What is the most efficient way to boil water?

Today, an experimental post. I was wondering what is the most efficient way to boil water, using a kettle or boiling on the gas hob? In order to determine this, I decided to knuckle down and experiment (for those of you who know me, you will realise this is somewhat out of character).

In order to determine how much energy it takes to boil half a liter of water, I turned the heating off and took a gas reading, including the little wheel reading that measures how many cubic feet of gas I have been using. I put the water in a non stick pot (lid on) and put it on the medium hob at max power. It took 5.25 cubic feet of natural gas to boil half a liter. Converting cubic feet of natural gas to Joules is not straight forward. In principle I would need to density of the gas, which might depend on atmospheric pressure and temperature etc. and also I would need to know the exact type of gas, it might in fact not be 100% methane. Luckily I found a website that does it for me. So to 1 significant figure, it takes 6 MJ to boil half a liter of water.

To work out how much electricity it takes to boil half a liter, I simply timed the kettle. It took 103 seconds. The kettle claims it is rated at 2000/2400 W.  This implies that (to one significant figure) it takes just 0.2 MJ using the kettle. This is actually a very small amount of energy. In fact, it is possible to determine the minimum possible amount energy required to raise half a liter of water from room temperature (20 degrees) to boiling (100 degrees). The specific heat of water is 4.2 J/g, so I deduce that the minimum amount of energy it takes to boil half a liter is 0.17 MJ. The kettle is approximately 85% efficient!

Comments on the validity of this approach. I wish I had a thermometer to determine the exact temperature of the water, rather than relying on when I think the water is boiling in the pot. I also wished I had a high voltage ammetre to measure the actual power usage of kettle.

The conclusion of all this is that the kettle is massively more efficient (almost 20 times more efficient) than the gas on the hob. I believe that even if we allow for the fact that electricity production is not 100% efficient the kettle wins. So long as you only boil as much as you need!



About jakirkpatrick

I am a researcher in solar energy at the University of Oxford. I am interested in mathematics, programming and trying to understand why things work. I also like the great outdoors and riding my bike.
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8 Responses to What is the most efficient way to boil water?

  1. rod says:

    How does a microwave compare? I can’t decide which would be more efficient. I think I am betting on kettle.

  2. Emma says:

    What about the insulation of the container? If you could do it with a really well-insulated saucepan, it might even things up.

    • I think that insulating the pan would mostly make it harder to heat it up: I guess the problem is that most of the heat from the flame is either reflected off the pan, or goes into heating the hobs and the pan itself.

  3. George says:

    That’s a really useful analysis. Even allowing for the cost and carbon intensity differences between gas and electricity the kettle still appears to come out on top.

    Once when my boiler broke down I decided that it would be more cost effective to heat the water for a bath using the gas stove. I will know better for next time!

  4. Michael says:

    I don’t surpass you have a kettle you can put on the hob for a closer comparison.

  5. Pingback: Stuff we linked to on Twitter last week | Highly Allochthonous

  6. Steve says:

    I found this post today because I was Googling to try to find out the same thing – how boiling water using a kettle compares to boiling it on a gas stove. I also did a similar experiement, but my figures are very different to yours.

    Here are the results of my experiement:

    I poured 1000mL of water at 17.1ºC into my electric kettle and boiled it. The kettle was plugged into an electricity meter (Brennenstuhl PM 230). The electricity meter gave a reading of 0.10 kWh.

    I poured 1000mL of water into a pot. After 5 minutes of stirring, I measured the temperature as 17.2ºC. I then took a gas meter reading, turned on the gas and heated the water until it reached 100.0ºC and took another gas meter reading. All other gas applicances including the boiler was switched off. The volume of gas used was 0.0074 cubit feet.

    Using the calculatioon from the gas supplier
    imperial units used * metric conversion factor * calorific value * volume correction / to convert to kWh = gas used expressed in kWh
    0.0074 * 2.83 * 39.2 * 1.022640 / 3.6 = 0.23319782603 kWh

    The amount of energy theoretically required to boil the water is
    E = 4.186 kJ/kgC x 1.000 kg x 82.8C = 346.6008 kJ / 3600kWh per kJ = 0.096278 kWh

    So in this case the electric kettle is definitely more efficient with the measured value being very close to the theroretical value (within 5%). But the gas usage was about twice the theroretical value. This is plausible as I can quite imagine half the heat escaping around the sides of the saucepan.

    For you to get a 20-fold difference in the gas used compared to electricity used suggests you have misread your gas volume used by a factor of 10.

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