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Does filling your fridge make it more efficient?

Posted by Alex Barrett on 6 January 2017 at 5:10 pm

It has been suggested that a full fridge is more energy efficient than an empty one. Many advice sites actually recommend filling up the excess space in your fridge with bottles of water in order to make it run more efficiently. Unfortunately this is a myth. Let’s examine why this is, and what it tells us about fridge efficiency.

A fridge consists of a well-insulated box, and a heat pump. Refrigerant is pumped through a series of coils at the rear of the appliance. It absorbs heat from the interior, and then transfers that heat to the outside [1]. The more heat you need to remove from the interior, the more work the fridge has to do. Consequently every time there is an influx of heat into the system it will require more electrical energy to cool everything down again.

The argument goes that when the door is opened the cold air will flow out, being replaced by warm air. Reduce the amount of air space in the fridge, and less air can be exchanged. Fill that space with water and you won’t need to cool down a mass of hot air every time you open the fridge [2]. This is true to an extent, but for all practical purposes it won’t work. What we need to think about is how much energy is lost when the door opens and how much is needed to cool down all of those bottles of water in the first place.

Specific heat capacity is the amount of energy that is needed to change the temperature of one kilogram of a substance by one degree Celsius. For water this is 4.18 kJ / kg °C [3], for room temperature air it is much lower; 1 kJ/kg °C [4]. Consequently it will take far less energy to change the temperature of the same mass of air than it would an equivalent mass of water.

This means that when you open the door of your fridge it is far easier for the air to warm up than the water (or the contents of the fridge). However it also means that water is much harder to cool down in the first place.

We need to consider the volume of air in our fridge, rather than the mass. Air is far less dense than water. One kilogram of water takes up a litre of volume. One kilogram of air takes up 817.7 litres. The exact mass of air that will be lost when you open the fridge door will depend on the size and model. However unless you have a really big fridge there will not be the same mass of air in there as there is in even one bottle of water.

Cooling down enough water to substantially reduce the air volume will require far more energy than repeatedly cooling down the air. It has been estimated that you would have to open the door of a fridge thousands of times for it to be worthwhile [5]. Unless you are going to keep the water in there for months without using it then it won’t really help. The only way to make this work would be to cool down the water before you put it into your fridge, maybe with liquid nitrogen, or using someone else’s fridge. Either way it isn’t really energy efficient.

So what can we do?

Clearly the less time you spend with the fridge open the better, but in practice the air exchange will happen pretty rapidly. So long as you are not leaving the door open for more than a few tens of seconds then you are probably ok.

It is best not put hot food straight into the fridge. If you let things cool down to room temperature before refrigerating them then you are introducing less energy into the system, and so forcing your fridge to do less work.

If you are not filling your fridge to capacity on a regular basis then you could switch to a smaller one. Buying a small fridge will be more expensive than filling the large one with water bottles, but will be much more likely to save you energy.


  1. How a fridge Works
  2. Dubious energy tips at How Stuff Works
  3. Specific heat capacity of water
  4. Specific heat capacity of air
  5. Calculation of the relative energy use of cooling air and water.
  6. Some further reading; a study on the rates at which food cools down when the fridge door is opened and closed. Tyrewala, A. S., Nelson, D. G., & Almanza, B. (2015). The Effects of Door Opening and Food Placement on Food Temperature within the Refridgerator when Power is lost during a disaster. (Vol. 1). (Links to pdf)

Image Credit: Alex Barrett 2016


If you have a question about anything in the above blog, please ask it in the comments section below.

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1 comments - read them below or add one

Malcolm M

Malcolm MComment left on: 1 February 2017 at 7:56 pm

I agree with everything Alex has said, but if you still feel compelled to fill your fridge to reduce the loss of cool air you could always use empty (i.e. filled with air) plastic bottles to fill the fridge

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