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It does seem odd, but it is _far_ better than doing it manually. The reason it's so much more efficient is because it takes so long - it can do a great job despite using very little energy and water because it just keeps going at it gradually

The main power saving comes from running at lower temperature.

Heating up water takes way more electricity than running the pumps.

Take a look at the Technology Connections video. Most people would be astonished how little water is used.

And considering that practically all of the energy use is heating the water, that translates into low energy use.

The eco modes (or just generally, modes on newer dishwashers) take longer because to balance out the small amount of water, they need to spray the dishes more times. The energy use is low.