What's better for heating a mug of water: The stove or the microwave?

I know CogDaily is supposed to be a psychology web site, but sometimes, you have a question you simply must know the answer to.

I frequently need to boil a mug of water, and I've often wondered what the most energy-efficient method is. Is it the microwave, or the good-old-fashioned teakettle on the cooktop? In these days of global warming, it's an important question. So I finally decided to do the experiment.

Method and results:

I filled a ScienceBlogs mug with 300 ml tap water at 62° Fahrenheit.


Then I heated it at 30-second intervals in a 900-watt GE microwave oven.


Results are charted below:

Time Temp
90 seconds 150°
120 seconds 175°
150 seconds 190°
180 seconds 200°

I put an equal volume of water in a teakettle and heated over a 14,000 BTU gas burner until the kettle began to whistle. This took 189 seconds. The water was poured back into the mug and the temperature was found to be 190°



To heat water to an in-mug temperature of 190° it takes 150 seconds in a 900-watt microwave oven and 189 seconds using a teakettle on a 14,000-BTU burner.

The question, then, is how much energy is each device consuming for those time periods?

Let's convert everything to kilowatt-hours.

The microwave:
2.5 minutes = .0417 hours
.0417 hours * 0.9 kw = 0.0375 kw-h

The stove:

3.15 minutes = 0.0527 hours
0.0527 hours * 14,000 BTU/hr = 737 BTU = 0.216 kw-h

That means the stove consumed over five times as much energy as the microwave!

But how much electrical power is lost during transmission from the power plant to my house? According to Wikipedia, not much -- about 7 percent. So the microwave oven is the clear winner for energy-efficient heating of a mug of water!

[fixed units 4/21 4:20 p.m. EDT]
[fixed equations 4/22 6:20 a.m. EDT]

More like this

7% for transmission losses, but there are going to be losses in the conversion of the fuel source to electricity. I think if you want a true compassion you have to look at total amount of gas.The electicity used at least twice that much energy to generate. Maybe more.

Good point, KJC -- though of course it depends on the source. If gas is only 50 percent effective for electricity generation, it makes me wonder what the carbon output of a coal-fired plant compared to a natural gas stovetop is.

fwiw I believe our power here actually comes from a nuclear plant.

Update: If the comparisons on this worksheet are to be believed, then by my thumbnail calculation, coal fired plants put out about 4 times as much CO2 per kilowatt hour than a natural gas stovetop. Which suggests that the stove versus microwave debate is essentially a wash, leaning perhaps slightly towards the microwave, especially if your power comes from anything other than a coal-fired plant.

Well you only have the choice to save gas, by using the micorwave. It isn't all about efficiecncy. If you had a wind turbine at your house with a 10% efficiency then using that to power the microwave would be the best bet even if its efficiency is low. So if you have nuclear power.... well that depends where you stand on the TCO of nuclear power plants. I think it shows how tricky the correct environmental decsions can be. The economy of scale of large power plants is meant to take advantage exactly of that. However what happens to the "wasted" energy of the stove top. If you are heating your house at the time then was it in fact wasted at all? The wasted heat from the power plant was definitly wasted. And for fossil fuel plants I think and average efficincey is nearer 30% but gas is the best of the bunch. If things are priced fairly (sans subsides) then it is my opinion that whatever actions you take, unless they have a noticable effect on your fuel bill you are making very little real difference. If they do have a noticable effect then, well, you hardly need the feel good of doing something good for the environment to know it makes sense.

I'd be interested to see how a water boiler matches up, seeing as it's a device specifically designed for the task in question. I know they're wicked quick, and maybe more efficient. The heating element is completely immersed in the water, as opposed to the large amounts of waste heat with a microwave or stovetop.

In an air conditioned house the gas range contributes greatly to the AC load, not a small expense, likely tipping things in favor of the microwave. In the winter the waste heat from the range is actually a benefit. Regardless, year round I heat my water in an electric kettle.

By Steve Sykes (not verified) on 21 Apr 2008 #permalink

I'd be curious how an electric teapot compares in terms of time and energy consumed. Also, I wonder if said teapot could run off of a small solar panel, making it an even bigger win. Of course, this implies consuming a lot of hot water. And let's not even get into the carbon cost for producing that tea bag, instant coffee, or hot cocoa. :)

In the end, it probably would have just been easier to replace all your 60W incandescent bulbs with 12W or 15W CFLs. Or you could take the same amount of money and send it to a charity that takes care of sick and/or starving children in Africa.

KJC says that the wasted heat at the power plant is definitely wasted, but this too is variable. Many newer and smaller plants collect what would otherwise be waste heat and use it for useful work. This is known as cogeneration.

