Electric or gas to boil the kettle?
December 7, 2008 4:14 AM   Subscribe

An energy efficient cuppa? I have a 1900w (240v 10A) electric kettle, and an enamelled steel Le Creuset kettle that sits on the natural gas stove. Which is more energy efficient?

When he visited today, my Dad said he would buy me an electric kettle because the gas stove was taking a while to boil. I repeated that I had read that gas was more efficient, and resulted in less CO2. He replied that the scale efficiencies at an electric power plant beat the heat lost around the sides of a kettle on the gas stove. I can see his point, but am pretty sure the received wisdom is gas is better. The gas does take longer to get to a boil, probably twice as long.
Which is more efficient if the time taken isn't an issue?
posted by bystander to Science & Nature (17 answers total) 4 users marked this as a favorite
Perhaps this isn't the answer you want, but I'd say the gas and the kettle you already have is more efficient, because you don't have to buy something new - think of the energy expended to extract the petroleum needed to make the plastic parts and metal mined to create the heating elements, get the electric kettle to your local shop from wherever it was made, keep the lights and heat on in the shop so your dad can get there, cut down the wood to make the cardboard box it comes in.

It could be that on a one-kettle level, all of this energy consumption and use is infinitesimal, but your kettle couldn't exist at the price and location it does unless there were a few shipping containers of them landing on your shores every so often. So while you can almost certainly find someone here who could tell you which method is more efficient at heating water, I don't know if you can calculate the energy used to create one kettle.

Furthermore, what if your local electricity is generated from natural gas?
posted by mdonley at 4:36 AM on December 7, 2008

Also, if you use gas heating, heat loss around the kettle is not really heat loss, after all, so the inefficiency is negligible.
posted by alexei at 4:46 AM on December 7, 2008

Response by poster: To be clear, I own two kettles (the second was a house warming present). One electric, one sits on the gas stove. Right now, heating the home is no asset, as it is summer here (and while we don't have AC it could be a negative), so it is wasted heat. If the difference is so small, maybe I should use the electric in summer and the gas in winter ;-)
posted by bystander at 4:55 AM on December 7, 2008

Well, if the question is purely about energy efficiency in your kitchen, then it all comes down to wasted heat. If we assume you are heating 1L of water from, say, 15 degrees to 100 degrees, then you're looking at 85,000 Joules of energy. In the case of the electric element, the heat is going directly from the element to the water, whereas in gas, the gas also has to heat up the metal of the kettle, which will radiate heat away, and you've also got to think of the radiant heat from the gas flames themselves.

However, looking outside your kitchen...there should be pretty much no energy loss from the gas. It comes from underground, travels through pipes to your house, you set fire to it, it releases a certain amount of heat. However, with electricity, you've got to burn some coal first (some of the heat from which will be wasted), boil some water at the power plant (wasted heat here, too), turn a turbine (friction, heat), turn a generator (not 100% efficient), run the electricity over powerlines (energy will be lost), through transformers (more energy loss, heat loss).

If you look at it this way, if your electricity was coming from a natural-gas fired power plant, then it would be a hell of a lot more efficient to use the gas kettle, rather than convert gas to electricity then to heat.

However, it is a complex question, and I'd love to see someone come along with some numbers.

Also...have you considered microwaving?
posted by Jimbob at 5:16 AM on December 7, 2008 [1 favorite]

I just remembered that watts = joules * seconds.

Therefore, if your 1900w electric kettle were 100% efficient, it would take 45 seconds to boil 1L of water. How long does it really take?

The same sort of calculation could be done with the gas, but I imagine we'd need to know the flow rate of the gas, and the energy released per cc.
posted by Jimbob at 5:26 AM on December 7, 2008

If you're worried about generated CO2, you'd have to know how your electricity is generated. Where I live it's all good old Coal but yours might be oil, gas or nuclear.
posted by octothorpe at 5:48 AM on December 7, 2008

Here's a decent read on Power plant efficiency.

