How much energy do you save by putting a lid on a saucepan

Someone tried to tell me the other day that putting lids on saucepans doesn't actually save any energy. I had to admit that when I did my experiements for the book, since I had been concentrating on comparing cooking on the hob and in the oven I did not actually test the effect of lid on/lid off. So I did another experiment today and here are the results.

I used the same method as I used in the book. In both cases I boiled 450g potatoes (2 servings). I weighed the pan, potatoes, and water before and after cooking, to see how much water had evaporated and calculated the energy needs from this and known heat capacities. The measured gas usage was double with the lid-off but the measuring unit is too large to make that accurate. In fact I estimate cooking with the lid off took 60% more energy. This pales into insignificance when compared with the extra energy to cook in the oven but it is still a significant increase. The difference was slightly less energy to heat the pan (without the lid) and slightly more to heat the water because there was more to allow for evaporation -- in fact there was less left at the end than in the lid-on experiment. However there was much more energy removed by evaporating water: 4 times as much.

I would do this experiment a few more times but I have enough potato salad for now.

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