Sunday, October 19, 2008

How to boil water

A friend told me he'd been fighting with his kitchen-cohabitants over the question of whether to simply use the gas to make hot water for pasta, or whether to use the kettle, then put it in the pot.
To answer this question quantitatively, I did some experiments and I've written a new webpage, How much is inside HOT WATER? The page assumes that the motivation is to save energy. The conclusions apply to Britain today and to similar countries.

My conclusion is that using the gas hob is slightly better in energy terms than using the kettle; but my recommended behaviour depends on the time of year. In the winter, if you would like to have more heat in your kitchen, then using the gas hob alone is definitely best; in the heat of summer, using the kettle may be preferable.

Disclaimers, small print...
If your motivation is to save money then the answer will depend on your fuel prices. If your motivation is to cook the pasta as fast as possible then you should use neither method alone - you should use both the kettle and the hob, with roughly half in each.


Iain Murray said...

A related common source of arguments is the question of whether to fill the pan and/or kettle from the hot or cold tap. However, as an article in the New York Times explains, the hot tap should be avoided for health and safety reasons.

Unknown said...

Nice research - but I have an electric (ceramic) hob, should I use that or the kettle?

David MacKay FRS said...

@gavin - I don't know; I expect it's very close; can you do the experiment and let us know?

Alex said...

Hi, former student here, I remember your lectures well, and am still a convinced Bayesian.

Anyway, how much water did you use? Can you say what % of the energy consumed actually was needed to heat the water from the initial temperature to 100C?

Also, in Japan they have kettles that keep the water hot at around 80C (or hotter if desired) and let you decant it as and when desired. This struck me as being ludicrously inefficient. Perhaps though with adequate insulation it might not be, compared to a method in which you boil a slightly excessive amount of water from cold for each pot of tea. Any thoughts?

Unknown said...


I have had the same debate, but with respect to cofee & tea, not pasta. I just repeated your experiment, but I used a standard hob kettle on my gas stove.

Measured 0.73 cuft = 0.22 kWh gas to take 1 litre of water from 12 degC to full boiling. Using your calcs, that is about 25% less than the electric kettle uses at the power station.

I reckon that maybe a saucepan (as you used)wastes about 30% more energy by heat flowing up the side. Note I used a fairly small gas ring - if I had used a larger one it would no doubt have been a bit more wasteful.

Anyway, on straight energy terms, the gas kettle is significantly better. Plus I get the bonus of an extra 0.1 kWh approx of heat in my home…….



Oliver said...

I've wondered about this on and off for ages: thanks very much

Oliver Morton

(on hot tap: surely relatively few homes have lead in pipes now?)

Ecorenovation said...

And what about the microwave!!

Jeremy Bickerstaffe said...

Your analysis doesn't include the efficiency of the gas distribution network. How much gas energy is lost by sending it to your house?

Chris said...

Microwave? I attemped a simple experiment, boiling a single cup of water (350ml) for my tea, using the same kind of energy meter that David uses.

My 3Kwh kettle consumed 0.04 kWh of electricity to boil the water poured into it from the cup.

The 900W Microwave consumed 0.07 kWh of electricity to boil the water in the tea cup.

Compost John said...

I love this blog and would like there to be a 'followers' button, like I have on mine
(scroll down, in the right-hand column)
When you have this application, I'll become a follower and will be notified when you post. Currently you are in my 'favourites' list!

I boil water on my smokefree woodstove, which uses virtually (fossil) carbon-free fuel (brought to my house using cycle trailer), chopped either by hand with bowsaw (embodied energy here) or with my electric chainsaw (embodied energy here too, but leccy from 'Good Energy'.. 100% renewables).

I use the microwave to heat water in individual cups when the stove's not lit, as the water heated is just that in the cup, whereas in a kettle, you heat usually more than one cup...
But, if I heat one cup to instant coffee temperature, it takes 2 minutes. Two cups take 3 minutes and three cups take just 3 and a half minutes... so this should be taken into consideration when deciding to use the electric kettle, gas hob or microwave to heat water.

I look forward to reading more posts on this important subject!
John Cossham, York, UK

Philip Machanick said...

