Dishwashing: man vs. machine

September 24, 2008

When searching online for information comparing manual dishwashing to dishwashing machines, a University of Bonn study is the most prominent point of empirical research that shows up (e.g., 1, 2, 3, 4). This study is usually interpreted as showing that dishwashers are more resource efficient than hand washing – using less work time, less energy and less water to wash the same amount of dishes.

Some commenters in the Treehugger post linked above showed healthy skepticism of this all-too-convenient claim. Fortunately reports from the University of Bonn study are available online (1, 2, 3) and the researchers were kind enough to include some data in those reports, making it possible to examine the results rather than rely on media reports alone. I thus decided to have a methodical look at the study – this post presents my conclusions on this matter.


Multiple weaknesses in the experimental setup make the interpretation of the study difficult. The data analysis carried out by the researchers seems tendentious. Claims that the study shows that using a dishwashing machine saves substantial amounts of energy, water and time as compared to hand washing are highly dubious. According to the study’s own findings, the most efficient handwashers used far less energy (actually, none, since these washers used no hot water) and about the same amount of water as the most efficient machines. Using no hot water had no negative impact on the cleanliness of the washed dishes.


General comments

  • The study was “promoted” (which I take to mean “sponsored”) by four household appliance manufacturers: Electrolux, Arcelik, Bosch-Siemens Hausgeräte and Merloni. This, of course, is a factor with a potential of introducing a bias in favor of machine washing.
  • The three documents describing the experiment linked to above use different tones in describing the conclusions of the experiment. While the “project description” talks enthusiastically about saving 200 tubfuls of water per household each year and 64 minutes of work per day by switching to from handwashing to dishwashers, the two reports are more cautious in their wording. The “Home Energy” magazine article merely states (at the end of a list of recommendations for economizing) that “[i]f you can afford a dishwasher, use one – preferably a new one. A full, energy efficient dishwasher cleans best and has the lowest environmental impact of any method.” The third report (which appears to be a white paper) takes a middle approach by making only some of the quantitative savings claims made in the project description and then tucking those into the middle of the “results” section rather than giving them headline status.

Experimental setup The experiment involved getting 113 volunteers from 7 European countries to wash and dry 12 place settings of dishes each (140 pieces – china, glasses and cutlery). The washing was done in a laboratory which was set up for the purpose. The time, water and energy used by each volunteer were recorded as was the level of cleanliness achieved. These are then compared to the quantities measured for two dishwashers complying with the highest European standard for efficiency set at that time.

  • The reports do not describe how the volunteers were gathered and who they were. If the purpose of the study is predict savings that could be achieved by having hand-washers switch to machine washing, then the population of interest is that of people who are regular hand washers. It makes sense that these people would, through experience, gain knowledge and habits that would allow them to be more efficient hand-washers than the average person. Therefore, not screening the volunteers to make sure that they are regular hand washers introduces a bias into any calculation regarding potential savings due to switching to machine washing.
  • The volunteers were apparently required to dry the dishes. The reports do not break the work time into washing time and drying time. In practice many handwashers use air drying and so avoid spending any time drying. Adding the drying requirement therefore increases the total work time significantly. In addition, the requirement causes additional resource consumption for those washers who leave the water running while drying the dishes (see page 737 of the white paper).
  • There may be many factors in the dishwashing setup that impact the resource consumption. The validity of any conclusions of the study depend upon how representative is the dishwashing setup at the lab since it is the setup at the average handwasher’s home that is of interest rather than the one that was used at the lab. Two major factors are the flow rate of the faucet and the temperature of the hot water. The reports do not state what were the settings for these two important parameters and how those were decided upon. In fact, one could consider an even more crucial setup decision – that of putting in a hot water tap as part of the setup in the first place. One of the conclusions of the study is that energy consumption (i.e., hot water consumption) is very weakly correlated with the level of cleanliness. A washing setup which provides only cold water would therefore reduce the average energy consumption to zero, while not having a significant impact on the quality of the work.
  • One specific and significant way in which bias is introduced into the interpretation of the experimental results is by comparing the average handwasher to an optimal usage of a mechanical washer. The washers selected were selected for efficiency; they apparently were in brand-new, optimal condition; they were loaded with exactly the load they were designed for; and they were (it seems – the matter was not discussed) loaded by an expert. A more informative study would compare the typical handwashing to the typical machine-washing. It is hard to tell what are the effects of the various atypical conditions listed above. For one thing, it turns out that many dishwasher users usually pre-rinse the dishes before putting them into the washer. Any water and energy used in the pre-rinse need to be added to the dishwasher account. At the very least, it seems that experimenters should have had some volunteers use the dishwashers rather than run the dishwashing cycles themselves. Alternatively, it seems that a fair comparison would be that of an optimal usage of the mechanical washer to the best hand-washer rather than to the average one. Such a comparison would yield very different results than those mentioned in the reports.
  • It is not clear why the experimenters assume that the 140 pieces washed in the experiment “represent the number of dishes that get washed in a typical home”. The authors present no evidence to substantiate this assumption.

