Best Way to Boil Water

When it comes to home energy use, we often find that conventional wisdom is wrong, uselessly vague, or outdated. So, we recently set out to put a bit of experimental rigor into the question of making our morning cup of tea.

Here are some questions we wanted to answer:
  1. What’s the best (i.e. least energy intensive) way to boil water?
  2. Is the answer different for 1 cup of water vs. 4 cups? What about 2 quarts?
  3. Most importantly, if I’m doing it wrong, is it worth changing the way I do things already?
We carried out 13 tests on a typical microwave (1.3 kw), a popular electric stove (1.5 kw element), and a standard electric kettle (1.5 kw). The results were a bit surprising. The electric kettle (unsurprisingly) performed great. The microwave, on the other hand, was shockingly inefficient while the stove performed surprisingly well (considering all of the wasted heat it dumps into the kitchen).

Question 1: What’s the best way to boil water?
Answer: The electric kettle won hands down. The real advantage with the kettle is on the first cup. It boils in half the time as the first cup boiled in either the microwave or the stove. Since (at 1450 Watts) it uses about the same amount of power as the others, halving the time halves the energy. Thereafter, the energy requirement for boiling larger volumes of water is nicely predicted by a linear function. Each additional cup requires an additional 25 Watt-hours of energy and about a minute of additional time. The stove had similar performance except that the first cup required twice the energy (and time) of the kettle’s first cup.


Question 2: Does the answer change based on the volume of water boiled?
Answer: For the first quart of water, the answer is surprisingly unchanged. The only thing that really changes is that the microwave performance degrades much more rapidly than the other two. So, while the decision to boil one cup of water in the microwave isn’t much worse than using the stove top, using the microwave to heat up 4 cups of water is a bad choice (from the energy and time perspective).

Question 3: Is it worth changing my behavior?
Answer: In the grand scheme of home energy usage, it turns out the savings you could gain by changing your habits on this one aren’t huge. In fact, even if you boiled a quart of water every day, the cost difference between the best performer (the electric kettle) and the worst performer (the microwave) would only be about $4 per year. But, what if we extend this lesson into other realms? We might consider reheating our soup in the kettle rather than the microwave or adding an electric kettle to the office kitchen for all of those cup-o-noodles (after all, the kettle is faster too). Furthermore, this is one of those rare energy decisions that can save time, money, and energy. Electric kettles are cheap, fast and efficient. Microwave ovens are expensive, slow and inefficient (for heating water).
The relative strength of the stove can also be seen as great news. If you size your pot appropriately the stove can almost rival the kettle… not bad.

*- To heat 8 cups in the microwave or electric kettle would take two batches. You can therefore approximate the time (and energy) by doubling the time it takes for 4 cups. The savings are small enough though that you might as well just use the stove for volumes greater than your electric kettle capacity.
Posted by Luke
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68 comments:

  1. Running the same test on one cup of water in an old-fashioned tea kettle on an 8000 BTU/hr natural gas burner resulted in boiling (whistling) in about 2:30. That burned 333 BTUs or 0.0033 therms or 97.6 Wh (equivalent) of gas. Let's compare the amount of gas burned to the amount of gas burned at a power plant to produce the electricity necessary for the other approaches. Factoring in a 60% generation efficiency for large-scale gas turbine utility generators, that same 97.6 Wh (equivalent) of gas could have produced only about 57 Wh of usable electricity. We'll ignore transmission losses for now. That's not enough energy to boil water in the best-case electric appliance, the electric tea kettle. So, at least for one cup of water, a gas burner and old-fashioned whistling tea kettle wins hands down.
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    Replies
    1. The real advantage with the kettle is on the first cup. It boils in half the time as the first cup boiled in either the microwave or the stove. Since (at 1450 Watts) it uses about the same amount of power as the others, halving the time halves the energy. Thereafter, the energy requirement for boiling larger volumes of water is nicely predicted by a linear function. Each additional cup requires an additional 25 Watt-hours of energy and about a minute of additional time. The stove had similar performance except that the first cup required twice the energy (and time) of the kettle’s first cup.
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  2. Greg, great comment! I like the idea of comparing natural gas at the home to power plant equivalent. Here are my actual electric kettle numbers:
    1C - 34 Wh
    2C- 58 Wh
    3C- 81 Wh
    4C-103 Wh
    If we factor in your 60% efficiency figure, it still would only require 60 or so Wh of equivalent power plant natural gas and the electric kettle still wins.
    If, however, we look at national averages for generation and transmission efficiency, your numbers become much more competitive. I think we're about 30% efficient on average (see upstream watt multiplier post). That takes my 34 Wh to about 110 Wh of equivalent source fuel. Yikes.

