How much energy does it really waste leaving the lid off a pan?
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Headlines
When boiling water in a pan on a hob (such as vegetables) or other very watery contents leaving the lid off uses around 50% more energy.
This is unlikely to be true for all hob saucepan cooking. Simmering pans and pans with contents that are less watery will probably waste less with the lid off though is still likely to be worthwhile.
Keeping lids on greatly reduces the amount of water boiled off into the air in the kitchen, and so reduces condensation and damp issues, another strong reason to kip lids on.
The recommendation that we put lids on saucepans as a way of saving energy is widely communicated. But how much energy does leaving a lid off actually waste?
Until now I've not seen any hard figures on this.
Very helpfully, a properly thought through test has been done by someone in this case testing boiling a pan of potatoes both with and without a lid.
The experiment showed that cooking with the lid off used 60% more energy.
Comparing to other types of pan cooking
Boiling anything such as water (or a pan contents that are very watery) it will always be more energy efficient to leave the pan lid on.
When cooking pan contents that are less watery (such as soups, porridge, gravy, custard, stews etc) I would expect there to be less energy wasted with the lid off, though still significant.
The reason is these sorts of foods don't conduct heat so well and so won't transfer the heat from the intense hob heat at the bottom of the pan to the top so easily and so won't encourage water to evaporate off the top so quickly.
Conclusion
So perhaps, until other tests or data emerge, a 50% is a good rule of thumb to assume for most pans that are boiling, cooking quite vigorously, or are simmering at a high temperature.
Certainly if you hover your hand over an open pan cooking you can feel the heat coming off, that is basically wasted heat and not going into your food!
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Mark Thompson
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