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Whenever I need to heat up water for a French press or hot chocolate, I do it in the microwave. Is there a good reason to use a kettle instead of the microwave to heat water?

  • Cleaned up the very long discussion. If you know of an argument for kettles, please add it as an answer, not as a comment! – rumtscho Nov 22 '16 at 19:57

12 Answers 12

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Bear in mind that I'm using an electric kettle, rather than a stove-top one.

First, the advantage of a kettle is that it is quite efficient, and turns itself off once the water is boiling. This as opposed to the microwave, which only stops after a set time, rather than relying on the condition of the water.

Second, a microwave can cause water to superheat, particularly in a ceramic or glass container. This means that there is a certain risk that the water will "jump out" at you once you add something to it or put a spoon in.

Additionally, most kettles have more power than the average microwave, so the kettle is probably faster to boil water.

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    I thought the standard fix for #2 was microwaving the cup with a spoon already in. – Federico Poloni Nov 20 '16 at 10:50
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    @FedericoPoloni I really hope this is a joke, and that everyone will get it... – Quentin Nov 20 '16 at 15:35
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    @Quentin It's not a joke. It is safe to microwave a cup of water with a spoon in, and apparently it prevents it from heating over the boiling point. See for instance physics.stackexchange.com/questions/234042/…, or do some more research yourself if you don't trust it. Metal without anything else to absorb the microwaves is dangerous, and metal objects with different shapes can cause sparks. But water with a spoon is perfectly fine. I have done it many times. – Federico Poloni Nov 20 '16 at 16:14
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    @FedericoPoloni alright, well I've learned something, thank you. I have been raised to the "NO metal whatsoever in the microwave rule" and never really questioned it, especially since the time I forgot a spoon and made my own fireworks show inside :p – Quentin Nov 20 '16 at 16:22
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    Whether a microwave oven can deal with a metal object at all is at best make and model dependent. It is unlikely to be universally safe. – rackandboneman Nov 21 '16 at 8:59
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I think the primary considerations are convenience (how much effort is it to set up and use the system?) and time spent (how long does the system take to heat the water?).

A standard electric stove can have 2500W elements, and most of this energy will go into a kettle sitting on the element and thus heat the water. Even a big built-in microwave won't be rated at much more than 1000W, and a fair percentage of that is wasted in heating the magnetron (I've found citations of 30% to 50% wasted power). So, all else being equal, a kettle on a stove will heat a body of water 2-4 times as fast as a microwave oven.

If you're instead talking about electric kettles, then in the US (120VAC) they're 1000W to 1750W with very high efficiency, so they'll still be faster than anything but a stove or a commercial microwave. In the lands with 220VAC power, electric kettles can be up to 3000W, which would be even faster than a stove-top kettle.

With a mug of water the convenience of a microwave may outweigh the lost time, but if you're heating a liter or more of water then you probably want to pull out that kettle.

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    Electric kettles have heating coils that are designed around convection currents to increase the rate of water heating. It is surprising how fast a smaller amount of water boils in those things. Definitely much faster than what is possible in a microwave. – Nelson Nov 21 '16 at 1:35
  • This answer is incorrect (in the first part, anyway). Electric stove is far lower efficiency than microwave. See this article for example. – Joe M Nov 21 '16 at 8:43
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    @JoeM Um, what? Your article clearly states the electric kettle is the most efficient... From the article: "The clear winner is the electric kettle, at 81% efficient, followed by the microwave, at 47% efficient, with the stove being the Hummer H2 of the bunch at 30.5% efficient. " – Nelson Nov 21 '16 at 8:51
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    Your first paragraph seems to imply that an electric stove is highly efficient. "Most of that energy..." Electric stoves are 30% or so efficient. 30% is not "most" by any effect. The electric kettle is clearly the superior choice, but your answer sort of tacks that on the end. You also misunderstand how a microwave's power is measured (or, advertised). – Joe M Nov 21 '16 at 8:57
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    @JoeM ... why? The original poster asked about a kettle, and hasn't posted since. – Daniel Griscom Nov 21 '16 at 15:25
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Convenience

Using an standalone electric kettle device provides for a convenient and safe way to boil water without minding.

  • Because of automatic power-off feature, you know the water will be at the correct temperature, not too cool and not too hot.
  • No danger of boiling dry in devices with a safety Off feature.