    So, if you're in a coal-powered area and have a natural gas stove, take Greg's advice and use the stove top.
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  3. This also goes to show how much more efficient it is to make tea with friends:

    alone - 34 Wh/cup
    1 friend - 29 Wh/cup
    2 friends - 27 Wh/cup
    3 friends - 34 Wh/cup

    It looks like more data might be necessary since the 4 cup pot seems to be just as (in)efficient as the 1 cup, but in general you're likely saving 10-25% just by making tea with your friends, roommates and/or family.

    Spending time with other people saves energy and money in more situations than just making tea: cooking, heating/AC, transportation, listening to music, and watching movies to name just a few.
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  4. Great point Zach. Check your 4C math. You + 3 others: 103/4 = 26 Wh... the most efficient per cup value.
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  5. doh. That makes way more sense.
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  6. Thanks for the info. My wife and I are constant tea drinkers (kicked the soda habit and my wife won't drink plain water). We have always microwaved our water, but I figured that it had to be pretty inefficient... After reading this, I will be headed to the store to get a electric kettle!
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  7. Excellent post, my girlfriend and I have been going back and forth forever about the merits of having a microwave and whether it's really worth it. Until now, I thought the one area where it really excelled was in boiling water. Looks like I will be experiencing a case of foot-in-mouth-disease in the near future.
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  8. What did you use for kettle on your stove top? A pan with or without lid, or an old-fashioned teakettle? Thanks!
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  9. Kaat,
    Good question. I used a medium pot with a lid. The pot was sized such that the stove element was slightly smaller than the base of the pot (i.e. the pot completely covered the element). A tea kettle would probably have performed even better.
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  10. I always heated my cup of water in microwave because it was the most convenient and quickest (2.5min). I knew it wasn't the most efficient way, but since the difference is very small to really care about. I really enjoy reading your articles. I started a similar blog of my own at smartenergypractice.com
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  11. Hi,

    Great test! I can add that a drip brewer can be even more efficient than the electric kettle. I have tested it to check how to do our morning coffee http://urge4lessenergy.blogspot.com/2009/11/most-efficient-way-to-boil-water.html

    Svein*
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  12. Any idea on how this data would change with the new glass cooktops?
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  13. Good question.
    If you're referring to an induction stove top, the performance should be about 20% better (20% less energy) than that of a conventional electric stove. We don't have an induction stove handy for testing, but theoretically it would even out perform the electric kettle, especially with large volumes.
    If you're asking about a halogen stove though, the results are only slightly better than those of a standard electric stove (we've verified this with testing).
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  15. I clocked my gas range doing 3c water in 5 minutes on a 12.5k BTU burner. Doing the math puts that at 305 Wh. Incidentally, gas costs 72c/CCF here, so that means my water cost almost a penny to boil. Electricity is about 13c/kWh net, so that would have been 2.3c--over 230% more expensive.

    Maybe next time I'll put a lid on it.
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  16. Josh,
    I'm with you up to the 305 Wh figure. Then, I'm not following. Please correct where I've gone wrong:
    12,500 BTU/hr * 5min / 60 min = 1042 Btu
    1042 BTU * 1 cf / 1028 BTU = 1.01 cubic feet
    1.01 cf * 1 ccf / 100 cf = 0.01ccf
    0.01 ccf * 72c / ccf = 72 cents

    3C in the electric kettle required 81 Wh.
    81 Wh * 1kwh / 1000Wh * 13 cents / kwh = 1 cent
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  17. Consider that the lower gas stove efficiency will mean heating the house, that in winter can be considered fully used heat, not waste. Isn'it?
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  18. Anonymous,
    Sorry for the delay. This is an interesting way to frame heat loss from the stove. The answer in my opinion is "kind of." Here's my thinking:
    - Compared to a proper heater, the stove tends to be an inefficient way to heat a space. For instance, heat pumps can introduce MUCH more heat per unit energy than the stove top. An air source heat pump can provide 2.5 units of effective heat per unit of energy (depending on outside temp). Natural gas and propane provide more like .95 units of heat per unit of energy. What's more, stove tops don't tend to be very effective at distributing the heat. In fact, ventilation fans can serve to exhaust it directly.
    - If you count it as useful heat in the winter, you need to doubly count the waste in the summer b/c its fighting with the AC. Right?
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  19. WOW, what a cool thread. I want to weigh in.

    Luke - It's worth considering that at $15/mcf natural gas costs $.05/kwh, and burns much more nicely than coal and other fossil fuels used for electricity generation. If you heat with it, it's actually more efficient (in terms of net heat/unit of energy) than your air-source heat pump because of the 30% conversion efficiency for industrial power plants.

    At my house, we can't get natural gas, but have a set of propane burners next to our grill on the back porch. In the summer, we cook (almost exclusively) with these. Consider that (because food and drink re-radiate the heat our bodies don't) fully 100% of the heat used in cooking will end up in your house. If you need to cool it down again, (assume COP of 3) you add another 30% to the energy cost of heating up your water inside.

