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AskPablo: The Dishwasher

| Monday January 22nd, 2007 | 16 Comments

Emma.jpg People in tie-dye everywhere will tell you that technology is the devil, but most of today’s “Cultural Creatives” will tell you that using a dishwasher is far more efficient than washing your dishes by hand. So which is it? Well, this week we will find out once and for all. I am joined by my friend Emma, a junior scientist in the third grade, who will help me conduct the necessary experiments to get you an answer.


When practicing the good old scientific method it is important to document everything. This way you can go back to check for errors, or to explain any anomalies. Ideally I would get several dishwashers to test side-by-side but I just don’t have those kinds of resources, so the one I have has got to do… We are talking about a GE GSD3000W16. This model was probably made in the late 1980’s and features several energy-saving functions. Changing the settings to the less efficient options can result in an 80 minute cycle! For this experiment I will set it to “Short Wash,” “Low Soil Level,” and “Energy Saver Drying,” which will take about 28 minutes to run.
When we started our experiment at 5:29pm my Kill-A-Watt meter read 123.8V, 59.9Hz at the wall outlet under the cabinet, where the dishwasher is plugged in. In addition to hooking up my Kill-A-Watt meter I have also disconnected the drain pipes of the sink and the dishwasher and have diverted them into a bucket for precise measurement with a 1 liter measuring cup. The control experiment will involve me washing the same exact dishes that are going into the dishwasher by hand, under running hot water.
With Emma carefully watching the bucket under the sink I begin to wash the dishes. I have allowed the water to run for a while until it becomes warm. I am also counting the water wasted in doing this since it was also once heated by the water heater. From a previous AskPablo we know a little bit about the energy required to heat water in a water heater. I figured out that we need 156.7 kJ of heat energy per liter, which is equal to 148.5 BTU per liter. I looked up that natural gas creates 117.08 lbs of CO2 for every million BTU. By using a couple of metric conversion factors and fancy number-crunching I arrive at 7.8876 grams of CO2 per liter of hot water.
My hand washing experiment resulted in 20 liters of water used for dish washing and 4 liters wasted to get the hot water to the tap. So 24 liters times our emissions factor above results in 189.3 grams of CO2 for a hand-washed load of dishes. But how does that compare to the dishwasher?
To avoid clogging the dishwasher outlet most people do a quick pre-rinse of the dishes. This amounted to about 4 liters. We loaded up the dishwasher and readied the bucket and Kill-A-Watt meter. The dishwasher drew and expelled water four times so we were busy measuring the contents of the bucket. The meter reached a peak 10.8 amps, although the nameplate on the unit says that the maximum is 12.5A. The total amount of hot water used by the dishwasher was 25.2 liters! Over the whole 28-minute cycle the Kill-A-Watt measured 0.38 kWh, roughly $0.06 worth of electricity.
Killawatt.jpg
The Department of Energy tells us that California’s electricity is responsible for 0.51 metric tons of CO2 per MWh, or 510g/kWh. Our energy use resulted in 193.8 g of CO2 (0.38 kWh x 510 g/kWh) and our water use resulted in 198.77 g CO2 (25.2 l x 7.8876 g/l). So, the total amount of carbon dioxide emissions from using the dishwasher is 424.12 g (almost one pound!) while hand-washing of the dishes resulted in 189.3 g of CO2.
The verdict is in! Even if you are using the super energy-saver mode on the dishwasher you are still using more energy, and releasing more CO2 than by hand-washing your dishes. Keep in mind that newer models might be more efficient with their water and energy use but probably not much. According to a Tampa, FL government website “the average dishwasher uses 15 gallons of water per load!” Also keep in mind that I was hand-washing dishes under running water. To save even more resources I could fill the sink with water to do the dishes in and then do a separate, quick, rinse.
I would like to extend a special thanks for my great Jr. scientist assistant, Emma. I look forward to hearing your comments and ideas over the coming week. Thanks for reading AskPablo!
Pablo Päster, MBA
Sustainability Engineer
www.AskPablo.org
Pablo(dot)Paster(at)gmail(dot)com
View Pablo Paster's profile on LinkedIn


▼▼▼      16 Comments     ▼▼▼

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  • Nick Aster

    Awesome post Pablo! I reckon I’m stingy enough with my dishwashing that my hand wash is even more efficient. Maybe the “dishwasher is more efficient” people are really, really wasteful with water down the drain? I know some people who are really obsessive with overly cleaning dishes…

  • T. Hernandez

    Great research. I would have never thought this to be the case. I don’t think you could have achieved this finding without your assistant Emma, I’m sure.
    Thanks for the info.

  • Anonymous

    Wow! I’ve been hoping you would do this comparison for a long time. I suspected the results would be as you’ve stated, but your research gives me great ammo for convincing others.
    Question: Were you excruciatingly careful about how much water you used during hand washing? That is, did you turn off the tap between reaching for new dishes?

