It can boost the heat of the water used in the machine to 140 F, the temperature required to clean effectively. Then you can lower your water-heater thermostat to 120 F and save energy. A lower setting will also reduce the danger of scalding. Also check to see if the dishwasher has an energy efficient no-heat drying cycle. It costs less to air dry dishes.
Compact models use less energy but hold fewer dishes.
A compact model may actually result in greater energy use if you have to operate it more frequently. Choose a dishwasher that has several wash cycle selections. If your dishwasher has a booster heater, turn your water-heater thermostat setting to 120 F. Scrape instead of rinsing off large food pieces from the dishes. Soaking or prewashing is usually not necessary unless food is burned or dried on the dishes. Operate the dishwasher with a full load and select the energy-saving cycle when possible.