    And the pleasure of cooking on an instantly variable, super-powerful heat source is inestimable for someone like me with an enthusiasm for cooking; hence the maxim 'Now you're cooking with gas!'
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  20. Griff,
    Great comment! Good point about more fully accounting for the heat pump energy. We'll dive into home heating and cooling in a future post.

    To be clear, I'm all for cooking with gas. As Greg's test revealed, it's efficiency can rival even the electric kettle... especially when we look at coal power plant losses.
    This post was intended more than anything to highlight microwave inefficiency for boiling water.
    Thanks for weighing in!
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  21. In case anyone is checking the math on the Josh Mac comment. It looks like I missed a decimal point.

    The bottom line is that for Josh's case we should be comparing 0.72 cents with his gas range to 1.01 cent for an electric kettle boiling 3 c of water. Gas wins (but not by a huge margin).
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  22. Burning gas on a stove may use less fossil fuel than burning it to generate electricity but why burn it at all?
    Buy your electricity from a company that only buys from renewable generators like wind turbines and photo voltaics. And uses a higher percentage of your bill to invest in new renewable generation than any other company. (In U.K. Try Ecotrcity or Good Energy).
    If you use a stove you may waste some electricity compared to an electric kettle but the world has no shortage of wind or sun and you save the energy used in manufacturing and then recycling the kettle as they tend to break down every couple of years, unlike a stove
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  23. It is a lovely and interesting experiment that you carried and the results are mind blowing.I always thought micro oven was the best of all.
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  24. Good question. I used a medium pot with a lid. The pot was sized such that the stove element was slightly smaller than the base of the pot
    benz
    Home Design
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  25. I have a question: Do micowaved liquids lose heat any faster than those heated on the stovetop?
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  26. Anonymous, the short answer is no. Thermodynamically, the mechanism for heating makes no difference on how fast the liquid loses heat. BUT, the container plays a big role in the rate of heat loss. For instance, if you pour the hot liquid (regardless of heat source) into a room-temperature container, the liquid immediately starts to lose heat as it warms the new container. I'll explore this more in a future post.
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  27. Very nice touch from you, I admire your dedication...
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  28. Re: loss of heat from microwaved liquids versus stovetop boiled. Thank you Luke. However, it seems to me that if you don't stir a microwaved liquid, there might be cooler spots that take the heat from hotspots, thus accelerating cooling.
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  32. Hi Energy Savers,

    I'd like to add my thoughts and observations that gives the advantage to using a microwave over a kettle or pot on stove, especially for heating small amounts of water and also larger soup or stew dishes.

    1. Unlike your findings, my microwave ("Panasonic 1200 watt High Power" "inverter") boils 1 cup of water (starting temp 55 degrees F. ~ 1000 ft. above sea level) between 1 minute and 30 seconds and 1 minute and 50 seconds. My method of heating is to place a thin, flat saucer on top of my mug and to turn the microwave off soon after I hear a rattle created caused by the steam escaping from the vigorously boiling water. (I assume the variation in boiling time is due to voltage variations depending on electrical demand at different times in the day.)

    2 Our microwave, heats my mug as well as the water. When I use boiling water from a kettle for my hot beverage, as soon as the water is poured, the mug saps a noticeable part of the heated water and my "hot beverage" is no longer hot enough for my liking. If boiling water is poured into my mug to preheat it there is a waste factor to consider.

    3. When my wife boils water for her cup or small pot of tea using our electric kettle, inevitably a good portion of hot water remains in the kettle i.e. She doesn't measure how much water she puts in the kettle accurately.

    Being a conservationist and an interested observer, I've noticed that many hosts do an even worse estimation of water than my wife when using their kettle to heat water - often boiling nearly a full kettle to make a pot of tea which might require 1/3 of a kettle's capacity. On the other hand, using a microwave the actual drinking or serving vessel is being filled to exactly the amount of hot water needed. For these folks using their microwave would be more energy efficient when heating a 1cup beverage or a small pot of tea.

    4. For heating liquid intensive dishes such as soups, a microwave has the added advantage of “contaminating” only the serving dish. As a result there is no waste of time and additional hot water needed to clean a pot.

    For heating 1 cup of water or up to 3 cups of soup I use our microwave. For larger amounts, I agree the electric kettle or stove is worth using, both for speed and efficiency - ESPECIALLY IF ONE TAKES THE TROUBLE TO MEASURE THE AMOUNT OF WATER BEING BOILED.
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  33. The original analysis is far too naive. As earlier commenters point out, the only meaningful comparison is not raw BTU consumption at the point of use but the net consequences, energetic and otherwise, of providing the desired energy service by one means vs. another. Electricity is well-transferred at the point of use (by stovetop coil or microwave) but, in a typical thermal power plant -- where almost all our electricity still comes from -- it is very inefficient to generate. And don't forget those 7% grid transmission losses.