The important question here: were you watching both of these containers while you tried to boil them? Because that's been established to increase the boiling time (see Zeno, G.P. et al.)...

From my _Home Energy Diet_ book, I offer this piece:

p. 70
By the time the electricity gets to your home, 70 percent of the energy that went into making it has dissipated as heat, making the national electric generation and distribution system about 30 percent efficient. What this means is that for every three units of fuel put into a power plant, only one unit makes it to your home in the form of usable electrical power. For every kWh of electricity you save at home, you're really saving over three times more in total primary fuel consumption.

It refers to a diagram I have seen elsewhere--here is a copy of it: https://eed.llnl.gov/flow/02flow.php

A teakettle with a wider bottom would have made quite a bit of difference. Judging from the photo, it looks like a lot of that flame wouldn't have been heating water. rb

I was under the impression that the base efficiency of fossil fuel fired plants was around 30% but with cogeneration it could go as high as around 50%. Still a lot of wasted energy (well maybe not a lot in terms of theoretical maximum efficicincy of chemical to electical energy at the industrial scale I am sure it is right up there)

Don't you philistines have electric kettles yet?

AMAZING! When I put hot coffee in my Sb mug, it split.

You've got your units mixed up. That first calculation should be:
.0417 hour* 0.9 kW= 0.0375 kw*hr

And the second should be:

.0527 hour* 14,000 BTU/hr= 737 BTU= 0.216 kW*hr

BTUs and kilowatt-hours are units of energy. BTU/hr and kilowatts are units of power, which is the rate at which energy is transferred or dissipated with respect to time.

This is high school physics. Confusing power and energy is also a characteristic mistake of "free energy" cranks.

In the future, if you're going to use quantities denominated in energy or power units in a blog post, you might want to get a physicist or an engineer to look them over first.

By Ktesibios (not verified) on 21 Apr 2008 #permalink

@McDuff: actually, I had a friend from Germany move here for a couple of years. We looked everywhere for a kettle like that, and finally found one in a specialty tea shop in the city.

You really have to seek these out here.

Ktesibios -- I actually was fixing the units as you wrote.

Chad -- Heh. I wasn't watching; I was writing this post. Perhaps that skewed the results.

Wow. In the time it took me to type that, you fixed the mistake. Cool.

BTW, the 900W rating for the microwave is most likely the RF power output, not necessarily the power input to the magnetron or the power taken from the wall socket. The RF power output would be equal to the product of the anode power input to the magnetron and the efficiency with which it converts DC to RF. The power input to the magnetron would be the product of the power taken from the wall socket and the efficiency with which the oven's power supply converts 120VAC to high voltage DC.

I would expect the power taken from the wall socket to be substantially greater than the RF power delivered to the oven cavity.

By Ktesibios (not verified) on 21 Apr 2008 #permalink

I recently did a Thermodynamics course that included a lab session on efficiency. We compared heating water in an electric kettle with a microwave oven, and found that the kettle was much more efficient than the microwave in getting heat in to the water: something like 85% vs 45%. A lot of the energy going in to the microwave was not getting in to the water, with motors and fan taking a hefty chunk, plus other losses.

This test was slightly stacked in the microwave's favour, too, because we boiled the water in the kettle but not in the microwave, and heat loss to the atmosphere is proportional to the temperature difference.

Most stoves in my state are electric, so maybe the results of this experiment don't really apply to my tea? ^_^

Much of the heat from the gas flame escapes around the kettle, warming the room.

I use a Sunbeam HotShot every day to heat water for my instant coffee. It is a 1450W device which heats 2 cups of water to boiling in about 80 seconds. That's faster than my microwave will do it. That's a lot faster than my ceramic cooktop will do it. Now, I suppose I'll have to start making some accurate tests to determine the power consumption, etc.

I would like to take a moment to point out an incorrect mathematical statement in the article:

2.5 minutes = .0417 hours * 0.9 kw = 0.0375 kw-h

In fact, 2.5 minutes does not equal 0.0375 kw-h. Such statements are called "run-on number sentences" in the world of math education. Many students do not receive full credit on extended responses because they included incorrect equalities (at least in New York State).

I advise the author, as I advise my students, to work "down the page:"

2.5 minutes = .0417 hours
.0417 hours * 0.9 kw = 0.0375 kw-h

By Bronx Math Teacher (not verified) on 21 Apr 2008 #permalink

Regarding transmission efficiency of electricity. Seems like everyone here is neglecting to mention the transmission efficiency of the gas. Fair is fair, right? I mean, he's not mining the gas himself on the premises, is he?

