When I was at school I had a Science teacher – Mr Glasby – who had compelling analogies for scientific concepts.
He would say things like:
You want to have a cup of tea, so you fill and boil the kettle. But you forget about the kettle. When you come back, the kettle is boiling furiously (for some, unexplained reason, it has not switched itself off). The room is full of steam. What do you do?
Answer: You make the cup of tea.
(The point here is that even a small amount of water can produce a lot of steam and there is still plenty of water in the kettle for you to use).
Other explanations he gave – which remain with me even after 40 years – include how people who slept beside lime kilns in the 19th century to keep warm at night would “wake up dead” because the risks of carbon monoxide poisoning were not fully understood at that time. Also how electrons in atoms behave like people getting on the bus: they don’t like to pair up (i.e. you won’t sit down beside someone else on the bus if there is a double seat free). But, back to the kettle…
Leaving a kettle boiling for longer than necessary is a waste of energy.
It’s also an extra cost on your electricity bill. That’s also bad for the planet.
But you can also add unnecessary cost if you boil too much water.
It’s inefficient to boil a full kettle of water if you’re only making a cup of coffee for yourself. But how wasteful is it?
This is not a novel topic for discussion, of course.
Here’s an article which is more than 15 years old – about “keeping your kettle in check”. It notes that a survey conducted by the Energy Saving Trust (EST) in 2006 found that 67% of people in Britain admitted to overfilling the kettle each time they used it.
And this article by EST from 2022 suggests that if you avoid overfilling the kettle you could save yourself £13 a year (at pre-“Cost of Living Crisis” rates) on your electricity bill.
My Dad (Dr Harry Brash) is a physicist by profession and we were discussing this issue generally and, more specifically, how you might best be able to illustrate the amount of energy which is required in boiling a kettle to make it more ‘real’ – and how that could then be translated into much bigger issues such as climate change.
So, here is what my Dad has devised – with no apologies for the numerous “workings”.
How many Alkaline battery AA cells would it take to boil a kettle?
This is mostly about battery capacity but it isn’t simple.
Battery cell capacity is usually rated in units of ampere-hour (Ah) or milliampere-hour (mAh).
For example, the test results apparently showed a well-known brand of AA cell as having a capacity of 2348 mAh at 0.1A load but also that it had a capacity of only 506 mAh at 1A load. Note that, although the open circuit voltage of a new AA is more than 1.5V, they considered the “end of useful life” voltage to be 1.0V which is pretty low for many applications. I would rate their capacity results to be very optimistic, in practice.
The above stated capacity of “506mAh at 1A load” means that if you draw a constant current of 1A (1000mA) from a new Duracell AA cell then it will reach its “end of useful life” after 0.506 hour or roughly 30 minutes.
Heating water in a kettle is about transferring energy into the water.
I put about 300ml of water (large mug) at 13.5 ˚C into a Russell Hobbs kettle and monitored it using an inexpensive (£10) plug-in wattmeter. To boil the water took 0.037 kilowatt-hours (1kWh is the normal unit by which we are charged for mains electricity) but, by the time the kettle turned off automatically, the total was 0.046 kWh.
The first question I asked myself was how much energy should be needed to boil my mug of water.
The specific heat of water around these temperatures is about 4.18 kj/kg/˚C so the energy required to raise 300ml (almost exactly 300g) of water from 13.5 ˚C to 100 ˚C is 4180 x 0.3 x 86.5 = 108471 joules.
We need to convert this to kWh.
1 joule is the energy produced by a power of 1 Watt applied for 1 second so 108471 joules is 108471 / 1000 / 3600 = 0.030 kWh which is quite close to the measured value (0.037 kWh) but makes no allowance for the energy required to heat up the kettle itself.
The measured value of 0.037kWh is probably about right.
Let’s go back to the results for an Alkaline AA battery with capacity 506 mAh at 1A load which means, roughly, that the battery could provide an average of 1.1V @ 1A for 30 minutes (see graph in above link) which is an energy of 1.1 x 1 x 0.5 /1000 = 0.00055 kWh.