    Boiling water on a gas-burning stove is more efficient than burning the same natural gas in a distant power plant and shipping the power to an electric stove or even a microwave oven. During the heating months, burning gas is 100% efficient in a very real sense because all the "waste" heat from combusting the gas for one purpose contributes automatically to a second purpose, i.e., heating the house: whereas all the waste heat at the power plant goes up the chimney all year round.

    Also, focusing on "energy" leaves out every other aspect of energy generation and use -- and what justification is there for doing that? In practice, about 50% of US electricity is made by burning coal, which is even more destructive to obtain than gas and infinitely more polluting (toxics plus greenhouse gases) to burn. (We can mix the combustion products of a gas stove, whether propane or methane, directly with indoor air, and breathe them -- CO2 and water vapor.) So as regards environmental cost, sucking electricity vs. burning stovetop gas to boil your water is a flat-out, across-the-board loss.

    And of course the cleanest kWh or BTU is always going to be the one you don't use at all. Right on, about not heating more than you need, no matter how you heat it.

    Take-home message: there is nothing "clean" about electricity per se. Think not of white, humming appliances powered by wind turbines but of coal toxins going up chimneys, strip mines, vast landscapes drowned by hydroelectric dams, and ever-accumulating nuclear waste. All these lovely visions should be kept in mind, too, when discussing all-electric cars. A gasoline-powered hybrid pollutes far less than an all-electric car simply plugged into our existing, real-life, 2010 grid.
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  34. Take-home message: there is nothing "clean" about electricity per se. Think not of white, humming appliances powered by wind turbines but of coal toxins going up chimneys, strip mines, vast landscapes drowned by hydroelectric dams, and ever-accumulating nuclear waste. All these lovely visions should be kept in mind, too, when discussing all-electric cars. A gasoline-powered hybrid pollutes far less than an all-electric car simply plugged into our existing, real-life, 2010 grid.
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  35. Yes you are right Larry, gas stove is more efficient than electric one, but again you need to take gas from the ground. It would be interesting to test efficiency of the induction stoves, they should perform better than any other.
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  36. Anonymous, great points about getting the quantity right, extra dishes, and cup warming. I'll explore some of this in a future post.

    Larry, I agree completely. In the case of this test, we were exploring the best way in an all-electric house to boil water. We dove into the specs on gas in the comments and came to the same conclusion that gas wins. We've discussed the inefficiencies of electricity generation and transmission in a previous post. This appears to be a case where the monetary cost alone makes gas a winner even before we get into the environmental cost (which as you point out are particularly bad for coal powered areas).

    allo allo, induction top testing is on the to-do list. We touched on it in a comment in which we claim that (in an all-electric house) an induction top ought to be more efficient than a conventional stove by about 20%, making it the most efficient electric option.
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  37. Hello Luke, It looks like you've done a little bit of work from a home perspective on boiling water. Can you try to help me out?

    I'm building a system to boil maple sap (sap is about 2brix or 2% sugar by weight) I'm planneng on using electric elements and I"ve determined that 200kw of heating will evaporate about 350gal/hr. I've managed to locate specialy made eletric elements that can be submerged in water(or solution). Can you advise me on what types of phenomena can effect the heating efficiancy? will vapor bubbles on the heating elements detract a lot from optimal heat transfer to the maple sap? The heating elements are capable of supplying up to 90watts/in^2 of heating. Would I see much of a difference if I use a few 32kw heaters spaced farther apart than many 5.4kw heaters spaced close together?

    Your resonse will be appreciated. Sorry for any bad spelling. ~f
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  38. Fran, this is a fun set of questions. Unfortunately, they have exceeded my depth of intuition and experimental capabilities. What's more, lessons learned on our tiny kitchen volumes of water probably don't translate well to the much larger volumes you're heating.
    Sorry to not be more helpful. Good luck and please do let us know if you find answers.
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  39. "A gasoline-powered hybrid pollutes far less than an all-electric car simply plugged into our existing, real-life, 2010 grid."
    I am not sure how much pollution and toxic will be created when thousands of batteries from the hybrid cars will be discarded
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  40. I GOT A KETEL
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  41. I recently witnessed an electric kettle and was duly impressed with speed of heating. I would like to see more discussed about heat transfer to the surroundings. Using a stove - gas or electric - heats up the surrounding area considerably and makes my AC work harder. (I live in an extremely hot climate.) I assume both the kettle and the microwave heat the surrounding area also. Any information on this? Maybe there's an outside source if you don't have the experimental capabilities?

