Does a refrigerator cool more efficiently when full?
When I told my wife that one reason our refrigerator was having trouble maintaining a low temperature was that it was overfilled, she instantly produced three household-tips books that said a refrigerator is more efficient stuffed to the gills than it is empty. This seems to contradict the first law of physics: You don't get something for nothing. If we take Heloise and her fellow hinters at their word, it costs less to maintain a warehouse full of fish sticks than a tray of miniature ice cubes in a motel-room fridge. Given two refrigerators, one jammed tight with Jell-O molds and the other empty of everything but air, which box works harder, uses more energy, and costs more to run?
You get married yesterday, Bruce? You should know that once your wife starts whipping out the helpful-hint books, magazine articles, etc, you and your half-remembered physics don't stand a chance. While the First Law of Thermodynamics holds true even in kitchens, you forgot about the laws of heat and mass transfer--no trifling matter. If we consider only appliances that most people are likely to use on a regular basis (no pool pumps or spa heaters, please), the refrigerator arguably sucks up more electricity than anything else in the house.
Here's the crucial factor you're overlooking: Every time you open the door to the refrigerator, you let out cold air and let in warm air--and when you shut the door, all that warm air you let in has to be cooled down. The more air space in the fridge, the more cold air that can be swapped with warm air from the kitchen. More warm air = more cooling required = higher fridge juice consumption = lighter wallet.
You don't want to cram in so much stuff that chilled air can't circulate; parts of the fridge will take longer to cool down then and the refrigeration unit will just have to work harder. But in general, yeah, keep that baby full. Some utilities recommend keeping gallon jugs of water in the fridge to fill up empty space, and ice bags in the freezer too.
It's true that when you initially load up your fridge with room-temperature food and drinks, the motor has to chug a while to cool it down. But once the contents reach the desired temperature, the motor doesn't have to work hard to keep them there. The fridge sees a steady transfer of heat from the room to its interior, but that doesn't appreciably change whether it's empty or full. Some online theorists claim that the larger thermal mass of a full fridge (as distinct from the reduced air space) increases efficiency, since the compressor will run fewer times but for longer periods--the flywheel effect, let's call it. But my guess is that the contribution of thermal mass to energy efficiency is relatively minor. For that matter, whether the fridge is full or empty is a secondary consideration in the grand scheme. On a day-to-day basis, the main way to save energy is to shut that damn door.
A word needs to be said about this. Cherished American tradition though it may be, parking yourself in front of an open refrigerator while deciding between French and ranch will cost you. Depending on the size and efficiency of your refrigerator, poor door-opening habits can increase energy use by 5 to 10 percent, and in one study as much as 28 percent.
If energy efficiency were the only concern, your fridge would likely look like an old coffin-style freezer, whose lid can be opened with minimal loss of efficiency because dense cold air doesn't spill out. But a horizontal fridge would hog floor space and be tough to get things in and out of. What about dispensers that enable you to get ice and chilled water from your fridge without opening the door--don't they save energy? Uh-uh. They actually can increase refrigerator energy usage by 10 to 20 percent or more, no doubt due to heat infiltration through the dispenser opening plus the energy consumption intrinsic to ice making. One convenience you see on newer refrigerators--small drink-access doors--is advertised as increasing fridge efficiency by reducing heat transfer to the inside every time you grab a soda. Makes sense, but I couldn't find a good study to prove it.
So your best bet is to make efficient use of the fridge you've got. Make sure the seals on the door are tight--take a dollar bill and close the door on it along all four sides. If you can easily pull it out, then the seal may need adjusting or replacement. Pack your fridge full of brats and beer, a good policy regardless. Finally, and I think this applies as much to geopolitical excursions as it does to refrigerators, decide what you want beforehand, then get in and out fast.