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Are kettles king at boiling water?


The headlines

The humble kettle is the cheapest way to boil water using electricity.

There is no such thing as a low-energy or more efficient kettle. All electric kettles are very similar.

Boiling enough water for one mug of tea costs around just 1 pence.....if you only boil the right amount of water.

Boiling a full kettle using an ordinary kettle costs around 5 pence.

Boiling water on a gas hob using a suitable gas hob cooker works out a little cheaper, under a penny a mug but takes MUCH longer.

Boiling a kettle on a gas hob too around four times longer than an electric kettle in my experiments.

 

The three questions I wanted to find the answers to were:

Is it most efficient to boil water in a kettle, on a hob or in the microwave?

  • Is it more efficient to boil a full kettle, then put it in a flask and use for the rest of the day?

  • Is a low-voltage or quick-boil kettle going to use less energy than a mains-powered kettle?

These questions appear on social media regularly.

This article answers all those questions.

Also see my article on instant boiling hot water dispensers and which ones are DEFINITELY more expensive to run than kettles unless you use a LOT of boiling water.

There are various ways we can get energy into water to heat it up:

  1. With an electrically-heated coil in the base of a kettle

  2. Using microwave energy

  3. Heating a pan up using an induction hob, halogen hob, plate hob or gas hob

I'm mainly going to talk about and compare the first two here.

 

Kettles

There are thousands on the market, of every conceivable shape, size and material. They all do the same thing, using a heating coil in the bottom to heat the water. The coil heats to around 220C which then heats the water above.

Trivia: Kettles make a noise because the water very close to the heating coil starts boiling within seconds of the coil heating up.

It is irrelevant whether the coil is mains voltage or a low voltage (such as a camping kettle). The coil still has to get very hot to boil the water.

Different kettle voltages and different kettle power or Watt ratings have an effect on how QUICKLY the kettle can boil water by heating the coil to higher or lower temperatures.

A so called quick boil kettle will have a hotter (and more energy hungry) coil than some which will boil the water quickly.

The opposite is true for a low voltage or low wattage kettle. It won't be able to get the heating coil so hot but will have to keep it hot for longer to get all the water in the kettle to boil. A low power travel kettle for example will take around 3 times as long to boil than a conventional kettle boiling the same amount of water.

 

Results of (exciting!) experiments

The table below shows the results of experiments I and others have carried out to measure the costs of boiling water to make a mug of tea.

Method

Energy used per mug

Cost per mug

Efficiency of method

Normal 3kW glass kettle

0.0375kWh

1 pence

86%

Normal 3kW plastic kettle

0.0375kWh

1 pence

86%

Small collapsable silicon travel kettle (800 Watts)

0.0345kWh

0.9 pence

96%

1kW microwave

0.07kWh

1.9 pence

48%

Gas hob

0.073-0.1kWh

0.5p-0.7p

32%-44%

As you can see, the travel kettle is around 10% more efficient. This is because it is physically much smaller and lighter than the other two, and so there isn't as much kettle material to absorb heat when it is warming up.

Although gas hob cooking using a traditional whistle kettles is lower cost, in my tests it took around four times as long for the water to boil. Cooking on gas also has impacts to health because of its effect on indoor air pollution.

 

Useful tips about kettles

  • Some kettles allow you to boil water just to 80 or 90C, which tea and coffee experts say is much better for flavour. This will obviously reduce the above figures by approximately 20% and 10% respectively, so if those temperatures suit your taste, you will pay even less for your mug of tea!

  • A kettle with an element covered in limescale will use more energy, as the limescale acts as a barrier between the heating element and the water. This simple experiment suggests it makes a difference of around 4% in efficiency. If you've never descaled a kettle, it is incredibly cheap and easy to do, and very effective. Descaling sachets cost around 50p per treatment - see here.

  • To ensure we are not wasting energy (and money) boiling more water than we need, we have put a felt-tip line on our kettle to mark exactly the right amount of water for two mugs. I know someone who fills the mugs they want to use with cold water, then tips them into the kettle to guarantee they aren't boiling any more than they need.

 

So why didn't the microwave do well?

It has been common to see information in the media suggesting that microwaves are very efficient.

Microwaves are typically around 50% efficient at converting electricity into heat (see the Energy Consumption section in this Wikipedia entry). They are certainly MORE efficient than some other methods, such as electric oven cooking, at least for some things you might want to heat or cook.

The reason a microwave oven is actually so inefficient is it uses a lot of energy to actually generate the microwaves it needs to do its job. This is why a microwave always feels very warm on the insides and around the back when it has been on, even just for a short time - it is lots of hot gubbins behind the scenes!

Nevertheless, the experiments above clearly show that boiling water with an electric kettle is far more efficient than using a microwave. They may well be other cooking activities that microwaves aren't the best at, too, but unfortunately quality information about the comparative energy costs of real cooking/ recipe choices is almost non-existent. That is changing, as more people start to do experiments like the one above.

Look out for a blog I hope to do soon on the real-world efficiencies and costs of different cooking methods.

 

Is it more efficient to boil a full kettle and use a flask to use the water through the day?

Yes. Well, sort of.

If you boil a full kettle, it does make the overall process slightly more efficient but, as you can see, the regular kettles are already at 86%. So it might lift the efficiency by 10%, but that is a pretty small saving.

In fact, if you put boiled water in a flask, its temperature falls throughout the day, as heat is lost through the flask, and this may affect the quality of your mug of tea. Also, if you throw ANY of it away at the end of the day, you've thrown away water that you paid to heat earlier in the day, which is not very savvy.

 

Other water boiling methods

Water CAN, of course, be boiled on hobs. This very recent properly conducted test concluded that the kettle was actually slightly more efficient than an induction hob, and also provided more proof that the microwave uses considerably more energy.

 

For fellow geeks* out there, my experiment conditions were:

The kettle tests were actually all done with TWO mugs' worth of water, and the results divided by two.

I have done tests boiling one mug of water, and kettle efficiency drops to somewhere around 80% in regular kettles.

The amount of 'physical' thermal energy needed to warm water from tap temperature to boiling is always the same. It needs 0.103kWh of thermal energy to heat one litre of water from tap temperature (say 10C) to boiling.

The water temperature at the start of my own tests was 21C. By 'mug' I mean a water volume of approx 325ml.

The accuracy of this data is clearly affected by the accuracy of the boil shut-off switches in each device, which are relatively crude items.

An energy tariff of 27p/kWh is assumed, the standard tariff from Oct 1st 2023.

The quantity of water used in each test was 650ml and the temperature of the water in all three kettles was 21.3C before they were boiled.

I HAVE done some measurements with lower quantities and the efficiency drops off, to somewhere around 80%.

The figures for a gas hob are based on a mix of practical experiments I've done on boiling kettles on gas and quality data I have found online about gas hob efficiency here. Gas hob cooking efficiency varies a lot with method.

*Thanks go to Simon Bingham for doing the microwave test.

 

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Thanks.


Mark Thompson


Get Energy Savvy - simple practical home energy efficiency information

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1 Comment


Guest
Feb 04, 2023

Thanks for your research, very interesting.

I believe some heating element kettles have to have the element covered with water to operate safely, so more than a mug full of water as a minimum.

Unlike bottom plate heating elements that can cope with one mug of water...

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