It depends upon the capacity of water which is found in the dishwasher. The average gallon is almost 6 that is enough for dishwasher under 500. Some time 4 gallons is enough for the dishwasher to wash the plates and dishes in 6 cycles of the wash. During the six cycles of washing, water is dirt more. The energy cycle is different small dishwasher has 0.87KW/H, and large dishwasher energy is almost 1.5KW/H. Gallon is used according to the capacity of the dishwasher.