KJC says:
If things are priced fairly (sans subsides) then it is my opinion that whatever actions you take, unless they have a noticable effect on your fuel bill you are making very little real difference.

The key word here is "fair."
I think a good lesson of the past 30 years of environmental movement is that it isn't just subsidies. There are all sorts of other externalities that get swept under the rug, in different ways for different industries. Things like health care costs from pollution, toxic spills, habitat destruction, etc. And what is a "fair" price for pillaging the natural resources of entire countries?

But heating water in a microwave ("nuking it") makes it radioactive. That's why I never hold the mug in my lap.

Interesting question, I've wondered about the same thing myself.

One thing that threw me a little was the mixed use of metric and imperial units (particularly "300 ml tap water at 62° Fahrenheit.") Ideally the temperatures would be in Celsius. As a bonus, that would help make it a little more obvious how near to boiling the water is. Also consider replacing BTUs with Joules (or at least noting the conversion), though I know BTUs are still widely used.

Since we're being scientific and all. ;) I try to use metric whenever possible, even though my country hasn't quite switched yet.

Interesting discussion. I would only add that you should have a disclaimer/warning prominently displayed, for any unwary individuals (kids or adults) who try to repeat this experiment. Heating water in a microwave can be dangerous, it can get superheated but still remain liquid, then the slightest disturbance (such as moving the container or plunging a thermometer into it) will cause it to flash-boil and explode up and out of the container. I see that you did it safely, with the thermometer already in there, and just did it for short periods with checking in between. But somebody may not be as patient and may just put it in for several minutes (sans thermometer) and then check it and get a nasty surprise.

Even though your power might come from a nuclear plant, using this power will require more power to be generated at coal fired plant somewhere on the grid to replace it. I live by a the reservoir behind a hydorelctric dam, but I still consider my marginal power consumption to be coal generated. However, there is the potential to sequester the CO2 from a coal fired plant which is not feasible for a gas stove.

By Militant Agnostic (not verified) on 21 Apr 2008 #permalink

BUT who wants to use a micro wave???? No one seems to have taken in to consideration exactly what a microwave does to our foods etc - there is a reason why mums are told NOT to heat their baby's milk in one - it rearranges the molecules.......

I expect an electric kettle would beat both, achieving an efficiency near-perfect - all energy used would be put into heating the water, though there would be some losses due to boiling around the element due to uneven heating and conduction. I can see three optimisations to produce an ultra-efficient kettle:

1. Optimise element shape for rapid convection and to prevent the formation of hot-spots which could boil before the water has reached desired temperature throughout.
2. Insulation. This not only reduces heat loss, but avoids those 'got distracted while it was boiling, now it's cold again' moments.
3. Replace the 100% efficient electric heating element with a >100% peltier bank. A rectifier and smoother cap provide 230V (110 for you americans), suitable for driving a set of five 48v (24 for americans) peltiers. Throw in a few heat-pipes, some warming fins around the outside... And you have the most efficient - and expensive - kettle ever built. With all those fins, it even has an stylish retro-futuristic look.

Regarding three, I think that would work much better for an electric hot-water dispenser. Maintain an insulated steel block at 80c or so, could be probably done with only 50W pelt and even that wouldn't need to be on most of the time (Assuming very good insulation - think aerogel). For instant hot water, just pass it through channels in the block. So long as you don't demand more heat than the block can hold, instant and efficient near-boiling water. An auxilary conventional heater would be included for those times when you need an unusually large amount of hot water in a short time.

Aaron: I would have converted the temperatures to Celsius if it had mattered for this experiment, but my thermometer reads in Fahrenheit, and temperature actually never comes into this equation; we just use it to ensure that the two mugs are heated to the same temperature.

Bronx Math Teacher: You're right. Normally I wouldn't bother to fix something like this on what I thought would be a toss-off post, but given that the post seems to have taken on a life of its own, I'll go ahead and express the math properly.

Everyone who suggested that boiling water in a microwave is dangerous: It's only "explosive" if the water is super-heated (beyond the boiling point). It's also a much stronger effect if distilled water, rather than tap-water, is used. I didn't do either of those things. But yes, kids, you should be careful when you're dealing with hot water.


Turning ever-so-sligtly to a psycho tack - I reckon I can taste a difference in water that's been heated by microwave. Just possibly it's to do with the receptacal used for heating, but I don't think so.

In terms of energy efficiency, the shape of the container and volume used affect results as well, of course. (And in practice, a major source of waste is heating half a kettle of water to make one cup of coffee; half a pan to boil an egg ... and so on.)

By Pete Berry (not verified) on 22 Apr 2008 #permalink

A microwave oven won't do anything to your water except heat it. Milk may be different, but I would doubt a significant issue there, either.

