Residential Dishwasher Introduction
Dishwashers represent approximately 1.4% of typical residential indoor water use and Americans use just 1 gallon per person per day on dishwasher water use (Mayer, et. al. 1999, Residential End Uses of Water, AWWA Denver, CO). New efficient dishwashers use significantly less energy, as well as water, compared with machines from 10 years ago or more so the benefits of installing an efficient machine will be realized through both energy and water savings. Studies have found that dishwashers are used between 0.1 and 0.2 times per person per day. Clearly the potential water savings from dishwasher efficiencies are greatly variable from different households.
When comparing only the water used inside the dishwasher, handwashing uses an average of 23 gallons more per session (Stamminger, et. al., 2004, European Comparison of Cleaning Dishes by Hand, University of Bonn) or up to five times that of a new dishwasher. However, this study was based on fully loading the dishwasher and did not include the water used in pre-rinsing or pre-washing dishes before loading them into the automatic dishwasher. There are not yet definitive studies on the average load level practice of consumers, and most dishwashers are designed to operate at only one water volume (though the quantity of cycles is variable). In addition, many consumers continue to pre-wash or pre-rinse the dishes, which was not included in the comparative study. While the practice of pre-rinsing is usually not necessary with newer dishwashers, the practice seems to continue based on old habits. The water and energy savings realized from new efficient dishwashers will vary greatly the frequency use and the practices of a household. That said, automatic dishwashers offer the potential for significant water savings over handwashing when dishwashers are fully loaded when operated and the users do not pre-rinse the dishes unnecessarily.
How many homes in the U.S. are actually equipped with automatic dishwashers? The American Housing Surveys for the U.S. (1985 to 2003) confirm an increasing trend in the installation of these appliances. According to AHS, 58% of all housing units possessed automatic dishwashers in 2003. At the same time, 88% of all new housing stock (4 years old or less) was being equipped with them. The 2007 U.S. Housing Survey report that 62% of homes now have an automatic dishwasher; a 4 point rise in only 4 years. Trends are upward, as shown in the chart below.
Energy Star U.S.
Energy Star Canada
The Energy Star Canada website (by Natural Resources Canada – NRCan) makes available the water use data for residential dishwashers available in North America. For the most current data and the ability to search their database, visit NRCan website.
When viewing the table on the Canadian website, note that ‘Hot water use per cycle’ is total water use (since dishwashers only connect to the hot water line). Canada and the U.S. have the same energy efficiency requirement for Energy Star dishwashers. Thus, if the Canadian site says it is Energy Star Qualified, it is qualified in the U.S. as well.
Water Factors of Machines in Today's Marketplace
Using the Canadian NRCan data, AWE plotted the water factors of the 963 different dishwasher models currently available in the North American marketplace. The following chart reveals that 92% of all AVAILABLE residential dishwasher models meet the old Oregon criteria of 6.5 gallons per cycle. (NOTE: This does NOT mean that 92% of all dishwashers SOLD meet those criteria. Sales volumes of machines within each of the different WF classifications are not currently available). It is important to note that compact dishwashers were included in this chart, and generally have higher water factors compared to full-size machines. It is not yet known if compact dishwashers are more often operated at full capacity (more efficiently) due to their smaller size.
Consortium for Energy Efficiency (CEE)
The Consortium for Energy Efficiency (CEE) list of efficient residential dishwashers provides information on the water use of each machine. The list of qualifying dishwashers includes energy use (kWh/year), the energy factor, and water use (gallons/cycle). Click here to download a copy of the list.
Residential Dishwashers as a Potential Best Management Practice
Should water-efficient residential dishwasher installations qualify as a Potential Best Management Practice for water providers? That question is addressed in this 2006 California study: PBMP Report- Residential Dishwashers (PDF)