This type of analysis falls under 'engineering economy', and the 'right' answer depends on the goals of the question. In some cases, the analysis is life cycle costs (which include associated non-recurring costs (e.g. manufacturing)) and in others, it is simply process costs (all of which are recurring (e.g. usage)). It all depends on what decision you need to support with the analysis. Some folks only use life-cycle costs, however, we mostly live in a world of operational costs.

IME, in most cases, electricity loses. The huge generation and distribution losses do not make up for the almost perfect transfer of power from the heating coils to the water. Point of use generation removes a bunch of those factors.

It's splitting the wrong hairs. Carpool one day each week and you'll make a much better impact on the planet then with a decade of coffee brewing.
posted by FauxScot at 6:12 AM on December 7, 2008

If your electric kettle has a concealed element the most efficient means of boiling water for drinks is to boil exactly the amount you need in an electric kettle.
posted by mandal at 6:46 AM on December 7, 2008

Just a thought, but electric kettles aren't often very well insulated. Put your hand on the side of one as it's boiling the water, and see how much heat is lost through the sides, and through the top as steam.

I'm not saying that gas is better, just that there are lots of factors to consider.

Personally speaking, I'd go with gas. Less energy to make, less energy to transport, etc. And the kettle itself is liable to be longer lasting, possibly?
posted by Solomon at 7:09 AM on December 7, 2008

the most efficent is to only heat up exactly the amount of water you require. the electric kettle may have a minimum that is greater than 1 cup
posted by mary8nne at 9:06 AM on December 7, 2008

The electricity used by all the people reading and replying to this on their computers might outweigh the difference in lifetime energy consumption of the two. :)
posted by paanta at 10:09 AM on December 7, 2008

Best answer: If we assume you are heating 1L of water from, say, 15 degrees to 100 degrees, then you're looking at 85,000 Joules of energy [...] I just remembered that watts = joules * seconds.[...] Therefore, if your 1900w electric kettle were 100% efficient, it would take 45 seconds to boil 1L of water. How long does it really take?

What value are you using for the specific heat capacity of water? Because I use 4.184 J/(g.k) and I get a pretty different result to you when heating 1kg of water from 15 degrees centigrade to 100 degrees centigrade.

Back on topic, though: Electric kettles are very efficient at heating water. However, gas power stations generate heat and use that heat to generate electricity, and that means their efficiency depends on the carnot cycle which means they are maybe 60% efficient at generating electricity from heat. This is why (at least in my country) a joule of electricity costs more than a joule of natural gas.

Some power plants use their left-over heat to provide free heating to local homes, which is more efficient and great for them but it doesn't cut my bills any.

Unfortunately I don't have any efficiency figures for stove-top kettles.

My personal efficiency advice is to echo mary8nne: Get your cup, fill it with water, pour that water into the kettle, and use a marker pen to mark where the water comes up to on the outside of the kettle. Then just fill the kettle to that line and you'll make exactly one cup's worth of water.
posted by Mike1024 at 10:36 AM on December 7, 2008

Best answer: David Mackay, professor of physics at Cambridge, has the experimental data you need. His recommendation depends on the time of year (and probably varies depending on where you live.)
posted by anadem at 11:00 AM on December 7, 2008 [1 favorite]

Science aside, I moved to electric kettles exclusively about 10 years ago because I didn't run the risk of burning my apartment down if I forgot that it was on. That's gotta count for something.
posted by rhizome at 11:53 AM on December 7, 2008

Mike1024 is right. Ecologists like me shouldn't try to answer physics questions.
posted by Jimbob at 2:40 PM on December 7, 2008

Response by poster: Hmm...I thought this would be more conclusive. anadem's link shows it is pretty much a wash in terms of energy use.
Checking my utility bills, I pay $0.155 per kWh, and $0.016 per MJ of gas, and 1kWh = 3.6Mj I suspect that in financial terms anyway, the gas will be cheaper.
And thanks to Mike1024 about the tip of marking the line on the kettle that corresponds to how much you need, simple, but for 35yrs I've been eyeballing it like a fool.
posted by bystander at 3:20 PM on December 7, 2008

I just remembered that watts = joules * seconds.

More bad news Jimbob:

Watts = joules/seconds (joules = watts*seconds)
posted by biffa at 2:18 AM on December 11, 2008

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