AskPablo: The Dishwasher

Emma.jpg People in tie-dye everywhere will tell you that technology is the devil, but most of today’s “Cultural Creatives” will tell you that using a dishwasher is far more efficient than washing your dishes by hand. So which is it? Well, this week we will find out once and for all. I am joined by my friend Emma, a junior scientist in the third grade, who will help me conduct the necessary experiments to get you an answer.

When practicing the good old scientific method it is important to document everything. This way you can go back to check for errors, or to explain any anomalies. Ideally I would get several dishwashers to test side-by-side but I just don’t have those kinds of resources, so the one I have has got to do… We are talking about a GE GSD3000W16. This model was probably made in the late 1980’s and features several energy-saving functions. Changing the settings to the less efficient options can result in an 80 minute cycle! For this experiment I will set it to “Short Wash,” “Low Soil Level,” and “Energy Saver Drying,” which will take about 28 minutes to run.
When we started our experiment at 5:29pm my Kill-A-Watt meter read 123.8V, 59.9Hz at the wall outlet under the cabinet, where the dishwasher is plugged in. In addition to hooking up my Kill-A-Watt meter I have also disconnected the drain pipes of the sink and the dishwasher and have diverted them into a bucket for precise measurement with a 1 liter measuring cup. The control experiment will involve me washing the same exact dishes that are going into the dishwasher by hand, under running hot water.
With Emma carefully watching the bucket under the sink I begin to wash the dishes. I have allowed the water to run for a while until it becomes warm. I am also counting the water wasted in doing this since it was also once heated by the water heater. From a previous AskPablo we know a little bit about the energy required to heat water in a water heater. I figured out that we need 156.7 kJ of heat energy per liter, which is equal to 148.5 BTU per liter. I looked up that natural gas creates 117.08 lbs of CO2 for every million BTU. By using a couple of metric conversion factors and fancy number-crunching I arrive at 7.8876 grams of CO2 per liter of hot water.
My hand washing experiment resulted in 20 liters of water used for dish washing and 4 liters wasted to get the hot water to the tap. So 24 liters times our emissions factor above results in 189.3 grams of CO2 for a hand-washed load of dishes. But how does that compare to the dishwasher?
To avoid clogging the dishwasher outlet most people do a quick pre-rinse of the dishes. This amounted to about 4 liters. We loaded up the dishwasher and readied the bucket and Kill-A-Watt meter. The dishwasher drew and expelled water four times so we were busy measuring the contents of the bucket. The meter reached a peak 10.8 amps, although the nameplate on the unit says that the maximum is 12.5A. The total amount of hot water used by the dishwasher was 25.2 liters! Over the whole 28-minute cycle the Kill-A-Watt measured 0.38 kWh, roughly $0.06 worth of electricity.
The Department of Energy tells us that California’s electricity is responsible for 0.51 metric tons of CO2 per MWh, or 510g/kWh. Our energy use resulted in 193.8 g of CO2 (0.38 kWh x 510 g/kWh) and our water use resulted in 198.77 g CO2 (25.2 l x 7.8876 g/l). So, the total amount of carbon dioxide emissions from using the dishwasher is 424.12 g (almost one pound!) while hand-washing of the dishes resulted in 189.3 g of CO2.
The verdict is in! Even if you are using the super energy-saver mode on the dishwasher you are still using more energy, and releasing more CO2 than by hand-washing your dishes. Keep in mind that newer models might be more efficient with their water and energy use but probably not much. According to a Tampa, FL government website “the average dishwasher uses 15 gallons of water per load!” Also keep in mind that I was hand-washing dishes under running water. To save even more resources I could fill the sink with water to do the dishes in and then do a separate, quick, rinse.
I would like to extend a special thanks for my great Jr. scientist assistant, Emma. I look forward to hearing your comments and ideas over the coming week. Thanks for reading AskPablo!
Pablo Päster, MBA
Sustainability Engineer
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