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:
- Does washing dishes by hand save energy compared to an automatic dishwasher?
- Does the use of paper plates and paper cups require more or less energy than using china plates and cups?
Hand washing versus automatic dishwashers
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 consumption, a first cluster in the distribution occurs around 8 to 26 gallons, while a second occurs around 34 gallons. But many test subjects consumed more than 53 gallons, and 1 subject consumed 118 gallons. Similarly, for energy consumption, a first cluster occurs between 1 and 2 kWh, a second around 3.5 kWh, and another significant grouping shows up above 5 kWh. The highest energy consumption was 16.6 kWh.”
If you think that people who use lots of hot water end up with clean dishes, you’re wrong. Stamminger wrote, “When we examine these data [for washing by hand], we find almost no correlation between the energy used and the performance achieved [that is, the cleanliness of the dishes].”
China versus paper
Most environmentalists are scornful of those who use disposable coffee cups or paper plates. Such people take pride in the fact that they always use a china coffee cup at the office (washing it out in the sink, of course, when necessary).
So is there any logic to preferring china plates and cups to paper plates and cups? Which approach uses less energy?
Several researchers have published papers that include energy life-cycle analyses of the china vs. paper question. The analyses vary in sophistication. One analysis I discovered online (“A Life-Cycle and Economic Analysis: Paper Versus Ceramic Plates”) appeared promising, until I read this sentence: “Assuming five loads are washed per day [in a restaurant] for 365 days of the year, $2651.16 will be spent on electricity. There is no information on how much hot water is required for each wash load, so energy required for heating water was omitted.” OK — we can throw that analysis out the window.
Another analysis (“A comparative analysis of the environmental impacts of ceramic plates and biodegradable plates (made of corn starch) using the Life Cycle Assessment tool”) is somewhat more rigorous. That report concluded, “The minimum number of reuses of the ceramic plate to make the associated environmental impact equal to or smaller than that associated with the single-use biodegradable plate was found to be 50. As the number of uses of the plate increased, the environmental impact decreased.”
One analysis (“Paper Plates?”) concluded that paper plates and bowls make more sense than paper cups and plastic forks: “Disposable cups and cutlery are probably out, but plates and bowls could be an energy and emissions saving option, assuming you compost all the disposable dishes you use. Taking all the plates and bowls out of your dishwasher could reduce the number of loads you do by, say, 40%. This would save around 9000 kWh, which converts to a savings of 5470 kg of carbon emissions from not using the dishwasher, while requiring less than 2000 kWh and 250 kg of carbon to produce the disposable plates and bowls you would use over 15 years. This translates to a savings of 5 [metric] tonnes of carbon over 15 years… It looks like there is an argument for biodegradable plates and bowls, but not cups or cutlery. If you don’t have a dishwasher, and are currently washing all your dishes by hand, biodegradable plates are almost certainly a good option.”
Another online article (“Coffee Cups: Spilling the Beans,”) cites an analysis made by the Dutch Ministry of Environment. “It seems that a ceramic cup is a real greedy vessel when it comes to energy and water consumption and not much better as a contributor to air pollution and solid waste. With energy you’d have to use the ceramic cup 640 times before it would equal a polystyrene cup and 294 times to equal a paper/cardboard one. With air pollution it takes 1,800 uses to beat the polystyrene and 48 to thrash the paper/cardboard. Likewise you would have to drink 126 and 99 cups respectively for the ceramic to compete with polystyrene and paper/cardboard on the waste issue. And water? Sorry, just the use of a ceramic cup totals more than the entire life cycle water consumption of the other two.” Nevertheless, the article concluded, “In this story the ‘good guy’ ceramic cup wins hands down in the functional use category. Designed for a long, durable life, it can be used for well over 3,000 slurping sessions.”
“The results are extremely sensitive to the amount of energy used for dishwashing”
One of the most credible analyses was made by a Canadian chemistry professor. According to “Reusable vs. Disposable Cups,” an article published by the Institute of Lifecycle Energy Analysis in Seattle, “This classic life-cycle energy analysis was performed [in 1994] by University of Victoria professor of chemistry Martin B. Hocking. Hocking compared three types of reusable drinking cups (ceramic, glass, and reusable plastic) and two types of disposable cups (paper and polystyrene foam).” Hocking calculated that “the energy of manufacture” (embodied energy) of a china cup is 14 MJ, while the embodied energy of a paper cup is only 0.55 MJ. Somewhat surprisingly, the embodied energy of a polystyrene cup is only 0.20 MJ — less than that of a paper cup.
The article continues, “The efficiency of the dishwasher, and the efficiency of the energy system that powers it, determines how much energy is required for each wash. Hocking assumed a new, commercial dishwasher requiring about 0.19 MJ/cup-wash.” Hocking developed a “break-even matrix” to show how many times you would have to use a china cup before the energy invested in manufacturing the cup will break even compared to the energy used to make lots of disposible cups. You would have to use (and wash) a ceramic cup 1,006 times to break even with the energy used to manufacture 1,006 polystyrene cups, but you would only have to use a ceramic cup 39 times to break even with the energy used to make 39 paper cups.
Since the energy needed to wash a cup or plate once is less than the energy required to manufacture one disposable cup or plate, using a china cup or plate wins the race, as long as it is used for more times than required to reach the “break even” point.
However, there is a caveat. The article notes, “The results are extremely sensitive to the amount of energy the dishwasher requires for cleaning each cup. … If Hocking had chosen even a slightly less energy-efficient dishwasher as his standard, then the reusable [ceramic] cups would never have broken even with the foam cup. … In situations where cups are likely to be lost or broken and thus have a short average lifetime, disposable cups are the preferred option.”
