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How to make an energy efficient cup of coffee

Saturday, January 10, 2015

Would you believe me if we said you could save £25 per year through slightly adjusting the way you and the person you live with make a cuppa?? No, well we’ll tell you how and why. There’s a little maths in this, but don’t be put off by it!

According to the Energy Saving Trust, we in the UK waste £68 million annually whilst making our beloved tea and coffee. With the average Brit boiling the kettle 4 times a day, we think there is a lot of scope for families to have an easy win by getting to grips with their humble kettle!

How does a kettle work?

Lets start with the basics. Kettles heat cold water until boiling temperature (100 degrees Celsius). Generally this is done by an electric heating element or by gas (if using a gas hob). The amount of energy needed to heat 1 litre of water by 1 degree is 4200 Joules (don’t know what a Joule is? look here), so if we’re heating 0.25litres (1 cup) from 10 to 100 degrees celsius then this needs 94.5 kJ.

All kettles will have an inefficiency in heating; with heat escaping to the air around the kettle, heating the kettle itself and turning some of the water into gas which then escapes (this energy is called latent heat energy). N-power estimate the average kettle to use 132kJ when boiling a cup of water (using a little maths from here) but you could quite easily measure your own by:

  1. timing how long it takes for your kettle to boil 1 cup (250ml) of water
  2. looking at the power of your kettle (Watts, often found on the base of the kettle)
  3. joules = wattage x time (in seconds)

Most kettles these days are about 3kiloWatt power rating. Therefore, in practice, the kettle should eat through 3000 Joules per second, meaning that our water should be boiled in 31.5 seconds! This would cost approximately 0.4pence depending on your tariff’s price per kWh.

Does your kettle take longer?

If your kettle takes longer then this will be for one of two reasons

  1. Your kettle is less powerful
  2. You join three quarters of the British public by overfilling your kettle (from the Energy Saving Trust report here)

Yes, thats right, we Brits have a bad habit of overfilling! But how much does this cost each of us?

Let’s assume you have a conservative 3 cups a day, this will cost you about £5 per year IF you just boil the amount of water that you need. That’s not too much right?


  • If you overfill by an average of 1 cup every time, you will be wasting £5 per year
  • Say you live with a partner and foot the bill, you may be wasting £10 per year!
  • How about if you and your partner fill the kettle when the kettle is empty, then re-boil it every time you want a drink until it’s empty? – £25 per year (based on a 1.5 litre kettle)

Now I’ve made some big assumptions here, like you have all your drinks at home, but this could easily increase as well as decrease for a home with a couple of kids and an inefficient kettle (electric kettles are the best generally).

Tea kettle with boiling water on black background

Now on to the coffee!

For all of you coffee connoisseurs out there, you know that water should never be boiling when brewing as you may burn the bean! Instead you should use water at about 91 degrees celsius (according to the National Coffee Association), which would mean that you would massively reduce the latent heat loss from the kettle, and reduce the amount of energy you would need to add by a 9 degrees!

A rough calculation, assuming 50% of wasted heat can be saved by a reduction in latent heat energy (the energy needed to turn water at 100 degrees into steam), would suggest that you could save a further £3 per year. So by making a better coffee you’ll save enough to treat yourself to a coffee whilst out and about.

The moral of the story

This is probably one of the easiest wins when it comes to energy saving. Not only will you save a decent chunk of cash a year, you’ll also get a quicker cuppa and cumulatively, we can all save a whole load of our precious energy!

Kettles that make it easy

Not all kettles are made equal! Most try to make it easy for you these days by having cup measures written on the side of them, but some kettles even allow you to have lots of water stored and only heat the right amount of water for you. We wouldn’t suggest throwing your current one away quite yet, but if you ever do find yourself needing a new kettle then get one of these ones as they’ll likely pay for themselves over a year or two.

  • Alex Buckman
  • Posted by
    Alex Buckman
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