Electric Kettle or Gas?

podcake

rogerblack wrote: »

Unfortunately, your clarification is mislead.
If you heat a half a cup of water to 100C, you will use about 25 watt-hours. (80C*4180J/C/kg*0.25kg=23Wh)

This takes the same amount of energy if it's heated slowly or fast, unless heat leaks out while it's heating.

How much energy would it take to keep a kettle warm?

To keep a 2l kettle, with diameter 12cm, height 20cm warm, with a wall thickness of 1cm, and an insulation quality the same as good as decent house insulation:
Area = .08m^2
DeltaT = 80C
Thickness = 0.01m
Thermal resistance = 0.02W/m/C. (equal to Kingspan, or comparable PIR/PUR foams)
*
.08m^2*80C*.02W/m/C / .02W/m/C = 12W.

So, you can boil half a cup of water every couple of hours on the losses from keeping a well insulated kettle hot. (I question if you can buy a kettle this well insulated)

Your points on the gas kettle also neglect the extra ventilation needed to remove the humidity from the air from gas combustion.

I see your point, and I apologize for not being clearer in my initial post.

You are correct, it takes X energy to heat up water to a specific temperature from room temp regardless of how you do it, fast slow etc..

you hold the key in your comment :

unless heat leaks out while it's heating.

Take the example that your heating element was 50w, it would take a LONG time for the element to heat the water to 100 deg C. In fact, it would most likely not get there, as the losses would mean that the temperature would be dropping (due to insulation inefficiencies) as fast as the possible rise in temperature.
Where your 3000W unit has the water boiled in 3 mins (say) with fewer losses.

In the same way as you could put a candle under a kettle and it would probably never boil, but you are pumping in heat energy en mass

Also, further to my comments I am not suggesting keeping the kettle warm electrically, I am talking about passive insulation (Imagine the combination of a thermos flask and a kettle - the patent actually exists, but there are not many on the market)


Regarding my points on the Gas kettle.. I'm afraid to say I have neglected nothing in that regard. This was an experiment using actual measured data, not theory.


many thanks for your comment though, I do appreciate people pointing out to me where I am mislead

Martyn1981

OK Podcake and Roger, new challenge.

Kettle or induction hob? And will it depend on the pan and how much heat that 'steals'?

Mart.

podcake

On a side note, experimental data collected by some of my colleagues suggest that as much as 15 degrees is lost by hot water when it is poured into a ceramic mug.

It takes something in the order of 4 hours for this temperature to be reached (85 degrees) from a 'thermos' style stainless flask if its set up correctly.

so use a polystyrene mug and a 'thermos kettle' and you could boil 2 litres of water first thing in a morning (off peak maybe?) and it would do the job for approximately half the day...

Also, not sure if you read it or not, there was an interesting study recently ( which I can't put my hands on atm) that suggested that you would need to use in the order of 1000s of ploystyrene cups before it equates to making, shipping and cleaning of a ceramic cup.

i'll see if I can find it for you!

podcake

I'm afraid, I do not have an induction hob with which to measure, if somebody else does, please measure it and lets see!

rogerblack

Martyn1981 wrote: »

OK Podcake and Roger, new challenge.

Kettle or induction hob? And will it depend on the pan and how much heat that 'steals'?

Mart.

There isn't going to be much in it.
Induction cookers are ~85% efficient, electric kettle elements perhaps about the same (some of the electrical energy is wasted heating the element to 100C)

A small, very lightweight stainless steel kettle on an induction stove, filled only to the amount required is likely to use slightly less energy.

Something similar to http://www.ebay.co.uk/itm/330751452508?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1438.l2649

For plastic kettles, if the plastic is heated up to boiling, it takes about half as much energy as the water does per kilo.
My kettle weighs about 500g, so that's roughly equivalent to 250ml of extra water in it - easily doubling one cups energy usage. (In practice, it's about 1/2 this, as not all the kettle is heated)

Steel is a tenth of water, so the above kettle would use the equivalent of heating an extra 20ml.

That is rather more than I thought.
At 5 cups a day, in principle I've wasted ~30 quid in heating and reheating the plastic of my kettle. (over 10 years)

Interesting where the little wastages that we would never think about creep in.

Martyn1981

Thanks Roger, very interesting. I assume that a microwave would have similar efficiencies.

We have an induction hob (since last June) with, I think, 2 1.5kW and 2 2.5kW hobs. Very impressive.

Not sure if I'm adding 2 and 2 and getting 5, but one thing that shocked me, was being able to leave a metal spoon in a pan, and still being able to touch it. Whereas on a gas hob the spoon would get very hot, presumably from the wasted heat travelling up the side of the pan.

As I have PV I could introduce all sorts of varying cost factors, at differing times of the day, weather, seasons etc, but to be honest, an electric kettle is such a simple device it's ease of use kind of trumps the alternatives.

