# Does an empty refrigerator require more power to stay cold than a full one?

Given that everything else is equal (model of fridge, temperature settings, external temperature, altitude), over a given duration of having the door closed, does it require more electricity to cool an empty refrigerator AND maintain that temperature, than a full one?

In real-world cases there can be a difference though. The difference depends on when the refrigerator decides to cycle on and cool. If the fridge cycles on a timer or based on heat energy then there will be a difference due to the added heat capacity. The outside of the refrigerator will acquire heat due to conduction, convection, and radiation from external sources.

All heat transfer depends on $\Delta T$. The greater the difference in temperature between two systems the faster heat will flow. When you add heat energy to a full refrigerator, the system has greater heat capacity so the temperature changes more slowly and $\Delta T$ is greater than it would be in an empty refrigerator. If the refrigerator could keep the temperature absolutely constant at all times the difference would not matter. Because a real system cools and then stops cooling in discrete steps, a loaded refrigerator acquires heat from the environment slightly faster because it stays colder for longer.

The difference is so small though that I'm sure it couldn't be measured.

But, if the refrigerator only cycles on at a specific temperature there will be no difference. I wrote a numerical simulation to test this. Here is the plot:

The simulation is a simplified, idealized, purely numerical one, hence the lack of units. The Y axis is temperature and the X axis is time. The red curve is the loaded refrigerator and green curve is the unloaded one. For the simulation I made the loaded fridge have double the heat capacity.

Assuming the refrigerator only cycles on when the temp reaches the top of the graph then there is no difference between the two. Although twice as much heat energy must be put into the loaded refrigerator, the simulation shows that it takes exactly twice as long. That is, the average rate of heat flow is identical.

So, the answer to the question depends on how the refrigerator works.

• If the refrigerator is time interval or heat energy interval based, a loaded fridge takes more energy to maintain a cold temperature.

• If the fridge is purely thermostat based, there is no difference in energy consumption.

• Oh, given that typical refrigerators have the cooling loop controlled by cheap thermocouples it can probably be measured because there is a finite difference between the target temperature and the temperature at which the compressor kicks in. But I didn't want to get into that. I declare that my answer relates to a uniform spherical refridicow. Yeah. That's it. Apr 17, 2013 at 23:08
• I disagree. The full refrigerator will cycle slower than the empty one but I think it's going to spend the same percent of it's time at each point in the cycle and thus have exactly the same heat loss. Apr 18, 2013 at 0:41
• @ClaytonStanley, for a real refrigerator the temperature is allowed to creep up a tiny bit before cooling starts again. If there is more heat capacity in the fridge because it's loaded then more energy has been absorbed from the environment to move the temp and more energy must be removed. For the ideal fridge where heat energy is removed exactly at the rate it enters then the overall loading of the fridge doesn't matter at all. Apr 18, 2013 at 4:18
• I'll phrase Loren's original comment in another way. Yes it takes more energy to perform a cooling cycle when the fridge is full, but a full fridge performs 'less' cooling cycles over time. The average amount of power required for each system is the same, since the system spends the same proportion of time in each temperature state. Apr 18, 2013 at 5:29
• Yes it stays colder for longer, but it also stays less colder for longer. This is what Loren what mentioning about how the proportion of time spent in each temperature state is the same for each system. The full fridge just has an elongated charging exponential of T(t), while the empty fridge is squished. But the average value of each curve is the same. At least that's my current thinking. Happy to be convinced one way or the other :) Apr 18, 2013 at 6:25

The heat loss (power) at a particular temperature is the same. So, No - the cooling needed to maintain the thing cold stays roughly the same.

However, the empty fridge has lower total heat capacity. So, it will get warm faster in the absence of power. So, it is worthwhile to fill your fridge and freezer with bottle of water a few days before a big storm that might bring down the power.

• Well, filling the fridge might help prepare for power outage, but this question is more about efficiency and the fridge has to do extra work to cool the stuff that you added to prepare for the outage.
– Tom
Jun 23, 2016 at 18:02

No. The rate of cooling must simply match the rate of heating, and heating rate depends only on the temperature difference you want to establish and on the thermal conductivity and surface area of the walls.

More stuff in the refrigerator would give it a higher heat capacity, so that it wouldn't warm up so much when the door is opened. However, it will take considerably more energy to cool it initially from room temperature.

Forgetting about the temperature change, you can look at the heat change. With the cold stuff being immovable solids rather than air (which tends to just spill out of an opened refrigerator, even before it heats up), there will be less total heat lost whenever the door is opened. In this sense, the empty refrigerator might take slightly more energy to reestablish equilibrium after the door is opened.

• also a bad seal can introduce room temperature air into the fridge which needs to be cooled, this is more significant when there is more cooled air to be exchanged with inside the fridge Apr 18, 2013 at 1:11

Short answer: because food is more dense than air, and thus can retain "coldness" longer, once it has been cooled down.

All refrigerators leak temperature, mainly through joints. Problem is, in an empty refrigerator, all you have is cold air (which is the main heat transmitter inside a fridge).

Look for heat capacity. Thermodynamics is really an eye opener on physics. Makes you think about many many things about things arounds you.