Last spec i saw of a dishwasher was 10 liters / wash. But i think that's pretty low because they were bragging with it.

Estimate how much you need by hand. Filling up the sink, 5-10L? And then rinsing it all afterwards, another 5 at least. Keep in mind that a dishwasher does about 4 meals while you usually do one at a time when you do by hand.