    I would think the best options would be a solar kettle that you keep in a sunny spot all day or an outdoor gas stove. In fact, in a hot climate an outdoor kitchen or separate "summer kitchen" makes a lot of sense.

    In a cold environment, it seems the indoor gas stove would be preferable.

    As a result of all this discussion I am thinking of purchasing a portable gas stove. It does get cold here in the winter.
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  42. http://www.grist.org/article/boiling
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  43. Great calculations Luke. However, I have always been a fan of the microwave (well actually I was born in the 80s so I am not sure what a kettle is!) and I feel you may be biased against it in your experiments-no no i am by no means saying that you rigged them. My major doubt is about the covering/non-covering of the cups and your use of the boiling point of water. I understand you did the experiments somewhat in parallel to practice of boiling water in kitchen. True your question is "the best (energy, time, cost) way to boil water" and your experiments if the data holds up answers this question well. However many have been interpreting these results in terms of the "most EFFICIENT (energy especially) way to heat/boil water with common kitchen appliances." To answer the latter question fairly you will need to 1)cover ALL cups including the one used in the microwave (it takes longer/more heat to boil if the vapor escapes-see definition of boiling). 2) container used in microwave should be of appropriate shape/size to minimize uneven heating 3) don't use time to boil-use TEMPERATURE CHANGE FOR A FIXED (APPROX) HEATING TIME FOR EACH-then do the math for efficiency etc or better time to arrive at a fixed temperature BELOW boiling (avoid bp since its a phase transition point and degree of vaporization given a fixed specific heat of vaporization may vary for the heating modes). This is appropriate since there is NO NEED too boil the water when using the microwave (who drinks 100 degree tea?-sure its not 99 degrees after thermal losses?) I HEAT MY CUP OF LIQUID TO THE ESTIMATED TEMP AT WHICH I WILL CONSUME IT (NOT LOSSES WAITING FOR IT TO COOL)-THAT THE POWER OF THE MICROWAVE! If I have the time I will try to reproduce your data- i still have confidence in the microwave!
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  44. I have one problem though Luke-do I don't own a kettle of any kind. Would someone really boil soup in a kettle? ... then have to clean it afterwards? (see post above on 4 conveniences of the microwave)- In other words as someone born in the 80s I am not sure what else a kettle is good for-but the microwave is so useful-that's an aside...
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  45. Always used a kettle..from 1958 to present..Some were so slow to boil you could walk to macdonalds and back with a hot coffee before it boiled..This pre microwave.. Todays kettles set on a small pedestal and boil water so fast that before you get all the ingredience into cup...its ready for pouring
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  46. Always used a kettle..from 1958 to present..Some were so slow to boil you could walk to macdonalds and
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  47. I have one problem though Luke-do I don't own a kettle of any kind. Would someone really boil soup in a kettle? ... then have to clean it afterwards? (see post above on 4 conveniences of the microwave)- In other
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  48. I assume both the kettle and the microwave heat the surrounding area also.
    -Regards
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  49. @ Tom:

    The microwave only heats the surrounding only very little, though. After all it is still an electrical device that uses some energy.

    However, the rays only have an effect on the meal inside. And as you know only certain materials / consistencies are effected.
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  50. Another factor is the energy consumed during the manufacturing and shipping process.

    I suspect if you already own a stove and microwave, the "return on investment" in energy savings of using a tea kettle would take quite a long time to payback the energy that went into manufacturing and shipping the tea kettle.
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  51. Mark,
    This is a great point that we have occasionally touched on but failed to dive into deeply. Accurate life cycle analysis proves to be a real challenge for most of our manufactured products. I do think it is worth noting though that very few manufacturers are going to sell these goods at a loss. Yes, the factory probably has cheap energy and this might be doubly true overseas, but it's unlikely that these products are being sold for a cost that is less than the cost of manufacturing and shipping.
    Energy is one of the costs of manufacturing, but certainly not the whole cost. So, let's say you purchase an electric kettle for $30. It seems reasonable that there are less than $30 of materials, labors and energy put into that kettle. What if it's 1/3 energy (pure speculation)? $10 of energy input = 125 kwh (approx). Let's say you boil 4 C per day. If my math is right, the kettle pays for the embodied energy in 3 or 4 years. It's not a home run, but it's not the whole picture either.
    Do you shorten the life of your microwave by running it this much? I'm sure the embodied energy of the microwave is enormous compared to the kettle. More microwave usage might mean more microwave failure and increased frequency of microwave replacement. Is this a stretch?
    I do like how the stove fairs on this front though. It's a sunk cost for most of us, it's life is probably least affected by usage (of the three), and the performance is pretty darn close to that of the kettle.
    So, if you have a stove but don't own an electric kettle, feel good about using that stove. I'm less inclined to go that route on the microwave though.
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  52. Wow -- great thread! And it cleared up an initial assumption I had -- I figured that since there was all that heat in the stove top the microwave *must* be more efficient. I will be looking into an induction kettle. Did you ever do the immersion heater test?
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  53. Anne,
    Great question! No, we didn't do the immersion heater test. My suspicion is that since an immersion heater allows you to completely surround your heating element with water, the efficiency could be great.
    If you have one, it would be awesome if you could run a test or two and post the results. The most helpful info would be:
    - Wattage: If you don't have an energy monitor, the rating (Watts or Amps) might be written on the cord. If not, the brand and model should be sufficient.
    - Time it takes to reach a rolling boil for any one (or all) of the following quantities of water: 1C, 2C, 3C, or 4C.
    - Container type: Do you boil water in a lidded, insulated thermos? Or a cup? Something else?

