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I've always heard the claim that a dishwasher is more water efficient. I can attest that I once washed the dishes with a clogged pipe, and put a bucket below the sink, and was appalled at the amount of water used, and this was while using low flows and turning off the faucet when not in use.

Now:

  1. The dishwasher can operate at higher pressures and temperatures, making it more efficient
  2. On the other hand, the angle between the dishes and the water can be pretty low, making it less efficient. When washing manually you can control the angle.

So, assuming you're an environmentalist dish washer, what should you use?

SIMEL
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Gilad Naor
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    This claim is pretty widespread, but please provide a link to an exact claim you are sceptical about. This is the way questions are asked here... – sashkello Jun 03 '14 at 05:39
  • If you are inside a closed water cycle, [reducing the amount of water circulated might not necessarily lead to any benefit for the environment](http://skeptics.stackexchange.com/questions/20803/is-it-actually-beneficial-to-reduce-water-consumption). – Simon Richter Jun 03 '14 at 08:41
  • "might not" is not the same as "does not". Minimising many small inefficiencies at the expense of increasing a few larger ones still provides simpler optimisation opportunities, and regardless, the overall efficiency isn't necessarily hampered. – Gilad Naor Jun 03 '14 at 10:01
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    Note: to compare environmental friendliness fairly, whole cycle should be considered (including environmental impact of the dish washer machine production and disposal, and of the detergents used). Even when speaking about water consumption only, one should at least consider water needed for detergent, salt and rinse aid, otherwise apples and oranges are compared. (I think even with this in mind the dishwasher is likely to win, but neglecting this seems like a bad method). – Suma Jun 03 '14 at 11:37
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    This would depend on how you go about washing dishes by hand. If you're running water more-or-less continuously then I expect hand-washing would use more water. However, if you partially fill a sink and add soap, partially fill another sink with fresh water to use as a "rinse" sink, then wash the dishes in the soapy water, rinse in the "clean" water (refilling the rinse sink when needed), I would think that the manual method would not use much more water than the machine would. Plus, you have the advantage of continually monitoring the wash process. Share and enjoy. – Bob Jarvis - Слава Україні Jun 03 '14 at 14:22
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    Does anyone wash dishes with continuously running water?! I've never seen anyone do that... – gerrit Jun 03 '14 at 14:30
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    Does anyone not? I for one can't use the two sink method, having only one sink. – Ryan Reich Jun 03 '14 at 15:09
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    @gerrit apparently one reason to run the water continuously is to keep the water hot. in our house the path from the hot water heat to the sink is rather long (it takes flushing several gallons of water through the pipes just to get the hot to the tap) and once it is there, turn off the hot for only a few minutes and the water left in the pipes has cooled to only lukewarm. – Michael Jun 03 '14 at 15:23
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    Why do you need two sinks? I have one sink, fill it with some hot water (I might need to waste some water waiting for the water to become hot), put some soap in, wash my stuff, dry it with a cloth (thus removing the soap). The second sink is not needed. – gerrit Jun 03 '14 at 16:38
  • Certainly newer, high-efficiency dishwashers save considerable amounts of water. Ours employs such techniques as discharging small amounts of water at intervals to flush screens, so that the wash water remains relatively clean. – Daniel R Hicks Jun 03 '14 at 20:45
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    @gerrit I wouldn't be able to repeatedly use a cloth for soapy/grimy water removal. I'd end up with a full load of laundry for all the towels to be washed and waste more water. May as well just have running water for actively rinsing. – agweber Jun 04 '14 at 14:28
  • I'll ask the wife when she's done cooking the dinner. –  Jun 04 '14 at 17:58
  • I haven't seen a dishwasher that can actually get dishes clean without a manual pre-soak and rinse to remove the bulk of the food remains. If you stick in dishes with dried-on food remains, a lot of that will stay. A lot of what comes off gets redeposited on other dishes, and also clogs the drain grating of the dishwasher, preventing good water run off and drying. – Kaz Sep 27 '17 at 23:09
  • Do all these studies assume that dishwashers are optimally packed? I highly doubt that people pack dishwashers fully. This will make a big difference to the answer – bondonk May 26 '19 at 13:12

2 Answers2

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Yes, a dishwasher almost always uses less water than manual washing.

According to a study published by the University of Bonn[1], both the energy and water consumption of a dishwasher is better:

As regards the normal household practice of washing small amounts of dishes and heavily soiled articles, our comparison confirms the advantages of automatic over manual dishwashing when comparing the average behaviour in manual washing with a fully loaded average dishwasher. These advantages can be identified as lower consumption of water and energy and especially as better cleaning results and significantly lower amounts of manual working time needed.

As they say, the reason is mostly that smaller amounts are washed when handwashing. The average water consumption for 12 place settings is on average 83-121 litres (depending on whether all the plates are washed together or in 2 portions) when washed by hand and 20 litres when machine washed. They do however say that there's a huge spread on how much water is used when washed manually:

The water and energy measurements (Fig. 5 and 6) again show a very wide distribution of consumption values, ranging from four to 90 l and from 0.03 kWh up to 2.6 kWh for washing a pair of place settings.

As this is for only 2 place settings, even the most efficient manual washers are unlikely to achieve the same efficiency in cleaning as a dishwasher.

