In Facebook groups I see many people using a sous vide stick in a pot or in a container without a cover or insulation. This made me wonder how much difference it actually makes for your electricity bill and your carbon footprint. And so I decided to do an experiment.
For this experiment I heated up 8 litres (8.5 quarts) of water with a sous vide stick to 74C/165F and then switched off the heating but continued the circulation, by setting the target temperature to a very low value. After an hour I noted the temperature that the water had cooled off to in that hour. This is a good indication of how much power would have been needed to keep the water at 74C/165F during that hour.
With a polycarbonate container without cover or insulation, after an hour the water had cooled off to 48.7C/119.7F. Because water has a heat capacity of 4186 joules per litre per degree centigrade, we can calculate that .24 kWh of heat was lost during that hour. (So if you cook something for 24 hours at 74C/165F, in a container without cover or insulation, approximately 6 kWh of power will be lost and need to be added back to the water by the sous vide stick to keep the water at the right temperature.)
Furthermore, from the original 8 litres of water…
…about 1/4 litre (1 cup) of water had evaporated. So for longer cooks the water level will become too low, and the sous vide stick will stop.
Then I repeated the experiment with a cover. Now the temperature dropped quite a bit less, so 34% of the heat loss was prevented by adding the cover.
The water level also stayed at 8 litres, as most of the water that evaporated condensed against the cover and then dripped back into the container. So a cover does not only prevent heat loss, it will also ensure that you can do long cooks without having to check the water level.
Next I added a neoprene sleeve to insulate the container.
The insulation and cover together reduced the heat loss to 51%.
Putting a layer of neoprene on the cover caused a further improvement to 62%.
As a final step I added sous vide balls, that float on top of the water. This produced a minor additional improvement to 66%. So two thirds of the heat loss, and therefore the power usage of sous vide cooking, can be prevented by adding a cover and insulation.
As a reference I also tried the sous vide balls by themselves, without a cover and insulation. The result of this was slightly better than the lid only, but close.
Personally I don’t like to use the sous vide balls, because when you empty the container they tend to bounce all over the place. And as you can see from the results of my experiments, they do not add significant value if you already use a cover.
I also tried a sous vide stick in an 8 litre stainless steel pot. The heat loss was 15% smaller than the polycarbonate container. This can be explained because on the one hand the stainless steel pot has a surface area that is 32% smaller than that of the polycarbonate container. On the other hand, the polycarbonate container insulates better than the stainless steel.
Finally I also tried my SousVide Supreme, which has insulation and a cover. The result was very similar to that of the fully insulated polycarbonate container.
I monitored the ambient temperature and relative humidity during all of the experiments, which stayed constant at 18C/64F and 55%.
This table shows the results of all the experiments. If you care about your electricity bill and/or your carbon footprint, you should at least use a cover, but it also pays off to invest in insulation. A cover and insulation together can save about two thirds of the energy needed for sous vide cooking. For the cooking result you will not notice any difference — unless you ruin the cook because the stick stops halfway through due to too much evaporation.