    Thanks!
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  54. These are great ways to save money. Great blog i like it!
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  55. informative post!
    i have a few questions:
    -do you think superheating played any role in the microwave's relative lack of efficiency (as measured)?
    -if an object was placed in the container of water to facilitate bubble nucleation before being placed in the microwave, might it have boiled in less time than without a nucleation aid?
    -if the measure of performance was actually time to reach 100 degrees celcius instead of time to reach a vigorous boil, do you think the results would have changed? i understand it's difficult to measure temp in a microwave without a thermal imaging device, but i suppose it's possible to measure several cups after different intervals of time and then try to interpolate this data.
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  56. Hi Luke,
    Just wondering if you had a lid on the pot you heated on the stove.
    Thanks
    Leona
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  57. Tea kettles offer their users the convenience of saving them time. A large majority of the electric kettles today have an automatic shutoff on them so that once the water reaches boiling point it will turn itself off automatically.
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  58. Very interested to discover this discussion!

    I always use an electric kettle to boil water for tea. Around here, in green circles, it is like a mantra to "only boil water for what you will immediately use" - in practice this means pouring your water into your mug, then pouring it into the kettle to heat.

    However, I've had a longstanding argument with my brother about this, and perhaps you can shed some light on it. In the morning, i would drink about four cups of tea, probably in the space of about an hour. I heat all the water at first, reheating it as and when i want another cup. When you reheat the water when it's been previously boiled, it boils very quickly, though of course the initial boiling takes longer (though not four times as long). I think it would take more energy to boil four separate cups of water from cold. Wouldn't it?

    Of course there would be an optimum - if you left it too long you would lose the initial boiling energy. What do you think?

    -Frances
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  59. Always used a kettle..from 1958 to present..Some were so slow to boil you could walk to macdonalds and back with a hot coffee before it boiled..This pre microwave.. Todays kettles set on a small pedestal and boil water so fast that before you get all the ingredience into cup...its ready for pouring
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  60. you didn't plot energy use via an induction cooktop
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  61. energyhawaii, thanks for the comment. we've received lots of induction top questions (see comments above). it ought to be the best electric performer by ~20% but we haven't yet run the test.
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  62. Before I start I’d like to explain a few things I’ve learned during my 30+ years of testing ideas using “science”: 1) The first rule of experimentation is that one needs to be able to replicate the results. 2) As many variables as possible need to be controlled to insure that you are actually measuring what you are trying to measure. 3) If your results are “surprising” you should check them. You may have measured something wrong or otherwise flubbed up.

    With that said I tried to replicate PlotWatt’s experimental results. For the stovetop boiling part my results coincide very well. For the microwave heated I could not replicate the PlotWatt results. That the Electric Kettle is the best way to boil water is a no brainer. The heating coil is as near to direct contact with the water as you can get without putting wires directly into the water and the Kettle container itself is usually made of a fairly good insulator (plastic) so little heat is lost to the surroundings via radiation off the surface.

    Reviewing PlotWatts results it appears that the 13 tests run include one rune each for 1, 2,3 and 4 cups of water in each of three “heaters” and one run for 8 cups done on the stovetop. That is none of the tests were duplicated. Ideally, in order to eliminate the effect container type might have on the experiment all of the test containers should be of the same material. Obviously for the microwave metal cannot be used, so unless we use a Pyrex glass or ceramic pot on the stove there will be a container effect in a simple boiling test as described. With the Electric Kettle, you get what you bought and most of those on the market today are a combination of plastic and metal so this is almost impossible to replicate in either of the other scenarios tested.

    The fundamental question posed here is how do we most efficiently transfer energy from a heat source to water so that we can raise its temperature from that of “tap” water to somewhere around 100 degrees Celsius the boiling point of water. Unless you are at sea level on a nice day the boiling point will likely be lower than 100 degrees.

    The variables potentially present in the form of container types and ability to observe the point at which boiling occurred in the tests for the microwave and stove raised a number of questions for me. Those are the options I tested.

