Sunday, November 1, 2009

How to boil water - the sequel

One year ago, I wrote a blog titled how to boil water, which linked to a short essay, "how much is inside hot water?". Over the subsequent 12 months, a flood of emailers have requested that I answer their follow-up questions: "does it make any difference if the lid is on the pan?" and "how does a microwave compare with the pan and the kettle?". Dutifully, I did experiments this Sunday, and this link describes the results in full.
The conclusions are that keeping the lid on the pan while boiling water saves about 3%; and that the microwave is a hopelessly bad way to boil water for making pasta.


Milan said...

Do you have a hypothesis on where the energy from the microwave is going, if not into heating the water?

You could also try using a Kill-O-Watt type meter to work out how much electricity the microwave is actually drawing.

David MacKay FRS said...

@Milan, yes I used a meter to measure how much electricity the microwave was drawing - and I said how much that was.
The extra energy is going into making the microwave oven's internal organs hot.

Robin Smith said...

If you cook a lot of veg, don't throw the water down the sink until its cold. Let the energy dissipate into the house first.

Its not that good a comment but might save some energy.

Robin Smith said...

Oops my apologies you do mention in part4? how the heated water warms the house in winter.

May I suggest you take into account the real factor here. How much each method costs? After all, isnt it true that we woudlnt be here if people cared more about the environemnt than feeding their familly tomorrow?

Lets say (I think these numbers are right):

gas = £0.03/kwh
lecky = £0.13/kwh

No need to do the maths!

Unknown said...

according to the instructions of my microwave, the water heated as described in the post can actually reach a temperature of 100C or even higher without any sign of bubbles. it seems that this can be dangerous and it's discouraged by the manual (you should put a special stick in the glas). on the web there is a number of stories about water "exploding" after being heated in a microwave...

Milan said...

Sorry, I thought you depended on the 900 W rating, rather than actually measuring the amount of energy drawn.

Brian O' Hanlon said...


I came across this blog entry today, and I remember you have crossed swords a bit with Howard Liddell etc on the subject of heat pumps. Many of the passive designers out there, aren't great fans of renewable technologies.

Anyhow, for your interest:

I honestly don't know myself whether heat pumps are daft or not at this domestic scale.

Unknown said...


I have replicated your tests but with an ceramic electric hob rather than gas. Kettle (2.8kW measured and on full 240V in this rural area) is definitely best at:
0.12kWh taking 2m 36s

Different sized pans on matching rings used about:
0.2kWh and took 8m to 9m

The microwave used about:
0.27kWh and took 14m

Kettle is most efficient and quickest.

Annz said...

Interesting. At a lecture in the Engineering department (in Cambridge) last Wednesday, I heard it suggested that a saving of 90% energy might be possible if a lid was placed on a saucepan, rather than using the pan without a saucepan.

I think your experiment (which is not necessarily entirely incompatible with the suggestion) illustrates clearly how in practice any potential saving from puting a lid on the pan does depend on quite a lot of things. These would include how long the saucepan is left on the hob, and what the contents are (eg a stew might take a long time to cook), quite apart from the saucepan used.

This might suggest that meals cooked from raw ingredients in a short time might tend to be most energy-saving.

Looking at the saucepan you used in its photo, it looked a reasonably solid heavy-ish construction (which tends to give better cooking results in most circumstances) with a relatively thinner, probably not very insulating, lid.

However, if I really wanted to boil water in a saucepan, I think I would go for very light pan, with a diameter no larger than the hob, and with high sides (the type traditionally sold as a "milk pan". This might possibly have given better results, although I would still expect the electric kettle to win on energy consumption.

I do think it is a credit to the designers of electric kettles these days that they are so efficient.

It would be interesting to know what the result would be if a whistling stove-top kettle had been used. I suspect it would have used more fuel, although conceivably it could have been cheaper because of the relative price of gas.

D said...

This prompts me to comment that it's misleading to count energy on a delivered energy basis and to equate 1 kWh of delivered gas or LPG to 1 kWh of delivered elec.

This isn't a valid convention for decades to come if ever. Our fossil & nuclear elec. gen. system inevitably rejects heat and has a first law efficiency of about 0.36 (more for gas, less for coal or nuclear) from primary to delivered energy. This issue also affects biomass-fired generating plant, e.g. landfill gas, biogas from digesters, the wood to be used in Port Talbot and being used in Drax.

In the past experts were careful to make this distinction. We must be careful today. Some in the above discussion are under the impression that 1 kWh electricity has the same environmental impact as 1 kWh fossil fuel, which is totally unrealistic.

Unknown said...

You mention about the losses from delivering the electricity but not from delivering the gas. Although I do suspect this will be small.

And to make things a little more complicated, the lid would increase the pressure in the pot and therefore increase the temperature of boiling. Also as Anna said it depends on when the lid is put on. If it is put on when it's boiling wouldn't it stop more heat (energy) escaping (via steam) than if it wasn't boiling. Please correct me on these if I am wrong