If you are delayed in using the water, just hit the button for a quick re-boil. Some devices can even maintain a desired temperature automatically.

picture of electric kettle with a glass carafe perched on its electric base showing boiling water

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    Also, you might have all 2, 4 or 6 stove tops occupied (or some unusable due to overhanging large pans) ; you might not have a microwave oven or not have it in convenient reach. And a kettle can simply be emptied and shelved after use, so you are not taking permanent counter space. – rackandboneman Nov 21 '16 at 9:02
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Where I live, natural gas is far cheaper than electricity. So, using a kettle on the stove is “best” from this point of view. I happen to have solar power, so in the daytime it’s cheaper to use my own electricity instead.

I used to have another item not discussed on this thread: an instant boiling water tap. That was certainly the handiest, until it broke.

  • Plus kettle lives on stove, where I always know where to find it. No need to dig up the Pyrex measuring cup for the microwave. Besides, when I'm boiling for tea, I'm using the micorwave and another burner for other stuff. – Wayfaring Stranger Nov 22 '16 at 0:43
  • Measuring cup? Why not just heat in the serving mug? – JDługosz Nov 22 '16 at 5:33
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The kettle is more efficient. Nearly 100% of the energy is converted by Joule's law into heat into the water (the kettle is also heated and it makes some noise so it's never 100%, but near). Microwave ovens at the other hand usually have a 60-65% power efficiency just for microwave generation. So if both electrical appliances have a same power, the kettle will be faster and cheaper in energy. Kettles and also cheaper, but microwave ovens can be used for more things that just boiling water. Having a more specific purpose appliance, a more generic one, or both, will be interesting according to your needs, just like bikes, cars, running shoes, tools or whatever you imagine: how often will you use it, can a more generic and less specific solution be enough, what's the price, etc.

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There are a couple reasons why people might heat water on the stove instead of in the microwave - not all of them are strong reasons, but they can influence people nonetheless.

One point is, there is a perception that the method of heating changes the water. I've heard that the flavor is affected, if water is heated by microwave or on stovetop. I've heard that there may be deeper changes in the water - there was an anecdote floating around about how plants watered with previously-microwaved and cooled water vs never-microwaved water did not thrive, implying possible health issues with microwaved water (generally issues assumed to be long term and long scale use of microwaved water, not casual use).

I am not saying there is a difference in flavor or in health benefits, but there is some kind of anecdotal, hand-wavish perception there that people may be drawing from.

On the more practical side, it can be difficult to figure out how much the water needs to heat. On a stove top, the water will simmer then boil in visible markers, in the microwave, it just sits still and can vary in temperature (random example, it took a minute and a half longer to heat my mugful of water in the winter as opposed to summer, in my old microwave. Haven't sussed out my new microwave yet). If you're not familiar with the microwave's quirks (or compensating for the conditions), water can easily come out a little cool, or superheated, which is kind of annoying - especially if you're using it for something where water temp actually matters, like brewing tea.

I suppose part of the difference is the microwave takes less time and is supposed to be more reliable, which is why the small differences between times with different models or conditions seems so much more annoying than stoves, which everyone knows will take more time.

Also on the practical side, it's usually easier to heat larger amounts on the stovetop. An occasional mugful is quick in the microwave, but if you're having several cupfuls in in a row (or for several people) it's easier to keep a big pot warm on the stove than heat several mugs in succession.

As for the difference between a pot on the stove and a kettle - well, you've got me there. I suppose there are benefits to having a pot dedicated to just boiling water (much easier to clean, I guess?), and the kettle is specialized for the task - but I've always just used a when heating water and never missed the difference.

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    "As for the difference between a pot on the stove and a kettle - well, you've got me there." The kettle has a spout! – David Richerby Nov 20 '16 at 11:09
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    Of course, knowing the quirks of your stove is also rather important. I moved recently and got an induction stove, which is ridiculously fast—and that’s dangerous! Step out of the room for a moment and come back to absolute mayhem. Oops. On the plus side, it’ll bring a pot with about 2.5 litres of water to boiling in less than three minutes, which is far better than a microwave or kettle (if I could find a kettle that big) could ever hope to accomplish. – Janus Bahs Jacquet Nov 20 '16 at 11:23
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    I should note that the thing about microwaved water killing plants/being less good for plants has been thoroughly debunked. However, I'm pretty sure there's a flavour difference between boiling water in an electric kettle and in the microwave. I suspect this might have something to do with the kettle's metal base/element and the microwave using plastic or ceramic containers. It's subtle, though. And as a Brit, we understand that microwaves are too slow when you want a cup of tea! – Matthew Walton Nov 21 '16 at 9:41
  • @DavidRicherby - I am shocked! This is genuinely new information! I did not know this! (actually the specialization tradeoff of easier to pour and harder to clean just balances out for me. shrug.) – Megha Nov 21 '16 at 16:35
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    The flavour difference might also depend from the fact that kettles are not usually washed often, and limescale tends to accumulate. – Federico Poloni Nov 21 '16 at 18:39
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The other answers so far have made it clear that energy efficiency is the main advantage of using an electric kettle. What nobody has mentioned, however, is the fact that most modern kettles have a "minimum fill" line which usually corresponds to several cups' worth of water. I live alone, so if I want a cup of tea I always microwave it, because it's still going to be more efficient than heating twice as much water as I need in a kettle. When people visit, I put the kettle on.