    Experimental conditions:

    Stove: 1500 watt burner with a 1 quart aluminum sauce pan weighing 520 grams. Power use measured by multiplying boiling time by burner rating.

    Microwave: 1200 watt 30 year old Panasonic counter top model with a 4 cup Pyrex measuring cup weighing 780 grams. Power use was measured by a Kill-a-Watt meter.

    All water quantities were determined using a digital scale and were measured to +/- 0.1 oz for all determinations - it is assumed that 8 oz. is one cup. Temperatures were measured using a class A laboratory thermometer. (I did this sort of thing professionally for many years so I have some high end lab equipment.)

    All tests were performed on tap water with both the containers water at 23 degrees C to start. (The most time consuming part of the testing was waiting for all the pans, stove and micowave to return to ambient conditions before starting each test to insure that residual heat did not lower the energy need of subsequent runs.

    I ran all tests at least twice and averaged the results for the tabulation below.

    Energy used to heat to slow boil – Watt-hours
    Stove Top Microwave
    1 cup 66 53
    2 cups 98 101
    3 cups 132 142
    4 cups 167 192

    I will follow up this comment with the results of further analysis of the data that takes into account the effect of heating the containers.
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  63. As is immediately apparent I was not able to replicate the results in the PlotWatt blog. In fact for the first two cups of water the microwave is at least as efficient as the stovetop. After that there appears to be a small increase in power consumed per cup but given the variability between the two runs for each volume it is probably not statistically significant. (I did not do significance testing but given that the test were only reproducible to +/- about 5 watt-hours experience tells me that doing the formal test on this small sample size would show no significant difference.)

    While handling the containers after running the first set of tests I noticed that while the aluminum pan was hot all over only that part of the Pyrex cup in contact with the boiling water was hot. Apparently the conductivity of the aluminum resulted in the entire pan being heated to the boiling point while only part of the glass was heated in the microwave. So I decided to see what the potential effect of the “pan” material might have. First, I found that the specific heat (the amount of heat needed to rais the temperature of a material) of Pyrex and aluminum are both about 0.2 cal/gram – degree C. Using a specific heat of 0.2 cal/gram – degree C for both of the containers and a temperature change of 75 degrees (at my altitude water boiled at 98 degrees C) it takes 10 watt-hours to heat the aluminum pan to the boiling point and 14 watt-hours to heat the pyrex cup. The table below adjusts the raw results above for the energy lost in heating the pans. I used the full 10 watt-hours for all stovetop results and scaled the Pyrex loss from 7 to 14 depending on the percentage of the cup below the waterline for each test.

    While my tests show a small increase in the energy needed to boil the larger amounts of water in the microwave, I do not believe these are statistically significant given the sources of error I know are present in all of the measurements. But more importantly the differences are nowhere near those reported by PlotWatt.

    Energy used to heat to slow boil – Watt-hours
    Stove Top Microwave
    1 cup 56 47
    2 cups 88 90
    3 cups 122 130
    4 cups 157 178

    The apparent effect of container type stimulated me to do additional investigation using stainless steel pans. This is of interest because stainless has a specific heat of 0.12 cal/gram – degree C. Theoretically at least a stainless steel pan should heat water much more efficiently than an aluminum pan because less heat is lost heating the pan. I ran three tests with stainless pans. All were done with 2 cups of water. One was run in a pan with a bottom that only covered about half of the burner. The other two were performed in a pan that covered the entire burner as my aluminum pan did. The results were interesting. The small diameter pan while theoretically needing only 1 watt-hour of energy to heat the pan, took 180 watt hours to get two cups of water to boil. This is nearly twice the energy needed to heat the same amount in the aluminum pan. The two runs with the larger bottom pan were somewhat better; they averaged 104 watt-hours for the two cups after subtraction out 3 watt-hours for pan heating. I attribute the higher energy need to inefficient energy transfer caused by the warped bottom on my ss pan that prevented tight contact between the burner element and the pan bottom thus reducing the efficiency of energy transfer.

    Conclusions to follow.
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  64. These are the concluding remarks for the results I posted in the two previous anomymous posts.

    To put all of this in perspective the theoretical amount of energy needed to boil one cup of water under the conditions of my tests, that is raise the temperature of one cup of water 75 degrees C, is just a shade under 20 watt-hours. Pushing water through the phase change from water at the boiling point to steam at the boiling point demands a huge energy input. It takes about 18 watt-hours to create only one ounce of steam. It does not take long with the water at a rolling boil to dramatically increase the power consumption. So if we assume that in addition to the 20 watt-hours of energy needed to heat the water we used an additional 10% to get the visible boil going at best we are getting only about 50% efficiency in energy transfer using either the stovetop or microwave; when boiling water for one cup of tea it is more like 40%.

