Kettle energy efficiency
A environmentally concerned consumer asks
To boil the same volume of water, which is more energy efficient: a 3000 Watt kettle or a 2000 Watt kettle? I’m sure in a perfect experiment they should be identical in power consumption, but we do not live in such circumstances.
August 18th, 2008 at 1:36 pm
It will not be the same because resistance increases with heat, so consumption to deliver the extra watts will increase.
August 18th, 2008 at 1:45 pm
Am happy to announce that this is an area I am a complete and utter expert in! Infact I would say that there are very few folk who will know more about this than me.
2kw vs 3kw - no measurable difference what so ever though we did find that 3kw suffered more from overboil than 2kw (see point below).
The kettle design issues are as follows
1. overboil. The amount of time it takes for the kettle to stop heating the water once the water is boiling
2. heat loss. some of the heat from the element heats the air beneath the kettle rather than the water. This is very minor compared to point no1 but strangely you will find that more modern kettles have a plate element that sits on the bottom with one face not in contact with the water while older ones have a shaped element that sits in the water.
In reality it is not a design thing but more an “in use” thing. The “in use” issues are as follows
1. Boiling the correct volume. Seems obvious but this is the single most important consideration.
2. Re-boiling. Many people will boil a kettle, walk away and then come back to make the cup of tea and re-boil the water to ensure it is “at the correct temp”. Very inefficient
3. Fresh Water. Many users discard the kettles contents prior to starting a new boil - inefficient for obvious reasons.
Other considerations should be the embodied energy of the kettle itself. Buy a simple plastic kettle if you want to keep this low (and also the cost down!). Avoid Chrome or metal kettles.
August 18th, 2008 at 1:52 pm
And who ever spod is needs to do a bit of research! Yes resistance does increase inefficiency but it is the resistance of the material not the total resistance and the materials used have the same resistance be it a 2 or 3 or whatever KW kettle. Anyway, you can’t destroy energy so unless the inefficiency came out as light (and yes some muppet did invent a kettle that used light bulbs cause it looked pretty) then it has not been wasted as the inefficiency would come through as heat also! Go back to the physics class spod.
August 18th, 2008 at 1:56 pm
Please see ‘I heard a rumour’ below - the presentation of inaccurate information and rumour is not helpful. Point proven.
August 18th, 2008 at 2:08 pm
I think Spod might be on a bit of a mental safari!
August 18th, 2008 at 2:15 pm
Spod. Are you saying by being wrong you have proved a point? If only life was that simple!
August 18th, 2008 at 2:17 pm
Welcome to Alex Horne - nice of you to drop by.
August 18th, 2008 at 2:35 pm
I was trying to point out how easy it is to start a rumour. However, you scorched my rumour too fast for it to get started.
November 3rd, 2009 at 7:28 pm
Dear experts in kettles…
Would you say a “flat element” kettle (that one being the very bottom of the kettle) would be more efficient that those with a sheathed element sitting inside the water?
On one point, we got the heat losses underneath (I actually ignore if these are of any significance)… but on the other hand, we got that we need to boil about 0.5L of water not to be used, just to keep the element wet all the time, and avoid limescale layers that reduce the heating efficiency…
Well, many thanks in any case…
November 4th, 2009 at 11:09 am
Thank you Eljormaz for the chance to go on a bit more about kettles. Our study really did bring out nothing new to physics and unsurprisingly followed common sense (given where we are discussing this).
Two things need to be considered when dealing with elements that have an influence on kettle efficiency (though the main issues are what have been discussed before). Thermal mass and residual heat which in reality are one in the same just at other ends of the boil cycle.
For the first few seconds of any boil cycle the energy put in through the element will be used to bring the element up to a temperature and not to heat the water. At the end of the boil cycle the element will hold the heat and continue to dissipate it into the water even though it has reached the desired temperature resulting in over boil.
To compensate for the two things above designs should try to have the minimum thermal mass and for this the sheathed element wins hands down (as it does not need to be welded onto a plate of metal). For the latter it is more to do with controls. Kettles used to have a single cut out in the neck that sensed steam as it left the kettle but things have advanced considerably since then with additional safety cut outs in other places. Most importantly they now actually cut the power when the water has not actually reached the boil point and thus utilise the residual energy in the element to bring the water to a boil. The issue with this is that the kettle cannot sense the volume of water within it and so has to assume it has a full load to ensure the water is boiled and consequently will often overboil on small volumes.
I realise to many this will be a backward step but the element in the water in kettle with a narrow base to allow smaller volumes to be boiled will be more energy efficient. Even so, it comes back to the original points mentioned above - we need to educate people on how to boil the correct volume and not re-boil as the above makes only minor differences to efficiency.
November 4th, 2009 at 4:17 pm
Dear K,
Many thanks for your unexpectedly prompt reply. Indeed it makes sense, common sense the issue of the thermal mass. I will then go for a typical immersed-element kettle.
Good day
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