# Does keeping a fridge or freezer full improve its energy usage?

It's sometimes cited that a full fridge or freezer helps reduce its overall energy usage (for example, this source says so, based on the amount of cold air that 'can escape' when you open the door). However, I'm doubtful of this, based on the fact that you would also expect foodstuffs to warm up when you open the door. Even if there is a difference, it seems unlikely that it would be that large.

So what's the story? Does the fullness of a fridge or freezer affect its energy usage (edit: to clarify, keeping everything else constant - i.e. how often it's opened, where it's situated, ambient temperature outside, etc.)?

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I've heard this as well before except they explained it differently: the claim was that the cold items help keep everything else cold and thus it take long to thaw out, kind of like the difference between a single ice cube and a whole back of ice. –  rob Aug 20 '12 at 18:17
Sounds plausible. If you think about air exchange, cold air would be rapidly replaced by warm air whenever you open the door. But cold solids will hardly be affected by the door being open for a few seconds because they are not exchanged for warm solids. Then when the door is closed again, there is far less air volume to cool back down to the target temperature. However if you never open the freezer door, I'd bet energy consumption is about the same. –  Alex Wayne Aug 20 '12 at 18:27
If you remove all the cold items out of your fridge whenever you open the door, then it will have the same effect as letting all the cold air out. I have to -1 for "does not show any research effort." –  Flimzy Aug 21 '12 at 5:04
Here's an important point to consider ... Assuming the fridge door is closed most of the time, a "full" fridge, with its contents stabilized at a certain temperature, takes about the same energy to maintain that temp as it does to keep an empty fridge at that temp. The real "loss of cold" (actually the introduction of heat) is via the imperfectly insulated walls and door and doesn't have much to do with the mass of the contents. BUT what is forgotten is how much energy did it take to remove the heat from all those items in the fridge in the first place? It simply takes more energy to remove th –  Bob A Aug 22 at 3:08
I've always thought it the other way. It might let less cold air free, but it might need more power to keep all the things in there below room temperature. Plus the fridge door will be open longer while you search and sort stuff –  Muz Aug 27 at 3:05

TL;DR version: You can probably accept that advice, although the extent of the effect is probably smaller than TLC is making it out to be.

Background Materials

First, a little "theory" just to set some ground rules.

Refrigerators effectively remove heat from items, and expel that heat (and additional heat generated in the process) out the "back". They do this by cycling "cold" air through the compartment, allowing that air to heat up (very slightly) from the contents (if the contents are warmer than the chilled air), and then cycling that warmed air across cooling coils. See http://home.howstuffworks.com/refrigerator2.htm for example. They regulate this process to keep the chilled-air temperature relatively constant - within a few degrees. Similar to your air conditioner, if the chilled-air temperature is above the allowed range, the compressor and thus cooling system will "kick on", and will stay on until the temperature reaches the target temperature at the bottom of the allowed range.

Air is a rather good heat transfer medium. The reasons for this are that (1) heated air can be easily physically moved to the cooling system, (2) turbulent flow is relatively efficient to achieve, and (3) materials tend to have high heat transfer coefficients with air. It's also (fortunately), relatively unlikely to contaminate the materials being cooled.

Defining the Question

Now, the question on "efficiency" hinges on what we expect our refrigerators to accomplish. If what we want is to just put stuff in it, wait some period of time, and efficiently keep that "stuff" cold, we will arrive at a different answer than if what we want is to efficiently make some "stuff" cold, regardless of how efficient it is to keep it cold. The reason is that in the former case the heat transfer we are worried about is between the chilled air and the insulated refrigerator housing, while in the latter case the heat transfer which matters is between the warm food items and the chilled air. A third case, alluded to in your question, is how well we expect the former case (keeping chilled foods cold) to behave when one of the six walls of the container is completely removed (the door is opened).

Scenario: Chilling a Warm Dish of Food

I'll hit the middle case first. Placing a warm dish into an empty refrigerator (filled only with circulating air) will chill that dish far more rapidly than placing the same warm dish into a block of "chilled" dishes in the fridge, because the heat transfer coefficient of warm dish to chilled dish is much lower (less heat will flow out of the warm dish) than the heat transfer coefficient of warm dish to turbulent air flow. However, this is a hard difference to observe without really trying at it, because even a relatively "full" refrigerator will have copious air flow around any inserted item; it applies more to a freezer where you might be able to pack a bunch of regularly-shaped boxes tightly together. So, in this case, the answer is that an empty fridge is more efficient at cooling, but that verdict is not typically relevant. Also, it doesn't figure into typical home refrigerator "efficiency" as related to your electric bill because the temperature of the chilled air is typically kept constant rather than acting in a blast-chiller mode (where the air is chilled far below the target temperature of the food for a short period of time to bring the food quickly down to temperature).