    So what does all of this mean? First, it means that a microwave is just as efficient at transferring energy to water as a stovetop. Go ahead and heat your water in the microwave guilt free, you really aren’t wasting energy. Second, the efficiency of a stovetop system is highly dependant on the amount of the burner covered by the pan and how “tightly” the pan sets on the burner. Unless you have a flat bottomed pan that covers the entire burner you are better off using the microwave. An old fashioned stove top tea kettle is probably the best stove top option. They are usually made of stainless steel, have nice flat bottoms and will cover most burners entirely.

    A few side comments about microwave ovens. I noticed that during the longer runs needed for the multiple cup heatings the average output of my old Panasonic decreased. That is when I ran the oven for 3 – 4 minutes to heat one cup my Kill-a-Watt meter showed an average output of about 1180 watts. For the longer 9 minute times to do the 4 cup runs that average dropped to about 1130 watts. I don’t know it this is a factor in all klystron tubes (the source of microwaves in your oven) or just old ones, but it surely is a factor in the efficiency rating – especially if you calculate the energy used assuming full power.

    I have a number of other ideas about why my results differ from those of PlotWatt but I’ve used up enough space already so I will save those for later discussions if needed.
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  65. Nice to read this indeed. I didnt experiment directly i just calculated on papers as i found some info.

    According to chemical guys A litre of water takes around 2400 Kilo Joules to boil. This is a more or less linear function. Depends on startig temperature of course which is in this case somewhere between 20 and 25 degrees.

    i found 2300kj for heating 1L water from 20-100C and 2589kj for 25-100C

    1Wh = 3.6kj
    2400 Kj = 666Wh is that realisic? That would mean i nead to run a 1000W Boiler for 40minutes? Where is the mistake?

    Anyways my plan was to build an Solar run Tea cooker. The idea is to generate Hydrogene through electrolysis directly from the panel. This process (if electrolyte and electrodes are cool) can be (not in my prototype i supppose) 60% efficient or even more. Efficiency is hardly of my concern though. Its nice that this does not matter at all as long as you are not limited by space restrictions. So i dont care about my panel efficiency and the efficiency of conversion. But to make it clear, My panel has maybe 14% if im lucky (they will be the crappiest ones i can find) and if conversion ends up at 60% thats probably a damn good result taken my unprofessional construction out of beer cans and stuff from the kitchen. So yea... i only get maybe 7% total efficiency until producing the gas, but what the heck, i gut free sun on my roof. BTW im living in Bangladesh, so gas and power shortage are a day to day problem. As is poverty, thats why im looking into the cheapest possible supply, not the most efficient one. I know that this will not be really cheap here as a water cooker though... on the short run at least.

    Alright, now here is the calculation:
    i install a 100Wp panel. This will then, in Bangladesh (4-4.5hp) produce around 400Wh of energy per day. As my process is 60% efficient, i will have 240Wh worth of energy in the form of Hydrogene, thats 864kj right?

    now according to those mentioned chemical guys that would mean that i will need 300Wp installed panels to produce enough gas to cook 1L of water per day.

    Now im confused. I am not sure what a cup is in the context of this Blog, but if its 200ml than 5 cups make a litre. If each cup consumes around 26-30Wh than this would be 130-150Wh per litre or 540kj? According to the chemical guys not possible... or im getting sth completely wrong.

    Final thing, no matter if i need 100 or 300W panels...
    100W would cost maybe 200Euros and run for 20 or more years (30 likely)
    So i would pay 200-600Euros for 7200-10.800L of hot water. Thats somewhere between 2 and 8cents per litre of boiled water. maybe even cheaper depending on panel prices.

    This might sound expensive, but usually people here would need 30cents worth of wood to boil 1 litre and suffer risk of eye and lung deseases because of smoke. Also 80% of the population use biomass for cooking which results in massive reduction of forests. So it would make sense financially for poor people here.

    All depends on how much energy i need now for boiling water... and to build a safe low cost hydrogene cooker ^^
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  66. The electric hob is easily the worst option, costing three times as much energy and CO2 as the kettle. Although the gas hob still results in almost three times as much energy (heat) being delivered to the home, this comes at very little extra CO2 cost. If that warmth is useful to you, then why not? On the other hand, because the hob takes longer you're more likely to forget about it and/or leave the water boiling for longer than necessary, which uses more energy and will also put more humidity into the house. Further considerations might include whether using a saucepan is going to result in extra washing-up, and whether you really want the extra humidity from the burner and drying the saucepan afterwards...
    טכנאי מזגנים
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  67. I am not a scientist, but I know that liquids that are more substantial (and/or contain fat content) heat up faster in the microwave than water does. That being said, I would think that the microwave would be much more efficient in heating other liquids, like soup, than an elective kettle would. And a question. I know there are products to "brown" food in the microwave. Is there any product or method to increase the efficiency of microwaves to heat up plain water?
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