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AskPablo: Microwave Efficiency

| Monday January 8th, 2007 | 15 Comments

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This week I will be looking into the efficiency of my microwave oven. This is part of an ongoing series in AskPablo where I am trying to determine the most efficient means for heating water. I would like to thank some of my readers for submitting excellent questions this week and I look forward publishing my answers in the coming weeks.


In order to determine the efficiency with which my microwave heats water, I will conduct an experiment. I will also try to answer the question “Is it more efficient to heat small batches consecutively or just one big batch?” First the due process: I am analyzing a Magic Chef MCD790SW, 900W microwave oven. Using my Kill-A-Watt meter (available from Europort, sales@europort.com) I have determined that the house voltage is ranging from 123.3 to 124.8 VAC today at 59.9 Hz. The ambient temperature in the kitchen is 17.4°C (63.3°F) and is expected to remain constant. I have designed this experiment to test two variables, volume and time. I have set out nine glasses in a 3 x 3 grid and have filled the top row with 100mL of water, the middle row with 150mL of water, and the bottom row with 200mL of water. After allowing the glasses and the water to reach equilibrium with the room temperature I will microwave one cup from each row at 10 seconds, 20 seconds, and 30 seconds, measuring the water temperature before and immediately after.
During the experiment I observed that, while the microwave is rated at 900W, the Kill-A-Watt meter registers between 1260W and 1314W (average: 1287W), 43% above its rating. One hypothesis that I can think of is that the 900W refers to the specifications for the microwave emitter and not the entire unit and the remaining energy is lost in transforming the 120 VAC to the proper voltage for the emitter.
With my experiment complete I now have a table of data, including beginning and ending temperature for each glass. The 100mL cups increased in temperature by 10, 22, and 35°C after 10, 20, and 30 seconds, respectively. And after 30 seconds the 100mL, 150mL, and 200mL cups were heated by 35, 25, and 20°C, respectively. These results are interesting but require further analysis in order to answer my question. I have decided to convert these values, which show a relative temperature change, to a measure of absolute energy in the system. Using the specific heat capacity value for water, 1850 J/(kg-K), I can determine, for example, that a 100mL glass at 17°C has 53.65kJ of energy (1850J/(kg-K) x 0.100kg x 290K, K=°C+273). By calculating the embodied energy in all cups before and after the experiment I can determine how much energy entered each system.
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In order to compare the results I need to normalize them. This means that I need to look at the results as if all the cups had been heated for the same amount of time and had contained the same mass of water. I have decided to normalize to 200mL at 30 seconds. I do this by multiplying the 100mL results by 2.0 (200mL/100mL) and the 150mL results by 1.33 (200mL/150mL), then by multiplying the 10 second results by 3.0 (30s/10s), and the 20 second results by 1.5 (30s/20s). This gives me nine values, in Joules, all for 200mL at 30 seconds.
From these results I can conclude that it is more efficient to do multiple small batches than one big one. This result makes sense since it takes FOREVER to heat up a large plate of leftovers yet smaller plates take proportionally less time to get sizzling hot. It also appears that it is more efficient to run the microwave longer, which is probably due to the microwave emitter warming up. These results almost seem to contradict each other since smaller batches don’t take as long and may not allow the microwave to reach its prime efficiency. To reach a more definitive answer, including a point of maximum efficiency, would require a much larger experiment with many more cups than I have.
By dividing my normalized results by 30 seconds I get J/s, or Watts. This now allows me to compare the product’s specified rating with the actual amount of energy put into the water. 100mL at 30 seconds uses 432W, while my Kill-A-Watt meter measures 1287W. If these values are correct, the best efficiency recorded in this experiment is 33.6% (432W/1287W). Keep in mind that a power plant is also roughly 30% efficient, so in order to put 13kJ of energy into your cup of water in a microwave, roughly 130kJ of fuel energy is burned at a power plant (33.6% x 30% = ~10%, 13kJ / 0.10 – 130kJ).
So sit back and enjoy a nice warm cup of tea. Don’t worry too much about the energy required to make it. Instead think about that fact that most microwaves use more energy (over any given day) telling you the time than they actually do heating your food or water.
Pablo Päster, MBA
Sustainability Engineer
www.AskPablo.org
Pablo(dot)Paster(at)gmail(dot)com
View Pablo Paster's profile on LinkedIn


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  • http://moldybluecheesecurds.blogspot.com jff

    I’m really impressed by the depth of the analysis (and even performing the experiment several times), but what about the big question? Is this better or worse than using a gas stove? An electric range?
    I’ve got my teapot and a mug ready, but which is the more energy efficient route? :-)
    Thanks!

  • Anonymous

    Pablo –
    Great, great article. One of the best you’ve done. As stated above, it would be great to know the comparison to gas/electric. I’ve a feeling the mic wins out. . .especially over boiling water on a range where steam/water loss must be considered.

