I've been considering a dishwasher, but there are a range of different opinions on if they save water (or even energy) or not. Do we have any sources one way or the other?
I disagree that dishwashers, compared to hand washing, will not save energy.
First of all, as with almost anything, it depends how it's done. If you run your dishwasher half full, that's going to be bad for energy and water use. Similarly, if you hand wash, with the hot water running the whole time, that's bad. So, either technique requires tuning the way you use it.
However, dishwashers have some inherent advantages, in that they're closed units. Being closed units allows them to build up heat and humidity. That helps the washing process. It also means that water sprayed on dishes in the upper rack can then filter down, and rinse dishes in the lower rack. The same goes for water that's sprayed horizontally.
Also, understand that a dishwasher is fundamentally different than a clotheswasher. A dishwasher doesn't need to spend a lot of energy physically moving around heavy clothes and a large tankful of water. Much of the energy a dishwasher uses is in heating the water. But, that happens with handwashing, too. Hopefully, you can intuitively see that if the issue is largely about heating water, it's easier to do so in an enclosed space.
Rather than try to reproduce a thorough analysis myself, I'll point to an article on treehugger.com. An important excerpt:
These numbers indicate that it's possible to be more efficient when hand-washing, but it's pretty tough. Can you successfully wash and rinse a soiled dinner plate in just over a cup of water? If you can keep the water use low, equal to an efficient machine, you'll require less energy, but doing an entire load of dishes in 4 gallons of water is roughly equivalent to doing them all in the same amount of water you use in 96 seconds of showering (using a showerhead that emits 2.5 gallons per minute).
So, as long as you don't often run your dishwasher when it's only half full of dirty dishes, or unless you are very miserly with your water use (or have an old, inefficient dishwasher), the automatic dishwasher is likely to be more efficient. That is to say, it's possible to use less water and energy by hand washing your dishes, but it's not easy.
I'll also address another point in the other answer.
When you automate things, you substitute electricity for manual energy.
"Manual energy" doesn't come from nowhere. It comes from the food we eat. Powering a human being involves converting sun, water, and soil into food, processing the food, then letting the human convert it into usable energy. The human body, combined with the agricultural inputs, do not represent an efficient "machine".
I'd also offer the somewhat economic argument regarding opportunity cost. The time you'd spend handwashing dishes might better be spent on something else. If you're interested in sustainability (yeah!), then consider what other sustainable projects you could work on with the X hours a month you'd spend handwashing dishes. Perhaps add some insulation to your attic? Add mulch to your garden? Brew biodiesel yourself?
All these things matter in the overall assessment of what saves energy, or makes energy usage more sustainable.
Anyway, if I had to be tied down to a short, simple answer, I'd say that yes, a dishwasher does (tend to) save water.
First of all, dishwashers, compared to hand washing, will not save energy. When you automate things, you substitute electricity for manual energy. At best you are substituting fuel for food (but more likely you'd use that food anyway).
For water, there are a fair number of factors. These include how you wash dishes, how greasy the dishes are, how much water your dishwasher uses, and much more. For places which have separate irrigation water lines, I would actually recommend finding out whether the irrigation water is of sufficient quality to use for washing dishes (the required water quality to safely wash dishes is lower than that to drink). I would suggest that if the water is safe to play in (keep in mind kids, particularly, tend to ingest small amounts of water as they play), it is probably safe to wash dishes in. Here dishwashers have an advantage if they heat the water up to disinfecting levels and may allow you to use less potable water.
I live in Mexico and use only water collected from the roof during the 3 month rainy season - we soap the dishes first, no runnning water, then rinse them all at the same time. For two people, mixed diet (some meat and fish) this rarely takes more than 2 gallons at a time, often less for a day's worth of dishes. Dishwashers are way to costly (purchase cost wise) to consider and hard to get - I'd have to drive 40k to look at one, and in any case, a cash expenditure like that is a heavy opportunity loss on it's own.
I'd say stay with handwashing and practice getting to minimum water use using pre-soap and scrub method followd by rinse unless you have already spent the money on an up-to-date dishwasher.
I come to the US quite often. Here I see folks pre-rinse under running water more often than no, just because that's their habit (maybe from having earlier dishwahers??). And I think - hey dude - you should try living with 40.000 liters for the year and see what happens then! You'd be out of water before 4 months is up! And, consider this too - half your potable water goes down the toilet too!
So, I guess it is a little contextual, no?
Dishwashers will not save any type of energy. We live off grid and only have rain water and there is no ground water available either.
Wash your dishes in a small sink, and only when you have enough dishes to clean to use the water you use in the sink.
The dishwasher also uses power to heat the water, which also uses more power than either gas or solar heated water.