I sometimes end up boiling water in a pot without putting a lid on top because I can't find the lid.

How much energy is wasted when bringing water to a boil without in a pot without a lid? Or differently stated, how much more efficient is heating water in a lidded container?

I'm mainly interested in the context of home cooking, but this definitely has application to my hiking trips as well.

I expect the answer will scale based on volume of water, but other variables won't matter as much.

  • My question here is slightly different from this related question: sustainability.stackexchange.com/q/815/1056 – Nic Mar 7 '20 at 17:30
  • Using a lid while boiling/cooking reduces heat loss, thus the item being boiled/cooked reaches the desired temperature quicker, consequently less energy is used in boiling/cooking. – Fred Mar 29 '20 at 9:47

We can try to make back-of-the-envelope calculations here, or try to find research but I suggest you do your own because it is quite straightforward.

Just maintain a simple table with

  • volume brought to the boil
  • lid on/off
  • time required to bring to the boil

My guess it that after approximately 10+10 measurements you can calculate the answer.

If it is not just about bringing water to the boil, but to cook food until done, you may want to add a fourth column listing the ingredients - because e.g. beans take much more time than potatoes (the saving with lid would be greater). The actual mass of ingredients does not matter much, because it's all about cooking time.

If your pots and pans have very similar diameters you can just write down the water level instead of volume.

  • I will try this. – Nic Mar 7 '20 at 18:44
  • 1
    The penultimate paragraph is crucial: Measuring the time to bring to the boil will underestimate the loss if you're simmering, even if you scale by the time, because loss to steam is greater at higher temperatures. You could weigh everything and work out how much water is boiled off, at 2.2kJ/kg – Chris H Mar 9 '20 at 10:54

TL;DR: power saving with a lid comes from the fact that you can use a lower power-mode on your cookstove. Energy used to bring the water to the boiling point is roughly the same with or without the lid, energy saving occur after that point.

Full answer

I'm afraid that the question and the current discussions are quite ambiguous on one capital point. In case it is not obvious to everyone, I will therefore make it explicit here.

First: using a lid is more efficient, no questions, but it's much better if you make full use of it.

What's the catch?

Unless you really optimize your cooking and do things faster when using the lid (which, from experience is not that significant, at least for me), then using the lid will save almost no energy [1].

This is simply because the cookstove is sending the same amount of power to your pan or pot whether or not the lid is on. Using the lid enables to reduce water evaporation and the associated energy loss, as well as some radiative loss (probably negligible).

Now, if you're cooking pasta, using the lid will usually make you save energy because you'll have to reduce the power to avoid overflow, but if you're frying stuff, then you might keep the same power as without the lid, in which case you're basically not saving much...

Some numbers

I have induction at home, and my small stove varies between 160 and 1400 W when I vary the power setting from 1 to 9. I'll assume here that the power varies linearly (might be wrong but I did not find an answer to that question).

So let's make some pasta (200 g) using 1L of water.

With the lid:

  • Power 9 for 4.5 minutes -> water boils, add pasta
  • Power to 4 for 10 minutes -> pasta cooked

Without the lid:

  • Power 9 for 4.5 minutes -> water boils [2], add pasta
  • Power to 6 or 7 for 10 minutes -> pasta cooked

Power consumption is P(N) = 160 + 1240*(N-1)/8 Watt
Energy consumption is E = (boiling_time * 1400 + cooking_time*P(N)) J

Here, this gives:

  • with the lid, 10 minutes at 625 W (power on 4) gives a total consumption of 753 kJ or 0.52 kWh
  • without the lid, at 935 W (power on 6, for me), one gets 939 kJ or 0.65 kWh
  • without the lid, at 1090 W (power on 7, for my girlfriend, who thinks it does not boil strongly enough at 6), one gets 1032 kJ or 0.72 kWh

So with the lid, lowering the power to 4 enables me to save between 186 and 279 kJ on pasta, without changing anything about the time that it takes to cook it nor the exceptional quality of the final dish!

To give an order of magnitude, the average energy consumption per day is around 10 kWh in Europe so this energy saving is roughly 2 percent of that value, but can get much larger if you simmer a dish for a long time.

[1] Note that this would be quite different if a pressure cooker was used, but I'm assuming a regular lid here.
[2] Note that the time necessary to make the water boil is the same with and without the lid, which is perfectly normal since, before it boils, we are not wasting energy into liquid-to-vapor transition and the other energy transfers are very small.

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