Running the numbers on dishwashing

Will measures water

For a long time, I’ve idly wondered how dishwashers compare to hand washing.  I don’t like washing dishes (the counters in our kitchen are low enough that my back starts to hurt after a while), but there are always some dishes that either just don’t fit or seem too dirty to clean automatically.

I guess I have numbers on the brain after thinking about travel (in specific and more generally), because the last time I hand washed, I decided to measure how much water I actually used.

First, I should explain how I wash dishes. Like most people, I fill our sink halfway with warm water and toss some soap in. Anything that’s really dirty sits in the water as I fill the sink for some extra soaking. I plug the other half of the sink as well to capture rinse water, which I keep cold. I still usually have to use the tap to get more rinse water for pots and pans, but small items like utensils or plastic containers I can just dunk, which is faster anyway.

My extremely scientific method of measurement was to use a measuring cup to put the rinse water into the rest of the water, then dump all of the water down the drain using the same measuring cup. Finally, I subtracted the amount of rinse water from the amount of total water. Okay, it’s not super accurate, but it’s accurate enough to give a reasonable estimate. In this particular situation, I was cleaning four or five very dirty pots/pans as well as some miscellaneous smaller items. Together, they probably would have made up a dishwasher load, but only because pots take up so much space.

For the entire process, I used 26 cups (1 1/4 gallons) of warm water for cleaning and 14 cups (3/4 gallon) of cold water for rinsing. For comparison, I use about a cup of water to wash my hands (turning off the tap while soaping, naturally). I can’t figure out a good way to figure out how much electricity was used to heat the warm water, since it’s not all hot and the water heater runs all the time. It seems pretty negligible, though.

Washing dishes in the dishwasher also uses a small amount of electricity, about 0.56 kWh (or over a day of running a CFL) as long as you air dry. Using the heat dry setting increases that a lot. Since we air dry, I’d call the electricity difference a wash.

A modern dishwasher uses about 6 gallons per load, while older machines can use as much as 8-10 gallons. Our dishwasher uses three times as much water as I did. That also doesn’t take into account pre-washing. We rinse anything with food on it before putting it in the dishwasher, which would make the dishwasher even worse. A larger family could fill and run a dishwasher in a day or two, which would make pre-washing less necessary. Since we only run the dishwasher once or maybe twice a week, the scraps begin to smell if we don’t pre-wash.

On the other hand, this analysis also doesn’t take into account that I wasn’t washing the items that I normally put in a dishwasher. It would probably take me at least twice as much water to clean all of the plates, silverware, and cups that fit into one load. Washing all of that myself would also take much longer and require a lot more kitchen space for drying.

I always thought that dishwashers had gotten pretty efficient and would give hand-washing a run for its money, but that doesn’t appear to be the case. Although dishwashing is probably about as good as hand washing for lots of dishes, it’s worse for larger items or if you use heat drying.

I’ll keep using the dishwasher for most things because it’s much more convenient and, as Maggie will attest, I hate washing dishes. However, I’ll make sure that the dishwasher is always full and I’ll continue to wash our larger, dirtier items by hand.

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8 Responses so far »

  1. 1

    Student Doctor Green said,

    April 16, 2008 @ 3:13 am

    Love the post! You are rocking with all this number crunching and home experimenting. Pots do take up too much space and I’ve often thought about hand washing them but I’d heard (as you did) that dishwashers were more efficient. I have an old model though so who knows what kind of efficiency or lack there of it has. I’m definitely going to dust off my drying rack and start washing larger items by hand.

  2. 2

    arduous said,

    April 16, 2008 @ 5:33 pm

    Yeah, this is a great point. Something really big like a big pot, probably doesn’t take much water to wash, and takes up a lot of space. OTOH, silverware is probably best off in the dishwasher. One of the keys though is this: if you handwash, handwash as SOON as you use the dish. It requires much less water than when the food has hardened. Also, now my head hurts from all the number crunching! 😉

  3. 3

    equa yona said,

    April 16, 2008 @ 5:54 pm

    Hmmm, Going Green wrote about dishwashers today, check her numbers if you haven’t already http://burbanmom.blogspot.com
    I do it a bit differently in that I start with an inch or two of soapy water and rinse over my dishpan. I save all my dishwater to flush the toilet and my hot water heater is set at tepid(or thereabouts). I usually toss the bottom cup or two of really scuzzy water although why i don’t just flush it all is beyond me. It can’t be too scuzzy for the crapper can it? Ok, no electricity used, about three gallons of water warmed w/ natural gas, no water wasted. Our counters are low as well and I am 6’3″.

    [Edited by Will to add a link to Urban Mom’s post about dishwashing]

  4. 4

    Andy said,

    April 16, 2008 @ 9:38 pm

    I think you have a good experiment here. I have often heard that it’s roughly he same impact whether by hand or by machine, but I disagree. I wash everything by hand and I would say that I average about half a gallon of water for the equivalent of a full load in a dishwasher. Half gallon?! I know it sounds like nothing but hear me out: I put about an ounce of water on the first thing I wash, as well as some soap. I then rub everything down using that water and my hands. The water is then poured onto the next item. So far I’ve only used an ounce. Then, if food is still sticking I use a sponge and scrape it off, with no additional water usage. I then rinse everything in running water which accounts for almost all the water usage. So half gallon and only electricity to heat and pump that half gallon is certainly more sustainable then running a load in a dishwasher.
    -Andy

  5. 5

    Will said,

    April 17, 2008 @ 3:01 pm

    SDG: Thanks! I think home experimenting is a really important part of going green. What works well for other people might not work well for you at all!

    arduous: We probably should wash sooner, but we have trouble keeping a good flow going. We always seem to have stuff drying in the drying rack when it’s time to wash dishes. When that happens, I put a little cold water into my pots and pans which I dump into the warm water when I do finally wash.

    equa yona: Urban Mom’s prewashing numbers are interesting, but we pre-rise just to keep food from smelling before we run the cycle. When I experimented with not pre-rinsing, we were still able to clean everything with the short cycle.

    Andy: Wow! That’s not much water at all!

  6. 6

    Dana said,

    April 17, 2008 @ 7:52 pm

    Via Plans, I found this link that references a study done by scientists at the University of Bonn that claims dishwashers are more efficient that the average hand dish washing person. It, in turn, links the actual PDF of the study.

    Of course, things to keep in mind include that this was 1) done in Europe, where perhaps appliances run on different efficiency standards, and 2) they’re talking about average people.

  7. 7

    Will said,

    April 18, 2008 @ 3:06 am

    Yeah, I found the Bonn study in my research and couldn’t get it to make sense, so I calculated it myself (and I’m glad I did, since my results are so different!). Their dishwasher number comes out to 4 gallons per load for normal use, which is about half what it is here, so that does make a difference. Even so, that would have been less than the most frugal of their human washers, so I’m not sure what’s going on there.

    It’s possible that the test kitchen was set up in a way that encouraged water use or that people were less careful when washing dishes they weren’t going to use. Another possibility is that the dishes used (place settings) were compact and thus favor dishwasher use. I suppose the average person might use 14 times as much water as I did, but that seems hard to believe too.

    I guess my next test should be to fill the dishwasher and then wash all of those dishes by hand and see how that comes out.

    Until then, I’ll continue to wash compact items in the dishwasher and pots/pans by hand.

  8. 8

    Green House » takes guilt out of your way said,

    April 27, 2008 @ 3:14 am

    […] I see that local green bloggers Will & Maggie have also had dishwashers on the brain, and their own experiment was less favorable to the […]

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