Again in the context of tea and coffee, microwaves have another slight efficiency advantage over what the numbers show, because you don't have to heat the water all the way to boiling. Remember that you can't actually drink boiling water - even after the time it takes to prepare your beverage, the cooldown from adding milk, etc., you may need to let it stand a while before you can drink it. After a bit of practice, a microwave user can hit the exact water temperature for instant enjoyment, and no more. (Plus, I'm convinced the tea tastes better when the water is microwaved - probably because of the slightly lower temperature; pouring water straight from the kettle onto the teabag gives it a weird flavour sometimes.)

I don't know if I've answered your question or the inverse of it, but those are my reasons for favouring the microwave for small volumes of water. (FWIW, I come from a family of mug-microwavers - everybody I know thinks we are weird - and in several decades of experience I've never witnessed the superheating problem that so many people associate with water in the microwave. Just use sensible heating times and it's not at all dangerous.)

  • The minimum line can often be ignored especially if you're standing right there. With the water just about covering the element the kettle will work just fine (though in some cases it appears to be a little slow to switch off. If you're waiting to make a drink you don't have to let it boil either – Chris H Nov 21 '16 at 10:39
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    most modern kettles have a "minimum fill" line - I find that false. Most modern kettles I've seen have flat heating element and no minimum fill. Obviously the same for kettles you use on the stove. Oh, and increasing numbers of electric kettles have temperature setting, so you do not need to heat your water all the way to boiling. – Mołot Nov 21 '16 at 10:40
  • @ChrisH True. But for those of us with short attention spans who like to walk away... there plenty of things you can get done in 2m 20s :) – xtal Nov 21 '16 at 10:43
  • I'm sort of skeptical that practice will make your water temp consistent - I know the time it took to heat my mugful varied quite a bit from winter to summer (by a minute and a half at the most), between the temp variations in the tap water and the apartment, and even if I already figured that out after lots of experimentation - that's just the most extreme difference, the actual temp and conditions keep changing over time. it would be very easy to overheat the water on a warm day or have it come out lukewarm on a cold one, or consistently over- or under-heat all of a season. – Megha Nov 21 '16 at 16:49
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    If you're making tea, the water does actually need to be very close to the boiling point for the infusion process to work properly. – zwol Nov 21 '16 at 19:43
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An electric kettle is about 80% efficient, for around 1200W of power, and takes around 2 minutes to boil 12oz of water.

An electric stove is about 30% efficient. A 1250W burner would take about 5 minutes to boil 12oz of water; even a 2500W burner (the highest I've seen) would take 2.5 to 3 minutes (you would lose some efficiency as the heat increases).

A microwave oven shows the efficiency directly: a 1100W microwave is not using 1100W but is outputting 1100W (using something higher, probably 1500W for example). A 1100W microwave would take 2.5 or so minutes to boil the water, a 900W microwave would take a bit over 3 minutes.

As such, the electric kettle is clearly the superior choice, but it's not necessarily that much faster for a small amount of water. The microwave may be better when you consider the amount of time it takes to get out the kettle, fill it, etc. compared to directly heating a cup of water.

Numbers largely come from this article.

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    Only if you put your kettle away! It usually has pride of place on a worktop near an electric socket in British kitchens because we use them so much. – Matthew Walton Nov 21 '16 at 9:42
  • Electric stoves are far more than 30% efficient. According to the California Energy Commission, induction stoves are 90% efficient, standard electric ranges are 65% efficient, and gas stoves are 55% efficient. And here is a source which says an electric burner uses 25% less energy than a microwave to boil a cup of water. – Daniel Griscom Nov 21 '16 at 14:15
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    Where does the other 20% go? I expect inefficiency to lose energy as heat, but when heat is what’s wanted how can you be inefficient? – JDługosz Nov 22 '16 at 0:09
  • Any electric resistive heater is 100% efficient by definition. – Agent_L Nov 22 '16 at 11:24
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    Some of the heat doesn't go into the water, though. – Joe M Nov 22 '16 at 14:21
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Automatic Temperature Control

This is the main reason I use an electric kettle instead of any of the other options, and has yet to be mentioned in any of the other (admittedly very good) answers.