Some, but not all, of the life-cycle analyses cited above tried to account for the economic and environmental costs of disposing of paper plates and cups. (I anticipate comments from readers who will mention our country’s looming landfill crisis. While a full analysis of the disposal question is beyond the scope of this blog, it’s worth mentioning that paper plates and cups can be composted or used for kindling in your wood stove.)
So which wins — china or paper?
Based on my reading of the above analyses, I would say that china plates, bowls, and cups win out over paper — but only just barely, and only under certain conditions:
- China fails this comparison if your cabinets are full of more china plates and cups than you use regularly. After all, if you have 40 coffee mugs in your cabinet, that’s a lot of embodied energy. You have to use each cup dozens or hundreds of times before you’ve broken even.
- China fails this comparison if you have clumsy family members who break china often.
- China fails this comparison if you wash your dishes by hand.
If you are ever berated by an environmentalist for sipping coffee from a paper cup, you might explain the above facts. (Or not.)
And if you keep a ceramic coffee mug on your office desk, and you wash it out in the sink in your office’s lunch room — it might be time for you to buy a package of Dixie cups.
Does it make sense to supply cold water to your dishwasher?
Dishwashers require 140°F water to operate. U.S. dishwashers assume that the incoming water temperature is 120°F; all dishwashers include an electric resistance element (often rated at 900 watts) to raise the temperature of the incoming water to 140°F.
If the incoming water temperature is less than 120°F, the dishwashers still work; however, the cycle takes longer, because the dishwasher has to wait until the necessary volumes of water are heated to 140°F.
Three researchers from the Florida Solar Energy Center — David E. Hoak, Danny S. Parker and Andreas H. Hermelink — have written an interesting paper that discusses the energy ramifications of the electric-resistance water heaters in dishwashers. In their paper (“How Energy Efficient are Modern Dishwashers? ), the authors write, “Given the small volume of the fills for the various draws and the length of the time between them leads to the question of how much of the heat in the water heated by the remote hot water tank actually reaches the dishwasher? This is a function of draw amount, length of plumbing to dishwasher, pipe insulation, temperature differences, and time between dishwasher fills within a cycle. … For instance, with a 30-foot plumbing run, it can be shown that only a portion of tank-heated water actually reaches the dishwasher. Some of the water drawn per cycle has been heated by the water heater, only to cool in piping, and then to be reheated by the dishwasher – inherently inefficient. … In large houses where 70-foot runs are possible, none of the hot water in each cycle would be directly heated. Also, in cooling dominated climates, much of the residual heat in the pipes is lost to the interior, becoming internal gains that must be removed by the cooling system. … One recommendation might be to plumb cold water to the dishwasher if the water heater is a resistance electric type and users are willing to tolerate the 20-30 minutes longer dishwasher cycle times. Cold water connection may use the least combined water heater and dishwasher energy since the dishwasher heats its own water only for the parts of the cycle where hot water is needed, but the energy (in the form of electricity) generally costs more. However, if one has a gas or solar hot water system, connecting them to hot will save carbon dioxide emissions and operating costs, particularly if plumbing runs are short.”
Minimum federal standards and the Energy Star standard
The Federal minimum efficiency standard for automatic dishwashers (effective January 1, 2010) requires standard-sized dishwashers to use no more than 355 kWh/year and no more than 6.5 gallons of water/cycle.
The Energy Star standard for dishwashers is more stringent; it requires standard-sized dishwashers to use no more than 295 kWh/year and no more than 4.25 gallons of water/cycle. Since the test method assumes that a dishwasher is used for 215 cycles per year, an Energy Star dishwasher must use no more than 1.37 kWh per cycle.
Which models of automatic dishwasher are most efficient?
The Energy Star program maintains an online list of Energy Star dishwashers. The most efficient models of standard-sized dishwashers are manufactured by Asko (the Asko D5894A) and Bosch (the Bosch 800 Plus). However, these models are costly; according to Bosch, the manufacturer’s suggest list price for the Bosch 800 Plus “ranges from $1,399 to $2,099.”
Another online list of energy-efficient dishwashers is maintained by the Consortium for Energy Efficiency (CEE). Appliances on the CEE list use less than 295 kWh/yr and 4.25 gallons of water per cycle (equivalent to the Energy Star requirements).
According to Consumer Reports, the Kenmore Elite 12783 ($1,200) topped their tests for quiet operation and low water use. However, the runner-up from Bosch (the Bosch Ascenta SHX3AR7US), came close in performance to the top-rated Kenmore, and was a bargain at $700.
Another recent list of efficient dishwashers highlights the Asko D5894A and the following Bosch models: SHE68E05UC, SHE8ER5UC, SHV68E13UC, SHX68E05UC, SHX68E15UC, and SHX8ER5UC.
Tips for using an automatic dishwasher
No matter what brand of dishwasher you have in your house, here are some tips to minimize energy use and improve performance:
- Don’t pre-rinse your dirty dishes. Just scrape off the food scraps instead.
- Don’t run your dishwasher unless it is full.
- Don’t use “enhanced” settings like “super wash” or “pots and pans.”
- Use a good detergent. Consumer Reports recommends Cascade Complete with Dawn ActionPacs, Cascade Complete All-in-1 Powder, or Walmart’s Great Value Powder Pacs.
- If possible, don’t use the heated drying cycle. Heated drying uses an electric resistance heating element. If your dishwasher offers it, select unheated air drying, or just open the door when the dishes are still hot from the rinse cycle, to allow the dishes to dry into the room.
Martin Holladay’s previous blog: “Zen and the Art of Grading.”