking into the fridge every time we needed something. We even just kept food on top of the fridge rather than bothering to open the door since the inside and outside were the same temp for months.
Fridges/freezers do not create heat (besides the heat generated by the compressor running). They simply ‘pump’ it around.
Simply said, you are not cutting the cost to 0 in the winter because you are simply using energy to move heat that escaped into the box that you want to be cold. An ideal phase-change freezer does not produce any heat, it simply moves it and that still takes energy.
I think that your argument for having ‘em inside still holds some water**–just not as much as you wish.
**(Placing the freezer indoors creates less space needing to be heated)
Hi Jeff,
The heat generated by the compressor running is exactly what I’m talking about. Any electrical energy you put into any appliance (for example to run the compressor of a freezer) is released into the home in the form of heat (assuming no net energy storage as in the case of charging a battery). The law of conservation of energy requires it. So from the perspective of the rest of the house, a freezer is just a big electric heater. If you put 30W of electricity in, on average you get 30W of heat out.
If you are heating your home with electricity anyway, it makes no difference to your cost whether you do it with an electric heater, or a freezer, or a computer, or a stereo, or a light bulb or any other electrical appliance, or any combination of them. They all produce heat exactly equal to the electrical power they “consume” (energy cannot really be consumed… only changed from one form to another). So whatever you can do with the electricity first (like running a freezer) is just a happy byproduct and has a net cost of $0 in terms of electrical consumption.
To state it yet another way… You might think you are paying for electricity to run your fridge, computer, stereo, tv, light, etc, but in the winter, you’re really only paying for electricity to heat your home. Attempting to reduce your electrical consumption by turning off any of these appliances is pointless. It will simply result in a corresponding increase in electrical consumption by your heaters in order to maintain the temperature at your thermostat setting.
This is a bit of a simplification, ignoring the fact that location within the home does have some effect. There’s little value in heating unoccupied rooms.
I realize this is a little late, but I figured I’d hop in anyway.
You’re wrong with your assumptions of how energy works. Here’s a thought experiment to show it:
1. Put an electricity generator that runs off of heat around your freezer (A thermogenerator, according to wikipedia)
2. Use that generator to power the freezer
3. If the heat output of the freezer is equivalent to the electric input, then your freezer will run forever! Yay!
Not really. Apply that to an electric motor in a car, and you’ll get a perpetual motion machine.
You’re right that a lot of devices put out a lot of heat as waste, but if electricity is the only energy going in, and heat is the only energy going out, then the device is doing nothing but generating heat. Outputting light (monitors), doing math (computer chips), spinning a motor (blender)…that all takes energy too. The heat output of a device = electrical energy going in minus the amount of energy used to perform other tasks.
That being said, certainly seems better to have the freezer inside anyway! I’m jealous that you have room to have a chest freezer at all. =P
Hi Isaiah,
I hate to harp on the issue and perpetuate a “You’re wrong”… “No. You’re wrong.” argument… but… well… you’re wrong.
In your thought experiment, the reason you can’t turn a freezer into a perpetual motion machine isn’t because the motor and compressor consume energy. It’s because a 100% efficient thermogenerator isn’t possible. If it were possible, and if you could capture all the heat released from the freezer and convert it back into electricity, you COULD quite literally run the freezer forever off it’s own heat.
Energy cannot be created or destroyed. It can only be converted from one form to another. I think we agree on that at least. So… here’s another thought experiment… If you run an electric motor you are converting electrical energy into kinetic energy. When you turn the electricity off, the motor will slow to a stop. When stopped, there is clearly no more kinetic energy. The slowing of the motor indicates the conversion of kinetic energy into another form of energy. So… to what form has the energy finally been converted? Answer: heat.
Likewise all the electrical energy you put into a freezer is eventually released as heat. Yes, it first gets converted to kinetic energy in the motor and compressor, but the kinetic energy is eventually converted to heat. The law of conservation of energy requires it. If electricity is the only energy going into the system, and heat is the only energy coming out (and no energy is being stored inside), then the heat energy coming out must equal the electrical energy going in. From the perspective of the outside, the device IS doing nothing but generating heat. The fact that other tasks are performed inside the system is irrelevant.
“Outputting light (monitors), doing math (computer chips), spinning a motor (blender)…that all takes energy too.”
Yes, they all take energy in the form of electricity, and give it back in the form of heat energy exactly equal to the electrical energy “consumed”.