Gavin, let's see how far we can go with David's numbers.

The total transaction with the kettle+gas stove is 0.36kWh. We should adjust this for the electric hob but the difference will be small (a fraction of 0.06) so let's use this as a baseline.

The total energy needed to make the water boil on an electric hob is the same as the kettle, less energy leakage from less efficient energy transfer. The first number is 0.3kWh, so as long as the energy transfer is at least 83% efficient (with respect to kettle=100%), the stovetop method will be competitive.

That number is not only a property of the stove but also the container, as a container with better thermal conductivity on the base than the sides and lid will give better results.

I suspect you would get better results overall if you trained everyone to like their pasta al dente, so they cooked it on average for 10% less time.

Unknown said...

I am having an 'on-going- argument... !! I have been happily using the Gas hob with a Prestige Whisteling Kettle for the last 5 years as the worktop space has always beens tres limitee in my kitchens in France.I am told I am mad..wasting MONEY and ENERGY 'cos it takes a)longer b) costs more to boil it on a gas hob.
I disagree! but then I am a female and English...and the stove top space is only used when cooking al dente pasta and making stock...petit pois spinache etc !! Help I'm fed up with being made to feel a total Idiot when I'm not so sure!! Mado

Unknown said...

I've been wondering about this for a while: theres a lot to think about when your making a cuppa!

So, Mado, and anyone else wondering...

Reveiwing whats been said above, the amount of energy people have measured in thier homes that it takes to boil a litre of water is:

Gas hob (small ring)= 0.22kWh
Electric kettle = 0.11kWh

In the uk gas costs about £0.05/kWh, and Elctricity about £0.14/kWh, so... thats going to cost you about:
1.1 pence (gas), or 1.5 pence (electric).

(nb. In France electrcity is about 4.3 Euro cents/kWh, so it will cost about 0.5 Euro cent to boil a litre in your electric kettle...)

If its only cost you're interested in then its as simple as that. (Assuming you only boil the amount you need and switch the hob off as soon as the water boils)
Energy, though, is not created equally - Gas normally comes straight from source, eg by drilling into the earth and taking it, whilst electricity on the other hand can be generated in many ways.
By far the most common in the world are coal or gas fired power stations.(about 75% of electricity generation in the uk)
France however, generates a 75% of its electricity from Nuclear power stations, whist some countries such as Norway generate 95% of thier electricity from Renewables (Hydro-electric).

So although your kettle is consuming the same kWh of electricity, the effect it is having on the climate, pollution etc will be very different depending which country you're in.

A coal or gas power station is about 30-40% efficient, meaning that for every unit of electricity you use in your home, at least 3 times as much energy has been used (in the form of coal or gas) to generate it!
So then the amount of coal or gas that was burnt to boil your litre of water in the electric kettle becomes about 0.33kWh and the "greener" option would seem to be gas.

Its a complex one. The economics are simple enough, but to really figure it out you need to know where the energy you use comes from, how its produced, transported, how much is lost along the way etc etc. However its all out there on the internet if you have the time/incination! Heres a couple of informative links:

As a closing point, just to put the numbers we're talking about into context, if you rigged up your bike/rowing machine or similar with a modern, efficient electric generator, assuming you're a healthy fit human, you could produce electricity at a rate of about 100Watts.
You would need to keep pedalling/rowing at this rate (hard!) for over an hour to produce the same 1.5 pence worth of electricity you're kettle uses in 3 minutes boiling a litre of water...

Hope that helps!

Mark, Edinburgh.

Justin said...
This comment has been removed by a blog administrator.
heavyweather said...

How about induction heaters...

Paul Mott said...

It's a shame that the appliance manufacturers, who sell "coolwall" toasters, don't sell double-insulated (physically, not electrically !) kettles. Many folk, myself included, have plumbing that means hot water for the basin takes a long time to flow from the hot tap, and entails the inefficient heating up of a long run of copper piping (and its contents) - undesirable in summer. So we use a part-full kettle, and boil it rather often as an instant source of hot water for cleaning crockery etc. This would be a far more energy efficient act if kettles were sold with good thermal insulation (e.g. two separate plastic skins).

Unknown said...