How Sustainable is your Cuppa

We all need our cuppa, but how sustainable is yours? we're not talking about where the beans, coffee or even the cup comes from but the energy you use making it

How Sustainable is your Cuppa


Just before a recent #Connectfriday Gareth Kane and I entered a conversation about (of all things) him making a cup of coffee. After dismissing questions about the coffee itself ( there is little point in being “green” if the product is in itself substandard was, I think, the consensus) we came to the boiling of the water – a simple task we all do frequently. Gareth suggested that because the specific heat of water is fixed (it actually isn’t but unless you (like the Queen) choose to use bottled water to make tea you can’t affect it) it didn’t really matter. I pointed out that actually the “Latent heat of Evaporation” was more important – more on that later.

However we didn’t consider HOW we boiled the water and that has significant sustainability issues.

There are basically two methods of heating water to boiling point: by external heat source or by internal source.

So as external is basically putting a kettle on a hob with maybe 30-40% efficiency of heat transfer and internal (electric kettle or microwave) has a heat transfer efficiency of close to 100% -which is better?

The two “internal methods” appear more efficient but ignore the losses in generation and distribution of electricity. Despite what the “greenwash” from the industry suggests at most 15% of UK electricity currently routinely comes from renewable sources (shall we ignore the embedded resources that green generation has to payback?) with some (maybe 20%) from Nuclear (does anyone else find the concept of harnessing the power that destroyed Hiroshima to make a cuppa absurd?) and the majority from Coal, Oil or Gas where the conversion in the power station is at best 40% efficient (the carbon efficiency does of course depend upon which fuel is used with, of that list, gas being best – I’ll skip over the comparison of old steam based coal power stations and modern CCGT).


Add to that around 10% lost in distribution and other waste and we’re back to the same sort of figure for delivered efficiency!

So unless you’re fortunate enough to be using a wood burning stove (I could consider how frugally you’re expending the “temporarily sequestrated” carbon in the logs …..) or extravagant enough to use an electric hob (induction and halogen are a bit better) the choice doesn’t matter much perhaps?

Well there are two simple things you can do to improve it:
1) less water – as Gareth pointed out the specific heat capacity of water in unchangeable but the total heat capacity depends on how much water you boil – less means both less energy into the water and less time to boil (suits impatient me!) which means less heat lost to the room.

2) Less time boiling. As I pointed out to Gareth the Latent Heat of Evaporation (to save anyone googling it that’s the heat needed to make water at boiling point into steam) is MUCH more than the Specific Heat Capacity (the heat used in increasing the temperature of the water) it is important to stop heating the water immediately it first boils. (Add to that the fact that according to Laws, first (I think) spotted by Isaac Newton, heat lost to the room is ­­­­higher when the water is hot).

So the answer is simple boil as little water as you need and turn it off as soon as it boils!

As most electric kettles turn off soon after the water boils and kettles on the hob aren’t usually closely supervised (unless you’re disproving the adage that “a watched kettle never boils”?) and could be left boiling -maybe that’s the best option?


And MY pre #connectfriday cuppa (tea – I don’t drink coffee) -will be made in the microwave (yes I know a teapot would improve the taste but….)

For those interested the Specific Heat Capacity of water is around 4kJ/kg/k (it changes slightly with temperature and the Latent Heat of Evaporation is about 2,270 kJ/kg about 567 times more! So if your kettle contents weigh 1kg (i.e 1 litre) it would take about 4kJx90 to take it’s contents from 10C to boiling point =360J and 2270 kJ to boil dry

Andy Clarke

Topics: Energy efficiency & low-carbon
Tags: coal | gas | heating water | nuclear | water
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