I use electricity to make my stovetop hot, run my microwave oven, and heat my apartment (I know, not the most efficient, but I didn't build the bldg). In the winter it doesn't matter how efficient I am with lights etc, as long as I keep the windows tightly closed. In the summer, I don't drink hot drinks as much, and don't have the lights on as much....I have an electrically powered ac unit.

If you want to increase the efficiency of your microwave, unplug it when you aren't using it. The little clock uses an appreciable amount of the total energy used by the unit due to duty cycle considerations.

BUT who wants to use a micro wave???? No one seems to have taken in to consideration exactly what a microwave does to our foods etc - there is a reason why mums are told NOT to heat their baby's milk in one - it rearranges the molecules.......

Oh man. That's the most obvious crank site I've ever seen. Is it a parody?

It defines "radiation" as "radioactivity"... Man.

I have never heard that mums are told to not heat milk in a microwave oven. All I can guess that you might be thinking of is the fact that if you heat milk too much, no matter by which method, the taste becomes awful, and if you keep going anyway, a "skin" develops. All children in their right mind refuse to drink that. With microwave ovens it's easy to heat milk too much. I always put it halfway between "thaw" and the next stage for two minutes, or on that next stage for one minute, depending on the make of the microwave oven. Inexperienced people like my grandmother then touch the mug, tell me the milk is cold, and suggest to heat it again -- the milk isn't cold, the mug is; the very trick of a microwave oven is that it doesn't heat the mug, but the contents directly.

By David Marjanović (not verified) on 22 Apr 2008 #permalink

In other words, the heat denatures the proteins, not the microwaves.

By David Marjanović (not verified) on 22 Apr 2008 #permalink

David MarjanoviÄ et prev. re: baby's milk in the microwave:

There's a more important issue here, which you just missed: The microwave heats more quickly, but also unevenly! You can end up with not only a cold container, but cool liquid on the top, even while the milk in the center is almost boiling. Of course, that will not make for a happy baby!

So, stir or mix the liquid before you test the temperature....

For the original topic: Did you really have that 14,000 BTU burner turned up to max flame? Funny, the pot doesn't show much soot!

By David Harmon (not verified) on 22 Apr 2008 #permalink

We never heated our baby's milk. They seemed to prefer it unheated, and it seemed like a lot of extra work.

Yep, the burner was maxed out. I don't know why the pot doesn't look sooty; it gets plenty of use.

There's a more important issue here, which you just missed: The microwave heats more quickly, but also unevenly! You can end up with not only a cold container, but cool liquid on the top, even while the milk in the center is almost boiling.

Yes. That's why you need to apply low power for a longer time

By David Marjanović (not verified) on 22 Apr 2008 #permalink

One thing we've left out here: how efficient is either one? 300 ml of water requires about 300 calories per degree centigrade to heat up; going from 62F to 190F is 71C, so theoretical energy requirement is 71*300*4.184 = 89 kJ = 25 Wh. Thus, the microwave was about 67% efficient, the stove was about 12% efficient.

It's notable that the rate at which the microwave heated the water varied as temperature increased; the rate of heating started near 1 degree per second, and dropped to less than half a degree per second at the top end. As a guess, this loss was caused by evaporation. Putting a simple cover on the cup might have a measurable effect.

As a gas engineer, I'm surprised at the comments about smoke. If your approx 4Kw gas burner is being supplied with gas at the correct pressure, and of the calorific value it was designed for, there should NEVER be any smoke.
Smoke is a sign of incomplete combustion, and a smoky burner, aside from being inefficient is a carbon monoxide hazard. Simple, eyeballing test... Are the flames predominantly blue?
If yes, then it's good. If predominantly YELLOW, turn it off, get it checked.
Kettles. In britain, kettles are everywhere. efficient ones, both stove-top and electric, in the US, less so. (btw, we call them kettles, not tea-kettles. We use them to boil water for many purposes, one of those being tea. We do NOT make tea in them). However, our kettles run 240 volts, not 110, and typically 3Kw, which translates into fast boiling. For eco-friendly efficiency, and carbon neutral combustion, why not try a Kelly-Kettle, (also known as a volcano kettle. Dry twigs, dung, grass, if it'll burn, you can boil water. Qiuckly. Faster than expensive high tec camping stoves. http://www.safariquip.co.uk/i_kelly_kettle.html

(Just for the record, all that's missing at the end of my last comment is a period. No HTML problem or anything.)

