There are a lot of questions about preheating dishwasher water. Here are the answers:
1. Should I preheat dishwasher water? No. It takes the same amount of energy to heat water inside the dishwasher or in the hotwater heater. Reduce hot water temperature as much as possible (target 120 degrees) in your hot water heater. This lets you still have a very warm bath or shower (hot tubs are less than 110 degrees). Heat the water in the dishwasher only. The goal is to heat water at the point of use as much as possible, not in a storage tank. The dishwasher is good at this. It monitors the water temperature and heats it at the point of use and only to the temperature required to get the dishes clean.
2. How can I reduce my dishwasher cost?
Use air dry only. Heat drying actually uses the dishwasher's heating element to heat the air in the washer. This drys the dishes but is very expensive. New dishwashers are very energy efficient. In fact it is cheaper to wash a full load of dishes in the dishwasher with air dry than to hand wash.
Wash with full loads only. It takes the same amount of water for a full load or a partial load.
Wash when you are gone or at night. The higher humidity from the hot water isn't as objectionable and doesn't add humidity to the house during the middle of a hot summer day. This also reduces air conditioning load.