Take the 2-minute tour ×
Skeptics Stack Exchange is a question and answer site for scientific skepticism. It's 100% free, no registration required.

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?

share|improve this question
1  
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 3 at 5:39
2  
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 Jun 3 at 14:22
4  
Does anyone not? I for one can't use the two sink method, having only one sink. –  Ryan Reich Jun 3 at 15:09
2  
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 3 at 16:38
2  
@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 4 at 14:28

2 Answers 2

up vote 43 down vote accepted

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

share|improve this answer
10  
Presumably, this is heavily dependent on whether you wash under a running tap or in a sink full of water. –  David Richerby Jun 3 at 8:10
6  
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 3 at 8:44
9  
@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 3 at 8:52
3  
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 3 at 8:53
11  
Who funded this study? –  Bob Jarvis Jun 3 at 14:23

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.

share|improve this answer

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.