If we put 300ml of water at 13.5˚C in a perfectly insulated container, to boil it would require the energy from fully discharging around 0.037/0.00055 = 67 Alkaline AA batteries and it would take 30 minutes.
To boil a partially-filled kettle would need 67 batteries and take 30 minutes?
If you wanted to reduce the boiling time, you might think of drawing more than 1A from each cell. However that would further reduce both the useful capacity of the cell and the available average voltage.
A better (?) approach would be to use 10 times as many cells each outputting only 0.1A which would increase the available individual cell capacity from 506 mAh to 2348 mAh and increase the average voltage to about 1.25V per cell. At 0.1A, each cell can provide about 1.25 x 0.1 x 2348/100/1000 = 0.0029 kWh compared with 0.00055kWh when drawing 1A from each cell.
With 670 battery cells you would be able to boil about 52 big mugs of water, each taking 30 minutes.
What if you did not want to wait 30 minutes for your cup of tea?
If you also wanted to reduce the boiling time to, say, 3 minutes you would have to use a further 10 times as many cells, each of them supplying 0.1A.
That would require a bank of 6700 Alkaline AA cells but would be able to boil more than 520 mugs of water before new cells would be required. The energy available from this bank of cells (when new) would be more than 19kWh.
How much would your ‘3-minute’ cup of tea cost?
You can buy Alkaline AA cells for under £0.50 each, so the 6700 cell bank comes in at under £3,350.00.
In “mug” terms, that is about £6.40 per mug of tea.
Compare that with the 0.037kWh I measured to boil 300ml of water from 13.5˚C in a mains-powered kettle. At £0.30 per unit (1kWh) that cost £0.011 (just over 1p).
How practical would a bank of 6700 battery cells be?
An AA cell is about 14mm diameter and 50mm long. Not allowing for clever packaging which could reduce the overall volume slightly, 6700 cells would fit into a 0.4m (40cm or about 16” in old money) sided cube and would weigh about 154kg (more than 24 stones or quite a large person).
We haven’t considered how this rather odd DC source (our bank of cells} would be organised internally or how it would connect to our kettle.
Internally there are many options.
The first thought might be to connect all 6700 cells in series but that would give an initial battery voltage exceeding 10,000V. Your first cup of coffee might be your last!
A more pragmatic approach could be to have 100 sections in parallel, each section consisting of 67 cells (about 100V initially) and a suitable series diode to prevent any section with a slightly higher voltage from trying to charge other slightly lower voltage sections.
Modern electronics could readily convert the variable DC voltage from our bank of cells to a reliable 240V or 110 V AC mains equivalent to run a standard low power kettle (one which takes about 3 minutes to boil a 300ml mug of water) with an efficiency exceeding 90%. The technology is already in wide use, for example with solar panels and other applications.
Thanks, Dad. 🙂
You might think of Alkaline AA batteries as “powerful” but they have negligible oomph in the Kilowatt hours universe – which is what matters when it comes to powering our daily domestic needs from washing machines to fridges to electric kettles.
You can waste energy by allowing a kettle to continue boiling (even though you can still make the tea). You can also waste energy by overfilling the kettle beyond your needs.
Save yourself money and time by pouring the required number of mugfuls into a jug and decanting it into the kettle. The “time-saving” part? All this measuring pallaver may feel like a waste of time but the time you take to do it will be more than saved by the reduced time required to boil less water.
Most people in the UK have access to an electric kettle (it was 97% of the population in 2008). Most people overfill the kettle. If everyone developed a habit of boiling only what water they need, that could be a significant CO2 emissions saving.
Links you might like
- Simple steps to help your business move towards net zero (by Anne Littlejohn of Raeburn Christie Clark & Wallace, Solicitors, Aberdeen).
- Join The Daily Difference at The Carbon Almanac (a daily email, 365 days a year, with tips and encouragement in ways you can make a difference in combating climate change – US-based but global in content and ambition).
Peter Brash is a member of the Law Society of Scotland’s Sustainability Committee.