Results and analysis

  • As noted above, the reports compare the average resource use among the test subjects (27 gallons of water, 2.5 kwh) to those of the most efficient washing machines (4 gallons of water, 1-2 kwh). The most efficient hand-washers, however, used 4-5 gallons of water and essentially no energy (presumably, they simply used only cold water for washing). Even when looking at summary statistics for the entire population of washers, the averages present a distorted picture since they are skewed by a few extremely inefficient washers. Using medians to describe the typical washer would yield about 18-20 gallons of water consumed and about 1.5 kwh of energy.
  • The data analysis compares the energy spent by the dishwashing mahcines – which is consumed in the form of electricity – with the energy used to heat water in a water heater – which is often consumed by burning natural gas. The former method of energy consumption is quite inefficient, since it involves significant loss (about 3x) at the point of electricity generation. Burning natural gas for heat involves much less energy loss, and hence the 1-2kwh used by a dishwashing machine could actually be comparable to 2-4kwh spent on heating water using gas, making the typical hand-washer significantly more energy-efficient than the most efficient mechanical dish-washer.
  • It seems unlikely that in the average hand-washing home 80 minutes are spent daily on washing, which is the typical amount of time used in the experiment. Even if 140 pieces are washed daily, this would mean that more than 30 seconds are spent on each piece (including each piece of cutlery). As noted above this high finding may be explained by the requirement that dishes would be dried manually rather than simply left to dry.
  • It is important to realize that the magnitudes of the natural resources consumed by dishwashing – using any method – are very small. The average US household of three consumes about 150KWh every day. The total energy consumption of those 3 people is still 5-6 times higher. The per capita consumption of fresh water in the U.S. is over 1000 gallons a day (most of which is used to cool energy plants and for irrigation). The cost of those resources is small as well – about 10 cents per kwh and 2 cents per 10 gallons of water. It would therefore take years for the average household to recoup the expenditure on a mechanical dishwasher which is over $100. The unknown impact of the manufacturing of the dishwasher on the balance of natural resource consumption (energy and water as well as other resources) could also be substantial.

4 Responses to “Dishwashing: man vs. machine”

  1. So why do we still use dish-washers ? Says:

    So why do we still use dish-washers ? My guess is that the single most likely motive for preferring the machine is the high value we assign to the alternative use of our time.

  2. yoramgat Says:

    Hi Dad,

    I think that it is true that there is significant time saving associated with using a dishwasher (although it is probably nowhere near the 64 minutes per household per day claimed by the “project description” above). See some data here.

    However, saving time on dishwashing may not be motivated by an alternative desired use for time. I have seen the claim that despite the introduction of various “labor saving devices” into the household, the time devoted to household chores had not decreased – see for example The challenge of affluence by Avner Offer. Of course, could be interpreted as implying that given a fixed amount of time people would like to perform as much household maintenance as possible. This is possible but seems unlikely. It seems more reasonable to me that the use of dishwashers, like many other human activities, is the result of a very complex decision process that cannot be easily reduced to a single motivating factor.

    By the way, it is much more accurate to say that “some of us” use dishwashers than to say that “we” use them. The penetration of dishwashers into the household is far from universal.

  3. Nil Einne Says:

    Your criticism on the energy usage is a little flawed itself. You say ‘which is often consumed by burning natural gas’ and link to info for the US. Yet earlier you made it clear this was a European study so the prevalence of gas water heating in the US does not make the study flawed. It may mean the study is less applicable to the US, but that’s a different point which you didn’t make. Now gas water heating is common in Europe too (although I believe there’s a strong move to solar including in Germany), so the point itself may remain, but this doesn’t change the fact you can’t conclude a study is flawed just because of what is common in the US (if the study authors claimed their results were applicable to the US then you could say that conclusion is flawed) when the study itself was looking at something somewhere else. Here in NZ for example, water heating in most areas is by electricity.

    In any case, while the researchers probably should have done a better job (presuming your description is accurate, I didn’t bother to look in to the study), this is a fairly complicated calculation. For example, most of the energy used by the dishwasher likely comes from the hot water heating. In the past some dishwashers came with both hot water and cold water inputs in some countries (and depending on the dishwasher and your supply, it may also be possible to connect hot water to the inlet), but from what I’ve read this is no longer common. As I understand it, one of the key problems is that most modern dishwashers use so little water that you save very little, in fact may spend more by using hot water as unless you have very good insulation on your pipes, the water the machine actually uses is not much hotter then the cold water. I’ve seen it suggested that even if you use solar water heating or a heat pump, it’s rarely useful to connect the hot water system to the dishwasher in terms of saving energy. Note that this also highlights another point, if you are using a stored electric hot water heater (meaning resistive heating not a heat pump) you can’t compare the hot water the dishwasher uses to that you may use directly because the dishwasher should have very low loss as it’s heating the water the same time it uses it which isn’t the case for you.

    One point I do agree on is the energy consumption and to a less extent water consumption either way is not that big a deal compared to the household’s total.

  4. Yoram Gat Says:

    Hi Nil,

    Thanks for your comments. I was not aware of the issue of heat loss that you mention. You seem to be well informed. Do you have a professional background in this area?

    I agree that (like in many other real life matters) there are many factors to consider in this matter. Regardless, based on the points I raised above I feel that the study was tendentious (as can be expected, given its sponsors). As I wrote, a more useful study would compare average hand washers to average machine washers, in their own natural environments.

    Regarding the method of heating water: you are right – it is indeed a factor that would change based on geography, and could also change from household to household within the same neighborhood. Ignoring this issue by measuring energy alone is one of the points of weakness of the study.

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