By David MarjanoviÄ (not verified) on 22 Apr 2008 #permalink

The energy efficiency calculated for the microwave (67%) compares favorably with figures (65%) from about 1970. See this article on the history of home microwaving:


Most of the energy in a microwave is "tightly coupled" to the "load" as they say ... whereas you can feel a lot of heat above a kettle over gas. But then, the microwave doesn't "sing" when the water's hot!

I should point out that if the kettle was singing, then you were boiling water instead of just heating it, so you need to take the heat of evaporation into account for the mass of water that you boiled.

And if you are concerned about waste heat, instead of using electricity to boil your water, walk over to your friendly neighborhood nuclear power plant, and just make tea from the moderator.


You may want to go over that page with a fine-tooth comb. Or better yet, find another source. The scientific ignorance of the writer is beyond overwhelming; "direct current microwaves" is a contradiction in terms.

Ew. You used Fahrenheit.

I second the recommendation for the Kelly Kettles! It will set off your fire alarms indoors, but in the back yard it will not contribute to global warming like any use of electricity, just add a little to the smog levels, and it's like a miniature camping trip. Plus, the leftover charred sticks and ashes are good carbon supplements for garden soil, so long as you don't overdo the ashes. Fun Fun Fun. In the USA, Kelly Kettles are available from a few importers on the web, but I got one of mine from the family in Ireland that makes them through their website.

As for the all electric home or office, I vote for the microwave, even if it takes a little longer. Why heat non-aqueous mass?

By Bill Gibson (not verified) on 22 Apr 2008 #permalink

In the end, if the water doesn't actually boil the tea will taste like crap. And I am not talking about teabags (which always taste like dishwater), I mean real loose-leaf tea. Microwaved hot water just can't cut it, I'm afraid. When 'energy consumption guilt' finally gets me, I'll reduce the number of pots of tea I make each day rather than make lousy tea more often...

IIRC, the main reason that parents are advised against using the MW to heat (infant) milk is the difficulty in controlling the temperature (or rather, the ease of making a mistake and making the milk hot enough to scald). Heating directly over gas or an electric element is also advised against - the standard recommendation is to immerse the bottle in hot water if warming is needed.

You are correct, outeast. As a parent with a recently-weaned baby, I can attest that the recommendations are to prevent burns. Some experts are also concerned about destruction of certain proteins, but this is not fundamental to the microwave but is simply a matter of overheating (which happens much more easily in a microwave than on a double-boiler).

This is also why microwaved custards are usually inferior. ;-) It's too hard to control the temperature, and custards go from "almost done" to "overdone" very quickly.

By Calli Arcale (not verified) on 23 Apr 2008 #permalink

For those who think microwave heating alters the taste of the water, how would you suggest the molecular structure of H20 changes to explain the difference?

Not sure if someone has suggested it already in the lengthy comments list, but one of those little spiral in-cup electric heating elements would seem to be nearly lossless due to minimal exposure to the room environment (compared to any heating pot made of metal or even ceramic).

900 cooking watts uses about 1400 watts of input power

yes its still more efficient than gas, but NOT as much as your initial calculations

all forms of power use have transport & conversion losses

the gas has to be piped to the hose under pressure, what does this, electric powered compressors/turbines that compress the gas to supply the line power

but the end user has to figure HIS cost per btu, for the gas or watts of electricity NOT for the transport costs

the transport cost is part of the over all cost to the user

now if your OFF-grid like me, and generate your own electrical power

GAS ( propane ) is cheaper than , burning gasoline to power a generator to run the MW, but sometimes you have NO choice..

if i'm low on propane, MY method of heating, i WILL run the genny for the MW and cooking, or vice versa

One thing that gets missed in the arguments about the waste of energy in incandescent vs. compact florescent, gas vs. electric vs. microwave is that the lost energy is normally released as heat in the house. I live near San Francisco, CA and most mornings benefit from some heating. So when the stove or microwave or electric range "waste" heat it is in truth reducing my heating bill by that amount. And please don't repeat my mothers admonishion about putting on a sweater.

By robert estrada (not verified) on 26 Apr 2008 #permalink

Doesn't anyone in America just use a kettle? It's the absolute first thing to go into anyone's house in the UK, usually before any of the furniture.

I'd think that some physicists have addressed this problem. I know a couple and will enlist their aid in finding the answer.

My answer before additional research is wide copper kettle, moderate flame in winter for gas stoves. immersion elements or electric kettles for those without gas or in AC season.

I suspect there's also a diminished fraction of the energy going to the kettle compared to the room at blasting high setting, compared to moderate or low setting. The ultimate optimal temperature is another variable. If you insist on going to boiling, much more of the energy goes into evaporation of even a small amount of water. A microwave can't beat an immersion heater. That would be magic!