Scenario: Door Shut, Foods Chilled; Keeping It Cool

When the system is in stasis - the solids inside have all been chilled - the effects of "empty space" are debatable. In a simple refrigerator with fixed-flow air circulation fans, circulating air through mostly-filled space will result in greater turbulence and thus higher heat transfer coefficients and higher potential loss of heat at the edges. However, the operational heat resistance of the refrigerator's shell should be primarily due to its insulation - the overriding barrier to heat transfer is not the transfer from the inner surface to the chilled air, but the transfer through the insulated foam layers between the "outside" and the "inside" surfaces. The exception here is where there are insulative weak points - for instance, poorly-fitting gaskets around doors and inlets which stop mass air flow but have much less resistance to heat transfer than the full gasket. Overall, though, this will be very minor, and unlikely to cause any measurable difference in your electric bill. At stasis, then, there is generally no difference in efficiency.

Scenario: Where Is the Mustard?

Last case is when the door opens. This is the main cause of lost "chill" / heat inflow in any real-world refrigerator. This is no longer a simple "heat transfer" problem, but a mass transfer heat exchange: chilled air flows out, and warm exterior air rushes in to replace it. Home refrigerator designs aim to minimize this transfer, even at the expense of their efficiency in the first case above (blast-chilling foods). They do this by keeping the circulating air currents parallel to the door so that an open door does not directly expel chilled air. They also limit the velocity of air circulation to keep it "mostly" still when the door is opened.

Assuming absolutely still air, though, there is still a significant amount of mass-based heat loss through the open door: cooler air is denser and will thus tend to flow out the (vertical surface) opening, allowing warmer exterior air to flow in from above. The impact of this depends on how long you keep the door open: eventually the cooling system will reach a new equilibrium, chilling air and sending it out the bottom portion of the door, while the inside of the fridge above the air circulator will become essentially room temperature (which itself is warmed from the "other half" of the coolant system in the back of the fridge).

Summary

So: does a "more full" fridge affect this mass air flow? Yes, it could. In the "still air" extreme, a "full" fridge will contain less air, and so less air will flow out, even after a few minutes of the door being opened. Unless you tip your fridge so everything spills out, the "cold sinks" of the chilled foods will remain inside even if all the chilled air around them evacuates; when you close the door again the coolant system has less work to do to bring the now-room-temperature internal air back down to chilled temperatures because there is just less air to chill.

In the "forced circulation" extreme, though, there is a distinct possibility that your food items are altering the designed air flow in the fridge, causing an outward flow of the chilled air rather than a door-parallel "curtain" flow. This isn't necessarily a factor of how much food is in there, but on the specific placement and shapes involved. Another factor is that if the fridge is virtually packed, the "chilled" air will be moving faster (assuming a constant-volume circulating fan, which is to be expected), and thus any leakage along even the door-parallel curtain flow will be increased.

We end up with a little bit of a mixed bag there. Having a more "filled" fridge will cause more chilled air to leave the fridge while the door is opened; however, having a more "filled" fridge will allow the replacement room temperature air to chill to refrigerator temperatures more quickly. My guess is that in "normal" circumstances it is probably a safe bet to say that the latter outweighs the former in terms of efficiency, and the advice is "correct". However, to get a real measure of how important it is for your fridge you'd need to run an experiment with a electricity meter.

My guess is that the effect of the fridge being filled is probably not the most important aspect of that appliance's electricity bill impact, but that it is also probably significant. If you want to take TLC's advice, fill the "empty spaces" in the fridge or freezer with low heat transfer coefficients which will thus retain "coolness" longer when the door is left open. But, any effect is secondary to the primary cause of losing your cool: leaving the door open in the first place. Get in; get out. Know what is in there, and where it is. If having a bunch of empty tupperware or campfire rocks in there gets in your way and makes your fridge-gazing take longer, then it's completely backfired. I think the more practical advice is simply: keep the fridge organized, and know when you are out of something. Then, you won't spend several minutes trying to find something that isn't there.

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Welcome to Skeptics! Please provide some references to support your claims. –  Sklivvz Aug 27 '12 at 21:44
Is there an "official" way to cite references? The above is just basic heat transfer / mass transfer thermodynamics, so aside from heat transfer links (ex, to Wikipedia) I'm not sure what references to provide. Basic source references suitable for online links: –  Tom Dibble Aug 27 '12 at 22:17
For example we expect references that show that the scenarios you mention actually happen. We are not interested at all in theoretical speculation on what could happen, but only in an empirical, fact-based report of what does happen in practice. That's the basis of scientific skepticism. –  Sklivvz Aug 27 '12 at 22:21
Basic source references suitable for online links: Convective Heat Transfer is essentially heat transfer by movement of fluids (air), Heat Transfer Coefficient describes how heat transfers between two things, such as a food and the air inside a fridge, or the inside surface of the fridge and the air inside ... –  Tom Dibble Aug 27 '12 at 22:28
Much as I hate to complicate things, if your fridge is very full then finding the mustard takes longer, meaning that you leave the door open longer. –  DJClayworth Aug 28 '12 at 15:26

Residential refrigerator efficiency is a very well studied field because of the impact these appliances have on overall energy consumption, but I have not found any evidence of a specific study on this strategy.

