Dishes must be pre-washed in older dishwashers. Pre-washing consumes 22,000 gallons of water every year. So, few years ago, hand washing dishes was shown to be less expensive in terms of water use. Dishwashers are now available that are both water and energy efficient. Because no pre-washing is necessary, less water is used. Simply remove the food off the dishes and put it in the dishwasher.