In a freezer, which would use more electricity?

Question

1- an empty freezer, just air.

2- (Same kind of freezer) filled with bottles of water. The bottles are swapped with unfrozen water daily.

Water will retain it's cold temperature better than air, but is this evened out by the fact that it takes more power to freeze it in the first place? What if you only swapped some of the bottles daily?

Answer 1

The simplest way to see this is to realize that every time you put in an unfrozen bottle of water into the cold freezer, you are addinng heat to the freezer, so the motor must run longer to remove that heat.  That is why when you have a power failure,  "they" tell you to keep the fridge closed as much as possible, so as not to introduce heat into it.

Answer 2

The second choice would certainly consume more energy.  The specific heat of air is much lower than water.  Therefore, to lower the temperature of the water requires a lot more energy than lowering the temperature of air.

However, if one left the frozen water bottles in the freezer, it would quickly save energy.  While it would initially take more energy to freeze the water bottles, those bottles displace air in the freezer.  Whenever the freezer is opened, a lot of that cold air is replaced by warm air that must be cooled.  The more objects in the freezer, the less cold air that is lost and replaced by warm air.  

Another advantage of leaving frozen water bottles in the freezer is that it protects the food there against a power failure.  The frozen water bottles will keep the compartment colder than air will.  The bottles could then keep the food cold until the power is restored.