Dishwashers: Green Machines: Nitty-gritty

August 31, 2011

Reading time min

Q: Dishwasher or hand washing?

Asked by Kim Stone, JD ’96, Sacramento, Calif.

Adding or upgrading a dishwasher

If you don't own a dishwasher, is a new one a sensible investment? The answer, from either an economic or environmental perspective, is almost certainly yes. As noted in the Essential Answer, annual savings on your utility bill will more than offset the price of a new dishwasher during the machine's lifetime. If you're concerned about the cradle-to-grave energy consumption of the appliance, rest assured that the life-cycle energy savings facilitated by a modern dishwasher outweigh the upfront energy costs of its production. A study conducted by scientists at Bosch and published in Appliance Magazine, found that the production phase accounts for only about five percent of the energy used over the lifetime of a dishwasher. That corporate research is supported by the Life Cycle Engineering and Management Research Group at the University of New South Wales. Their researchers attested that "the environmental effects of manufacturing and transporting a dishwasher are minor compared to the effect of its improved power and water use."

You'd also be wise to upgrade a dishwasher built prior to the early 2000s. Doing so will save you $25-$30 annually on utility bills, and though you're starting from a better place than a hand-washer, your energy savings will still more than balance the production and delivery costs. Dishwasher manufacturers have embraced a range of energy- and water-saving features over the past decade. These include soil sensors and cycle options, which adjust water volume and temperature to the requirements of a particular load, and booster heaters, which reduce heat lost between your home water heater and the appliance by heating water the final few degrees in the dishwasher itself. With these additions, new models may be as much as 40 percent more efficient than the machines of the 1990s.

If you're interested in the energy-efficiency benefits of replacing other household appliances, take a look at this previous SAGE column. A glance at the chart below will tell you that your dishwasher is far from the biggest energy hog in your kitchen, so make sure you're considering the whole picture!

When replacing your dishwasher, make sure you dispose of the old one appropriately. If you're as lucky as my mom, you might have a hand-washing friend for whom even your old dishwasher constitutes an upgrade. Habitat for Humanity also accepts donated appliances. If you can't find a friend in need or a donation drop-off site in your area, try putting up a listing on Freecycle, or consult Earth911 to locate a program that will recycle your old dishwasher. And whether you're upgrading or buying a dishwasher for the first time, think carefully about your particular needs. If you own few dishes, consider a smaller, more efficient compact model. If you have a large household or cook and entertain frequently, find a dishwasher that can accommodate a full day's worth of dishes and look for cycle options that will allow you to boost cycle length and water temperature for especially grimy loads.

Making the most of your dishwasher

No matter how impressive the efficiency ratings on the label, any dishwasher can guzzle water and energy if used improperly. The American Council for an Energy-Efficient Economy suggests a few simple rules to ensure that your dishwasher lives up to its potential:

Never pre-rinse dishes—doing so may use up to four times the water that it takes to run the dishwasher. Instead, give bowls and plates a dry scrape before loading them.

Run full loads. A half-empty dishwasher uses the same amount of water as a full dishwasher, so running only full loads will minimize your water and energy use per dish.

Choose the lowest-energy cycle for your needs. You may need to spend a little quality time with your machine's user manual to make sure you understand the intended function of the various options. In general:

1. Select lower temperatures settings for lightly soiled dishes;

2. Use the half-load option if you need the run the dishwasher before it's full;

3. Choose the air-dry function over heat-dry, and

4. Look for time-delay functions that allow you to run the dishwasher during off-peak hours

What if a dishwasher isn't an option?

If, like me, your living situation doesn't permit you to install the dishwasher of your dreams—or if you own fragile dishes that can't go through the dishwasher—you can make hand washing more efficient. Start by thoroughly dry-scraping dishes. Let particularly crusty items soak in a little soap and water before scrubbing: This will minimize the time, effort and water ultimately needed to get them clean. When you're ready to wash, focus on minimizing time spent with the faucet running. The average faucet flows at about two gallons of water per minute, so if you're washing only a few dishes and can leave the faucet on for a total of three minutes or less your hand washing might approach or even exceed dishwasher efficiency. Fill sinks or dishpans with wash and rinse water, and start with the cleanest dishes so that you can re-use the water several times before running a fresh tubful. And don't overdo it on the dish soap—too much soap can cling to dishes, making them harder to rinse and resulting in increased water consumption.

Saving energy upstream of the dishwasher: hot water heaters

According to the U.S. Department of Energy, dishwashers "require the hottest water of all household uses, typically 135 to 140°F." That's up to 30°F hotter than the water coming out of your shower head. About 60 percent of the energy that a dishwasher consumes goes toward heating water. Although newer models include built-in booster heaters to take care of the last 15-20°F, the bulk of the heating takes place in your home's water heater, meaning that a more efficient heater can help maximize the energy-saving potential of your dishwasher.

Gas-fired water heaters are most common, and are easier on the environment and your pocketbook than electric versions. Most homes today use gas storage models—the kind with a burner on the bottom and a 40-gallon or larger tank above. If yours is due for an upgrade, check the Energy Star ratings for the most efficient options. Tankless water heaters, which cut energy use by 10-15 percent over storage models by heating water only as it's needed, are a great choice if you are able to make a bigger up-front investment in long-term durability and energy savings. Builder magazine reviews the pros and cons of tankless and conventional water heaters. You should also be on the lookout for Energy Star-qualified gas condensing heaters, due on the market this year. Similar in most respects to gas storage models, these new systems recapture and recycle the heat emitted by the central burner, achieving an efficiency improvement that could shave 30 percent off your energy use and about $100 per year off your utility bill.

If you're truly interested in the "greenest" water heating option, think about a solar water heater. According to U.S. Department of Energy estimates, these roof-mounted systems can save 50 tons of CO2 emissions—roughly equivalent to the annual emissions from nine passenger vehicles—over a 20-year period. And when combined with an efficient gas storage backup, a solar heater could also save you close to $200 per year in energy costs. Although upfront installation costs are higher, solar systems last longer than gas-fired systems, and most are eligible for Federal tax credits. The Energy Star website describes the different solar heating systems currently on the market, and provides some great suggestions and resources to get you started on installing a solar heater.

Water and Energy Used in Dishwashing: Summary

The chart below illustrates the relative efficiencies of various dishwashers.

dishwasher efficiency
Infographic: U.S. Energy Information Administration

In general, you'll see an improvement in efficiency as you move from a standard to a compact model, or from one that meets the minimum federal requirement to one that adheres to stricter Energy Star specifications. If you're willing to spring for one of the top performers, such as this $1,200 Bosch, you may be able to cut the water and energy use of the minimum federal standard in half. No matter which dishwasher you choose, though, you'll be doing yourself and the environment a favor—notice that the "hand washing" dot didn't even fit on the graph.

Kate Johnson, '10, plans to receive her master's degree in Earth Systems in 2011.

Additional Resource: Water heaters overview from the American Council for an Energy-Efficient Economy

You May Also Like

© Stanford University. Stanford, California 94305.