  • http://www.AskPablo.org Pablo

    Thanks for the positive comments. I am sorry about the suspense but I do state above that this is “part of an ongoing series.” So you are just going to have to wait a little bit longer. Thanks for reading AskPablo!

  • Anonymous

    Shouldn’t we be asking what the microwaves actually do to the food/liquid? Microwaves degrade and destroy the genetic make-up of whatever is being exposed, thus rendering the goods dead and void of any nutritional value. Microwaves are bad people! Rid yourself of these energy manipulators! Seek the truth for yourself and you’ll be surprised…

  • Boxer Faul

    Genetic makeup? Microwaves don’t do anything to genetic makeup, and even if they did, it wouldn’t matter because whatever in them is dead. Whether they degrade nutritional value is probably debateable, however I would agree that they generally lower taste and quality!

  • Ryan

    What good do the genetics of what you are eating do for you?! You are more interested in the molecular composition, which may have been affected by the genetics of the material while it was alive, but this is unharmed in microwave heating. Microwaves work by passing radio waves at approximately 2.5GHz through materials, which are absorbed by water, fats and sugar; and reflected by metals. This wave energy is translated into erratic molecular motion which in turn raises enthalpic energy (temperature) through friction. Essentially, you are merely heating foods more evenly than surface conduction heating, so long as the water, fat and sugar content is isotropic (evenly distributed) within the substance.

  • jeff

    Pablo I googled steam production with micro wave and found my way to your site. I live in Iowa, my house has a one pipe radiator steam heating system. I made a cheap steam producer for heating with a garage sale microwave and some pyrex beakers intigrated with my boiler. I run this thing for about 15 cents an hour…do a study on this, Gas fired boilers are on there way out for sure.
    Jeff

  • rema

    Pablo, do microwaves heat evenly? It has not worked for me. The upper surface heats up faster and evenly, but not the immediate lower layer or middle layer. I often have to stir it up and reheat so I get food thoroughly heated.

  • Steve in W MA

    I had ben tempted to ignore the microwaves-make-your-food-poisonous weenies, because about 99% of them couldn’t present an argument as to why and were extremely technologically and scientifically ignorant.

    However, we do know that cooking is a chemical process aided by energy in the form (usually) of heat.

    Charring meat and fats on a fire produces chemical byproducts that are known to be deleterious to health, as do most forms of cooking. However, most forms of cooking use energy only up to the infrared spectrum, and many forms of cooking use a high proportion of convective heat as well.

    Steaming at temperatures below 212 F has been shown to produce less toxic byproducts than cooking at higher temperatures, and especially grilling and frying, which has the highest rate of heat transfer of all conventional cooking methods due the the high molecular density of oil and its ability to transfer large amounts of heat quickly to food.

    High intensity microwaves as found in microwave ovens provide an energy “mix” that is almost 100 percent in the microwave energy range, which may be expected to make possible chemical transformations in food that are unique to microwabe cooking, *and in fact has been shown to do so*.

    That being said, I do use my microwave but use it primarily to heat liquids, most commonly water for tea or coffee. Liquids in general, may be the safest “foods” to cook with microwaves because of their extremely high heat transfer capabilities.

  • Dermot Cunningham

    I have always believed that the specific heat capacity of water is 4182 J/kg K. Kelvin is usually used for used for temperature change in these type of calculations. Therefore, for example, if we 0.1 kg heat water from 17C to 30C the embodied energy increase (as you call it) or enthalpy increases by 0.1 x 4182 x (30 – 17) = 5436.6 J (or 5.436 kJ).

  • Vincent

    Hi Pablo, this is great but I agree with Dermot: it would be better to consider 4182 J/kg K for the specific heat capacity of water.

    1850 J/kg K is the specific heat capacity of STEAM.

  • peter springer

    I did my own experiment comparing the power used to boil 16 ounces of water from 67.3F to boiling. 1500Watt electric kettle used 0.05kwh. 2 different microwaves used 0.11kwh to do the same job. electric kettle is much more efficient for boiling water!

    • haertig

      It's alawys good to remember that whatever waste heat is generated by boiling your tea water goes to heat your house. If you live in a hot climate, this isn't good. If you live in a cold climate, it reduces your heating bill.

      Gray

  • haertig

    It's alawys good to remember that whatever waste heat is generated by boiling your tea water goes to heat your house. If you live in a hot climate, this isn't good. If you live in a cold climate, it reduces your heating bill.

    Gray

  • Vasile

    Your experiment is interesting. However this is called calorimetry. Two basic principles of the calorimetry are:
    1. the supposition that entire microwave energy is transformed in heat
    2. the heated probe (water in your experiment) has a homogeneous temperature.
    Unfortunately a microwave oven does not heat homogeneously the probe [even ovens having both upper(the blade) and bottom (the rolling glass plate) microwave stirrers], so before measuring the temperature in every vessel you have to stir the water until it reach the same temperature, else your temperature measuring errors will be huge, because the temperature gradient into the 100ml or 200ml vessels can be as high as 5C or even more.