The amount of water used by a countertop dishwasher varies by model. Take the Ventray countertop dishwasher as an example. These countertop dishwashers are designed to be water-efficient and use less water than full-size dishwashers. It uses between 2-3 gallons of water per cycle. This is significantly less water than a traditional dishwasher, which can use up to 15 gallons of water per cycle.