However, some reputable organisations do support the claim. According to the Consumer Energy Center of the California Energy Commission:

A full refrigerator retains cold better than an empty one. If your refrigerator is nearly empty, store water-filled containers inside. The mass of cold items will enable the refrigerator to recover more quickly after the door has been opened. On the other hand, don't overfill it, since that will interfere with the circulation of cold air inside. The simplest solution is to buy the right size for your family in the first place.

Portland General Electric agrees:

Keep it full.
Full refrigerators run more efficiently. To fill space, you can fill empty milk jugs with water and place in the refrigerator.

So the basic theory is that having a cold mass inside will help when the door is opened (which I find easy to believe, similarly to how the sea makes temperature milder), and that this saving will pay back the cost of making such mass cold in the first place (which I find possible, but nowhere certain).

These are the best references I could find and while they do come from reputable organisations, they don't seem to be based on experiments.

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Sklivvz, thanks for your answers - these do seem to be the best references I've seen so far. The California Commission answer makes me suspicious ("don't overdo it" implies there is a point where it becomes less efficient with more contents, yet they don't provide any evidence for that or suggest how that could be determined). Still, these seem to be the most reliable information I've yet seen. –  Andrew Ferrier Aug 27 '12 at 23:20

I apologize for muddying the original answer by having it seem to focus on the door opening and closing, that was not my intent. Here is a cleaned up answer:

There are many factors that contribute to the effiency of a refrigerator. Determining the impact of one factor, the fullness of the refrigerator, would be quite difficult. If you are looking for a hard and fast numerical answer, you won't find one here. I am provided an experiment someone could do (but is beyond my expertise).

According to the sources that I can find (see below), what is in the fridge is just as important a factor as how full it is.

A fridge requires a certain amount of air flow in order for the fridge to maintain a temperature efficiently. If what you have in the fridge impedes the air flow, then the fridge might actually be less efficient.

Humidity levels are also a factor, so if you have things in the fridge that can change the humidty (fresh fruit/vegetables, uncovered pictures of liquid), that may also contribute to less efficiency.

Ideally, an experiment could be done that could give a rough idea of the impact of things in the fridge (but that is beyond the scope of what I can do):

• Start with an empty fridge, allow it to get to a set temperature, and then monitor its energy usage over a period of time (say 24 hours).
• Repeat the experiment with a half full refrigerator and see if there is a difference.
• Repeat the experiment with a full refirgerator
• Repeat again with an overstuffed, no airflow fridge.

In order to limit variables for these tests, what goes into the fridge would have to be the same for each step, and go into the fridge at the same temperature (i.e. boxes of juice that start out at room temperature or something similar that allows you to test various airflow configurations).

However, I do not know if the results would by typical of real world usage. The contents of a fridge change daily (at least in my house). Getting a real world result would mean that we would have to know how all of the other factors also affect efficiency.

US Government Refrigerator Fact Sheet

Here is some advice on different factors that can affect efficiency from Frugal Living.

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This is not an answer to the question. Does the amount of stuff in the fridge have any influence? It's self-evident that leaving the door closed helps, but that's not the question being asked. –  Sklivvz Aug 22 '12 at 20:12
As I said, there are many factors that influence a refrigerators efficiency. The most common is how long the door is open, but what is in the fridge and how full it is (and what it is filled with) also impacts efficiency. A fridge needs to be able to circulate air, so if it is too full, that is a factor; if there are open containers of liquids, that will change the humidity and be a factor, etc. There are too many factors to limit it to just how full the fridge is (which is what is reflected in the links I provided). I focused on the door as that was what was linked to in the OP. –  Ruminator Aug 23 '12 at 13:14
The question is whether the amount of stuff in the fridge has an impact or not -- all the other factors being constant. If you don't know the answer, then don't answer. –  Sklivvz Aug 23 '12 at 19:32
All of the other factors are not constant in the real world. I did update my answer to better reflect the intent of what I was trying to say, and provided an experiment that someone with the right equipment could do that might provide a result. I don't think that a hard and fast numerical answer is possible. –  Ruminator Aug 24 '12 at 13:05
Welcome to Skeptics! We are looking for definitive answers. Merely posting a description of a hypothetical experiment doesn't give us an answer. Finding someone who has done a similar experiment would. –  Oddthinking Aug 24 '12 at 14:04