You're right about the 'little wastes', they do add up!

Mart.

rogerblack

Martyn1981 wrote: »

Thanks Roger, very interesting. I assume that a microwave would have similar efficiencies.

Alas not - as I understand it, the best microwave ovens are ~60% efficient, on a par with gas.

At least for non-inverter ovens.
http://www1.eere.energy.gov/buildings/appliance_standards/residential/pdfs/cooking_products_tsd_ch5.pdf

http://ees.ead.lbl.gov/projects/residential_cooking_products_including_microwave_ovens - however this is now a somewhat dated study program - and the few inverter microwaves they list hit efficiencies comparable to the best conventional.

I note that there are panasonic ovens claiming 90% - https://panasonic.ca/english/appliance/microwave/inverter.asp which considerably exceeds anything I've managed to find non-manufacturer data for.

Ben84

podcake wrote: »

Hello All,

I wonder if I can help at all here.

Firstly, let me say theres a lot of confusion flowing through this board regarding this topic.

To offer the OP a definitive answer:

1) the ELECTRIC kettle will be MORE efficient than the GAS
2) depending on your various purchase prices the GAS will probably be cheaper. (this is because you will use in the order of twice the kWh but gas is often 1/3 of the cost per unit)

Basically, choose: Cost or 'Ecofriendlyness' (yes, i made the word up)

Source: Experimentation for my Postgraduate degree in 'Advanced Engineering'


now to address some of the confusion:



I'm sorry to pick on you Ben84, but I have to say that you seem to be terribly confused here. (although your maths corrects itself along the way, and your result is correct).

Watts is NOT a measurement over time in the way that you say. A Kettle rated at 3000W does not use 3000W in an hour. It uses 3000W CONTINUALLY. if it ran full pelt for an hour it would use 3000 Watt hours.

Also, in reality, boiling the amount of water you want for JUST the cup of tea you require is not always the best way to go about it, mainly because of the laws of thermodynamics. In reality, you are best off heating the water to boiling AS FAST AS POSSIBLE and keeping it there through heat insulation. If anybody wants me to explain that further, ask and I'd be more than happy to go into some depth with you.

That's ok, I'm glad you mentioned it because I now think I have confused the terms and how to describe things. My understanding of electricity is ok, but English is not my first language and I sometimes see things and get some idea of how the words work which isn't always correct.

So, I had thought in English the time value is only used when talking about kWh and everything else is watts - because I have never normally seen another term used? Well, you have mentioned 'watt hours', but is that a normal unit people understand?

I'm also curious about your suggestion the electric kettle is more efficient. While in terms of the appliance this is true, there's a lot more energy conversions going on to make electricity than with gas. In fact, when you use an electric kettle you may be heating the water with gas - just it is burning miles away in a power plant with significant energy losses at the plant and in the wires between there and your kettle. There's also a reasonable chance the power plant is burning a more polluting fuel than gas like oil or coal. So, electric isn't likely I think to be the more environmentally sound choice for everyone. It may however be for me as I live very close to a nuclear power plant, which I believe supplies most of the electricity to my house. Anyway, circumstances vary, but unless you think your electricity is largely from a low carbon source like nuclear or geothermal, I expect gas to be a good choice for the environment.

There may also be a case for me not using an electric kettle on environmental grounds, as using less of the low carbon nuclear electric would mean it could be used elsewhere on the national grid to displace more polluting energy sources like oil fired power plants.

However, this may all be worrying too much about how to make coffee Like plastic bags and light bulbs, kettles seem to have attracted some extreme environmental focus - but they are not and never have been significant energy consumers in the average household. Better to worry about insulation, that can save a lot of fossil fuels every year.

Ben84

Martyn1981 wrote: »

OK Podcake and Roger, new challenge.

Kettle or induction hob? And will it depend on the pan and how much heat that 'steals'?

Mart.

I hope you don't mind me adding something, but I think it's pretty much got to be the kettle.

Because the electric element in the kettle is almost entirely surrounded by the water, it heats the water before it heats the kettle itself. Induction hobs, although more efficient that other hobs, still have to heat the pan before the water inside it.

Cardew

Ben84 wrote: »

So, I had thought in English the time value is only used when talking about kWh and everything else is watts - because I have never normally seen another term used? Well, you have mentioned 'watt hours', but is that a normal unit people understand?

Although watt hours is a perfectly acceptable term, as we are billed for kWh it tends to be a convention when talking of electrical consumption in domestic properties to use kWh even when fractions of a kWh are used.

i.e. a 60 watt bulb on for two hours would be described as using 0.120kWh instead of 120 watt hours - although both terms are obviously correct.

In the same way the average annual consumption of a domestic property is 3,300kWh. It would be unusual for that to be described as 3.3MWh(3.3 megawatt hours)