I primarily boil water for steeping tea, and as tea aficionados will tell you, many types of tea taste better when steeped with hot but not boiling water.

Traditionally you would either have to wait for the water to reach the desired temperature (say 180 degrees Fahrenheit for example), or boil the water and then wait for it to cool to the desired temperature, either by guessing the temperature or using a thermometer. With my electric kettle I can simply select what temperature I want to produce and my kettle will heat the water until it reaches the desired state. Of course with the microwave you can't leave a thermometer in the water while heating (at least I would be wary of doing so), so you would have to either frequently pull the water out to test it, or boil the water and then wait for it to cool back to the desired temperature.

This has the following advantages over the microwave or stovetop:

  1. You can't forget about it and either overheat the water or wait too long and have it cool off too much (great for forgetful people like me)
  2. It is much lower effort since it handles the process automatically
  3. When you want to do another batch it is easy to heat back to the same point repeatedly or increase the temperature by small amounts as desired.
  4. You don't need an external thermometer since the kettle has one built in
  • My microwave has a load sensor, so adjusts to the size of the bowl or plate I’m heating, automatically. “reheat soup” gives consistent results. – JDługosz Nov 22 '16 at 0:14
  • @JDługosz That's interesting, does it let you pick the temperature you want to heat it to? – Kevin Wells Nov 22 '16 at 19:18
  • No, it’s not configurable. – JDługosz Nov 22 '16 at 19:27
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1. Efficiency

A microwave first converts the electric energy to microwaves, then send those waves into the chamber and there they are absorbed by an item inside. All of those processes are lossy, that is some of the energy is wasted as heat not in the food. You can hear a microwave fan that cools the electronics inside. That is the heat that's blown out into surrounding air and not into your water.

An electric kettle on the other hand, is a simple resistive heater. It means that all the energy going in gets converted into heat where you need it, that is on the heating element submerged in water.

This means that microwave will use more electricity to heat same amount of water to same temperature:

The magazine’s researchers discovered that an electric burner uses about 25 percent less electricity than a microwave in boiling a cup of water.

2. Power

Microwaves usually top at 800W range, while for a kettle, 800W is rather small one, a 1500W is common. This means that safe to assume that typical kettle will heat same amount of water in half the time.

Those are 2 biggest reasons why kettle is obviously better. There are many others, some mentioned here, although they are less prominent and often depend on your personal style or beliefs.

But - there are also reasons why sometimes it's better to use microwave:

  1. People tend to overfill the kettle. Even when they need just a cup, they tend to fill it to the max (let's say 4 cups). This means that heating of remaining 3 cups is wasted, as this water will be left standing and cool down. That drops the efficiency to abyssal 25%! If you can't judge the amount of water before pouring it into kettle, then microwave can be more energy efficient after all.
  2. Need of a certain temperature below boiling point. If you need water eg for melting chocolate, and you need it at certain temperature, then it's much easier to do it in a microwave. You can learn that eg 1 cup at 800W and 25 seconds yields the exact temperature you need. This is repeatable. Trying to do it in a kettle would require you watching it with a stopwatch, much less convenient and prone to errors.
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Take into account steeping temperatures for tea and coffee, as well as the brand of hot chocolate you're using, as in its melting point and solubility in water/milk/cream. (TLDR How hot do you need the water to be and how much water do you need to heat). Electronic kettles and a pot on the stove give you temperature control but 1-2 minutes in a microwave will get you boiling point water temps (212 Fahrenheit/100 centigrade), this is mainly because microwaves are built to heat water molecules in food, to heat/thaw the food. So your microwave will heat the water faster than the stovetop and electric kettle.

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No-one mentioned that you are supposed to pour the hot water onto the target (tea-bag, usually, these days).

In addition to being easier to control the temperature, easy to know when the temperature is correct, and not heating the drinking receptacle, a kettle is also easier to pour out of.

If you heat the water in a microwave in the cup, do you then try to dip your teabag into the hot cup and hot water, or do you pour the water carefully from that cup into another one with the teabag in it? In which case, why not just use a kettle?

  • Interesting, but the original question mentioned coffee and hot chocolate, not tea. – Daniel Griscom Nov 23 '16 at 12:22
  • Hah - touché. However, I think the first part of my answer still stands. – GreenAsJade Nov 24 '16 at 0:13

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