QUESTION: People sometimes say it is better to finish the last of the milk when there is little left but why is that? Is that more of a courtesy or does the refrigerator use more energy to keep a near empty carton cold than a relative amount of energy related to the volume of milk left?
ANSWER: Hello James,
One argument I heard from my wife (don't tell her I reported this) is that it is a matter of courtesy. Something about when she checks what we need before going grocery shopping. I wonder: would she think we needed a new milk jug only if no milk remained and the jug had been discarded? If so, would she have me drink a second full glass when one was all I needed? Again, if my wife hears about this, I deny saying it.
Regarding energy use: that supports keeping the old jug, even if empty, until a new one is obtained and will use the space the old one occupied. Every time the door is opened, cold air falls out of the refrigerator and is replaced by warm air from the room. That air then needs to be cooled.
The only argument I see in favor of finishing a small amount is in the courtesy category. If the motivation for leaving it in there is to postpone doing it in hopes that someone else will do it, then let your conscience be your guide.
I hope this helps,
---------- FOLLOW-UP ----------
QUESTION: So assuming someone also puts a full milk behind the near empty one to make sure there is no need for a quick trip to the market, lets say it's a gallon container.
If you kept the old one in there and it contained only a pint would the fridge only use energy needed to cool the carton itself and the 2 cups of milk left? Nothing really wasted energy-wise?
Right, as long as that milk stays in the cold fridge, the fridge is not required to work to cool the milk. The fridge runs only to keep the air cold. It's just that anything the raises the air temperature (like putting in some warm food), forces the fridge to cool the air again. That pint of milk is affected almost not at all when you open the fridge to get out the leftover pizza from yesterday. Then if you take out that nearly empty jug of milk, pour half of it in a glass, and promptly return the jug to the fridge, the remainder is still quite cold so that action has only a slight affect on the temperature of the air (ignoring for the moment the warm air that enters while the door is open).
There are 3 things that force the fridge to run for a significant time:
heat that gets in past its insulation and gaskets with the door closed,
cold air falling out and warm air replacing it when the door is open, and
putting in warm food.
I hope this helps,