An automatic dishwasher uses hot water and electricity, so it makes sense to choose an efficient model. But before discussing the question of which dishwasher you should buy, we need to address two questions:

The definitive research on the first question was performed in 2004 in Bonn, Germany, by researcher Rainer Stamminger. Stamminger reported his findings in an article for English-speaking readers (“Is a Machine More Efficient Than the Hand?”), published in the May/June 2004 issue of Home Energy magazine.

Stamminger found that a dishwasher uses about half the energy and one-sixth the water used by the average hand-washer. (The appliances used in Stamminger’s study used an average of about 4 gallons of water and about 1 kWh of electricity per load for a standard washing cycle.) Moreover, the automatic dishwasher used less detergent and got the dishes cleaner.

“To rule out possible regional differences in dishwashing techniques, we persuaded 113 people from seven countries in Europe to participate in the dishwashing experiment,” Stamminger wrote in the Home Energy article. “The subject was asked to clean and dry the dishes as he or she would do it at home. … For manual dishwashing, results below 3.5 were regarded as ‘really dirty’ or ‘not acceptable to be placed on a dinner table.’ Clearly, about half of our test subjects did not achieve an acceptable level of cleanliness.”

If you are washing dishes by hand, it’s extremely hard to do a good job with the same amount of hot water used by an automatic dishwasher. “To clean the 12 place settings of dishes, the 113 test subjects used on average 27 gallons of water and 2.5 kWh of water-heating energy,” Stamminger wrote. “The amounts of water and energy consumed by the test subjects to clean the dishes do not fall into any clear pattern. … For water…