[1] http://www.landtechnik.uni-bonn.de/research/appliance-technology/publications/07-02-03-dishwashing

drat
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    Presumably, this is heavily dependent on whether you wash under a running tap or in a sink full of water. – David Richerby Jun 03 '14 at 08:10
  • Yes, I guess that is what makes the difference between the 4 litres and the 90 litres. Unfortunately they don't address this issue in the puplication. Seems however that even a sink full of water (assuming those are the low-end points) is more wasteful than using the dishwasher. – drat Jun 03 '14 at 08:12
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    For reference, I just measured my kitchen sink and it's about 40x40x15cm, which is 24L filled to the brim and, say, 12-15L filled to a more sensible amount. Washing 12 place settings in a single sink-full would be pushing it but 6 goes just fine. Also, note that the comparison relies on the dishwasher being fully loaded, which is fine if just run it every few days when it's full (or have a family of 12!) but not if you wash more often. There seem to be lots of this kind of caveat, which need to be made clearer for the comparison to make sense. – David Richerby Jun 03 '14 at 08:19
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    Yes, but by saying this you are comparing the most water-saving hand washers with the most wasteful machine washers. A water-saving convert is unlikely to become totally wasteful and let the machine run empty. Also only 1 person in the study used less than 5 litres, so for most people even a half-empty machine will use less water. If you read the study, you'll also see that they did use different modes for hand washing (fig 7, 3*2 place settings + pots, 6*2 place settings or 1*12 p.s.), this explains the spread from 83 litres (3 p.s. + pands) to 121 litres (6 * 2 plates) mentioned above. – drat Jun 03 '14 at 08:44
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    @David: I think you're on to something here in the interpretation of the evidence. The "best" manual washers *in fact can* beat the dishwasher (albeit modestly). What's unlikely is that any given person is among the "best" manual washers and knows it with sufficient confidence to eschew the dishwasher. Especially given the Dunning-Kruger effect. – Steve Jessop Jun 03 '14 at 08:52
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    I think it's a little bit like saying walking is faster than a bike while reasoning that a marathon runner is faster than a 3-year old on a trike. – drat Jun 03 '14 at 08:53
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    @drat: it's a little bit more like you're saying "the fastest runners are unlikely to be faster than a cyclist" when in point of fact the fastest runners in a small sample have been measured to be about the same speed as a typical cyclist. The correct interpretation is that the fastest runners in the population are likely even faster than the fastest in the sample, while cyclists (dishwashers) don't show much variation from their designed speed (water consumption). The overall conclusion about averages is pretty sound, but talking about the "best" based on a small random-ish sample is suspect. – Steve Jessop Jun 03 '14 at 08:55
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    Who funded this study? – Bob Jarvis - Слава Україні Jun 03 '14 at 14:23
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    @drat i have to flush more than 5L of water through the pipes just to get the water hot! (plus the dishwasher doesn't care, it heats the water up itself) – Michael Jun 03 '14 at 15:35
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    @BobJarvis the Dish Washer Makers Association ;) – Braiam Jun 03 '14 at 19:15
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    @Michael: depends on the brand of dishwasher. Our Braun insists on having the incoming water hot or it won't run the cycle - which is a royal pain in the drain! (But this way the manufacturer can claim lower energy usage). :-( – Bob Jarvis - Слава Україні Jun 03 '14 at 19:30
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    @DavidRicherby Our family of 5 filled the dishwasher every 0.8 days pretty regularly (that is, every couple of days we had to run it twice in a day) ;) – Izkata Jun 04 '14 at 00:39
  • Dishwashers have also improved recently in terms of water consumption - some of them even store the final rinse water from the previous wash to use as the wash water in the next. My dishwasher uses 13.9L for a full standard wash (14 place settings). – John Lyon Jun 04 '14 at 03:34
  • 121l is nearly a bathtub. If you use that much water to wash your dishes, you have a problem. – Martin Schröder Jun 06 '14 at 18:39
  • @MartinSchröder the 121l is for 6*2 place settings, so it's just about 20 litres per washing-up which is not so much. – drat Jun 06 '14 at 20:24
  • That themselves give away that it is not a fair comparison: “comparing the average behaviour in manual washing with a fully loaded average dishwasher”. – PJTraill Aug 22 '22 at 10:03
  • [Here](https://www.researchgate.net/publication/339222159_A_guide_to_household_manual_and_machine_dishwashing_through_a_life_cycle_perspective) is a fairly neutral life cycle analysis of various methods in the US including observed behaviours and best practices. The emphasis is on greenhouse gases but also looks at water, energy and finance. Conclusions: running tap just terrible; manual best practices yield lowest emissions; machines use less water than even best manual methods; manual cheaper unless time monetised. – PJTraill Aug 22 '22 at 11:00
  • Another often overlooked consideration: we often have some items that seem unsuitable for the dishwasher, so do a manual wash anyway. In this case the lower _marginal_ resource costs of washing more items become relevant. – PJTraill Aug 22 '22 at 20:15
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Okay, a bit of searching found an answer.

The International Journal of Consumer Studies found that:

The study shows that these consumers, on average, used 49 l of water and 1.7 kWh of energy, whereas the dishwasher used 13 l of water and 1.3 kWh of energy on average for the same amount of dishes under the conditions tested.

It appears that I neglected the fact the modern dishwashers can recycle the water for several cycles.

Gilad Naor
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