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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
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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
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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 –  user15105 Aug 22 '13 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 '13 at 3:05
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1 Answer

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.

Consumer Energy Center – California Energy Commission

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.

Kitchen Appliances: Refrigerators | PGE

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
    
As a side effect, having your fridge (and especially your freezer) packed full of water (ice) will keep your food cold much longer in the event of an extended power outage. Most of the contexts of our freezer and about half the contents of the fridge were salvageable four days into a week-long power outage we experienced after a hurricane. We opened it at the four day mark because that was when ice became readily available again and we could move everything to coolers. –  dmckee Dec 28 '13 at 3:24
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