For a while, when it comes to green impacts, the prevailing wisdom has been that built in dishwashers beat hand-washing dishes, in a runaway. By the numbers, according to one study at the University of Bonn in Germany, the dishwasher uses only half the energy, one-sixth of the water, and less soap, to boot. That sounds easy enough, but there's a lot more to it than and black-and-white comparison between your faucet and sink and the appliance under your counter.
For example: How do the results vary with model of dishwasher? What hand-washing habits are people using? How do you heat the water in your home? And how often do you do the dishes? Turns out all these factors can change the impacts; keep reading to learn what goes in to calculating the greenest way to do your dishes.
Water use, energy use, and carbon footprintThere are three big factors we'll consider: water use, energy use (for heating the water, largely), and the carbon footprint that results -- we'll save things like soap and dishwasher cooking for another post. And, of course, following energy-saving tips like running the "light" cycle and turning off the "heated drying" option will change the way the numbers work.
Built-in dishwasher efficiencyThe average dishwasher uses 6 gallons of water per cycle; the average Energy Star-rated dishwasher uses 4 gallons per cycle, and their energy use ranges from 1.59 kWh per load down to 0.87 kWh per load. Using the Department of Energy's carbon dioxide emissions numbers of 1.34 pounds of CO2 per kWh, that's 1.16 to 2.13 pounds of carbon dioxide emitted per load, to go along with 4 gallons of water.
Energy Star assumes each load in a "standard" dishwasher (usually 24 inches in size) has "a capacity greater than or equal to eight place settings and six serving pieces," so we'll go with that when considering how many dishes need to be washed by hand.
Can hand washing be as efficient as dishwashing?The short answer: maybe. First, let's look at water usage alone. The average faucet flows at 2 gallons per minute, so if you can successfully wash and rinse eight place settings -- plates, bowls, forks, knives, spoons, glasses, etc. -- and those six serving dishes that your dishwasher can handle without running the faucet for more than 2 total minutes, then, you might be better off hand-washing. Assuming you're washing 54 pieces of dishware (that's 48 pieces of dishware -- 6 pieces per setting -- and 6 serving dishes), you've got about 4.4 seconds of wide-open tap water per piece, or about 9.5 ounces of water to wash and rinse each dish.
Impacts of heating the waterLet's assume you use warm water for both washing and rinsing -- half hot water and half cold water. Heating 2 gallons of water with a gas hot water heater (from about 60 degrees as it enters your house to, say, 120 degrees, set by the thermostat on your hot water heater) takes about 960 BTUs, or about 0.9% of one therm (100,000 BTUs), assuming 100 percent efficiency.
Gas storage tank water heatersGas water-heaters are usually more like 65 percent efficient, so it really takes 1477 BTUs, or about 1.5 percent of a therm, to heat that water. One therm emits 11.7 pounds of CO2, according to the EPA (pdf), so heating the water with gas for each 2-gallon load emits about .17 pounds of carbon dioxide.
On-demand (or tankless) water heaters are closer to 80 percent efficient, which changes the numbers a bit; it works out to about 1200 BTUs, or about .14 pounds of carbon dioxide.
Electric storage tank water heatersThe story is a bit different when considering an electric water heater; while most electric water heaters use between 86 and 93 percent of their energy for heat (compared to between 60 and 65 percent for gas), electric heaters aren't as efficient at heating water. It still takes 960 BTUs to heat that much water; it just takes about .28 kWh (since, according to the EIA, 1 kWh equals 3412 BTUs) to heat 2 gallons of water at 100 percent efficiency, or about .30 kWh at 93 percent efficiency. Each kWh emits 1.715 pounds of CO2, on average (thank you, EPA), so heating water with electricity for each 2-gallon load emits about .51 pounds of CO2.
Built in dishwasher vs. hand-washing: And the winner is...These numbers indicate that it's possible to be more efficient when hand-washing, but it's pretty tough. Can you successfully wash and rinse a soiled dinner plate in just over a cup of water? If you can keep the water use low, equal to an efficient machine, you'll require less energy, but doing an entire load of dishes in 4 gallons of water is roughly equivalent to doing them all in the same amount of water you use in 96 seconds of showering (using a showerhead that emits 2.5 gallons per minute).
So, as long as you don't often run your dishwasher when it's only half full of dirty dishes, or unless you are very miserly with your water use (or have an old, inefficient dishwasher), the automatic dishwasher is likely to be more efficient. That is to say, it's possible to use less water and energy by hand washing your dishes, but it's not easy. Of course, if you do it just right, it might just be a wash.
article found here.