  • http://www.AskPablo.org Pablo

    Anonymous- I was pretty careful with how much water I used while hand-washing. I did turn the tap off between dishes and didn’t have the water running at 3 gallons per minute. But like I said, I could probably have saved more water by using a soapy tub and a rinsing tub.
    Thank you all for your great comments! Emma and I had a lot of fun with this experiment and we’re glad that you appreciate our hard work.

  • http://www.SustSolutions.com Janice Neitzel

    Interesting research, as I keep reading that using a dishwasher (assumption is 4-8 gallons of water) is more energy efficient. Hmm…our’s is only a few years old – I’ll have to check how much water is used.

  • Eve

    Pablo and Emma has proven me wrong. I was so sure that the dishwasher was more efficient. The only concern I have is how sanitized are those hand washed dishes?

  • Anonymous

    Wow I am Emma’s teacher in 3rd grade but it looks like she is teaching me things too!

  • ecogal

    Hi Pablo and Emma! Great topic for discussion and nice research! Interestingly, the study at http://www.mtprog.com/ApprovedBriefingNotes/BriefingNoteTemplate.aspx?intBriefingNoteID=286, released last year, came up with the opposite result. They concluded that water consumption (in typical UK dishwashers) use 3-4x less water than washing by hand. Of course, differences in dishwashers and dishwashing habits can make all the difference. Just though I’d share similar research with an opposing conclusion!

  • Anonymous

    I’ve been wondering about this issue for a long time. As you showed, I assumed that handwashing was more efficient. You don’t use electricity (other than the hot water) handwashing afterall…
    Thanks for doing the experiment.

  • Sam

    the super interesting thing about this issue is that the dishwasher is allegedly “easier” and hence, it exists.
    Never mind energy for a moment:
    So all the dishes are piled up next to the sink. I have to use water to rinse essentially all the food off of them so that the outlet will not be clogged. I want to use warm water so that my hands will be comfortable.
    Why not just put soap in that water, give the dishes the extra second to scrub them completely, and rinse them.
    the alternative is bending over forty times to load them into the dishwaser, and then bending over forty times more the next morning to put them all away.
    It seems intuitive to me that if the dishes must be rinsed by hand, they might as well be washed by hand, as 50% of the job is done at that point, and the loading and unloading calories can be burned on a hike or a bike ride.

  • fefrie

    One thing that you forget to factor in is the fact that MOST people aren’t really concerned about using as little water as possible, they just have the taps on full (for the pressure)while they rinse, and rinse, and rinse untill they think their dish is clean.
    I’d say that the average hand dish washer uses way too much soap and way too much water. I’ve got roommates, I’ve seen how they wash. And the way that people are living these days in our SUV mentality, I’d rather they use a dishwasher.

  • Brian

    I’ll stick with the scientific study thanks anyway. http://environment.about.com/od/greenlivingdesign/a/dishwashers.htm

  • Mark White

    Your study was not very scientific considering that you at the end ssaid that you were running water and not filling a sink. I think 20 litres is about right tho to fill a sink. Most dishwashers now use under 20 litres so you dont really have to be a rocket scientist

  • Cathie Tongue

    I would also question how scientific this study is as one not actually filling a sink which most people do. Even if based 10litres per sink then 2 washes a day = 20litres conservatively.
    Also most dishwashers today use between 12-20 litres to do the same job so it is quite an easy
    solution that dishwashers are in fact more economical.

  • http://greenwashed.blogspot.com Buff

    I loved your mini study and how it reflected something more realistic. The comments posted to your study are revealing. What should be clear is that people don’t use dishwashers because they are more efficient, they use them to save time. The study out of the University of Bonn, Germany, is what started the long list of sites claiming dishwashers are more efficient.
    The study has many flaws of which the most obvious is using 12 place settings comprising 140 pieces as a test basis. That’s four people have three complete formal meals in one day and not washing those plates until after dinner. Most people don’t have 12 complete place settings and would have to wash dishes just to have clean ones for dinner. But the study was conducted with all the dishes at once so no dried on egg yolk from breakfast and no sticky cheese from lunch. The human testers all seemed to have the worst washing techniques averaging almost 30 gallons of water. That would be like filling and draining my kitchen sink 6 times or using my bathtub to wash those dishes. The study also doesn’t claim which model of dishwasher was used unlike your study. This is important because the Bonn study claims the machine used only 4 gallons of water but I can’t find any such machine in the United States, let alone one for sale.
    For more information and links to the German study read my blog at: greenwashed.blogspot.com

  • Anonymous

    was your dishwasher energy star rated? you said it was at least 28 years old – i like to think that we have made some drastic energy saving measures since then. also – did you try it with turning off the heated dry cycle in the dishwasher? i use the dishwasher with that turned off – run it in the evening before bed – and the next morning the dishes are dry and clean… without the heated dry. i will – as well – stick to the ‘scientific’ research.

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