Contrary to popular belief, many studies have proven that using a dishwasher is better than handwashing dishes. These studies cite comparisons of water and energy usage and the emissions from that usage to show how dishwashers are more efficient. I’m here to challenge the results of these dishwashing v. handwashing tests.
These studies fail to look at the broader picture of dishwashing v. handwashing. There are many more factors that should be considered: manufacturing and transportation emissions, end of life disposal, environmental effects, and even just practicality of always being able to fully fill a dishwasher.
In this post, I will not only take a closer look at these studies and examine what factors they conveniently exclude, but I will also perform my own experiment to see how efficiently I can wash a dishwasher’s worth of dishes by hand.
Study Says The Dishwasher Is Better
The average dishwasher uses 6 gallons of water for its entire cycle. Energy Star-rated dishwashers use 4 gallons. The water is heated by an electrical heating element in the dishwasher to around 140 degrees Fahrenheit.
In contrast, a normal kitchen faucet pours out 2 gallons per minute when fully open, and household water heaters are usually set to a max of 120°F (anything above this can scald/burn skin). So one would think there’s no way to wash all those dishes in just 2 to 3 minutes with the same effectiveness as an hour-long (or longer) cycle of the dishwasher.
Most articles online on the dishwashing v. handwashing debate are based on one of two things: water usage or monetary cost. Which is cheaper? Which is faster? Or which uses less water? There seems to be a shortage, however, of studies on which is more environmentally friendly (beyond water usage).
Bonn University Study
The same 2009 Bonn University study pops up frequently as “proof” that dishwashing is better than handwashing in multiple categories. It has been the only study I can find that actually performed an experiment to compare the two methods.
In this experiment, people from across Europe were left in a kitchen setup to clean 12 place settings of dishes in whatever manner they usually wash at home. The water use, energy use, soap use, time, and even “cleaning index” were all compared across nationality and then aggregated and compared to the average dishwasher. The results show the dishwasher beating out humans in every category.
The results of this experiment concluded the following averages for handwashing 12 place settings:
- Water usage: 103 liters (27.2 gallons)
- Energy usage: 2.5 kWh
- Soap usage: 35 g (1.2 oz)
- Time: 79 minutes
Here’s how those numbers compared to a modern dishwasher:
The cleaning index referenced in the table above is a measure of how clean the dishes were after washing. A higher number means a cleaner dish, so this experiment concluded dishwashers using a normal cycle did about the same as a person, and an intensive cycle resulted in a cleaner dish.
So it seems the dishwasher is both more water efficient (1/6 of handwashing) and energy efficient (2/5 of handwashing). It also is faster in terms of how long the person spends on the dishes (only loading and unloading). But is this study telling the whole truth about the water and energy usage?
Research Shows The Study Ignored Important Factors
What’s missing from this comparison? The study never looks at carbon or dioxin emissions caused by production, transportation emissions from factory to store to home, and the effects of disposal of the dishwasher versus a sink in its analysis.
Production, Transportation, and Energy Use
To take a closer look at what’s missing in the dishwashing v. handwashing debate, let’s look at emissions. An assessment in Appliance Magazine from 2003 states it takes 4,300 MJ to manufacture a dishwasher. This is 1,200 kWh, which converts to 1,608 lbs of CO2.
In addition to production emissions, it takes about 18 MJ/cycle to run a dishwasher. According to Treehugger.com, a household dishwasher is used 215 times per year on average. With a 15 year lifespan, the average dishwasher will emit over 21,600 pounds of CO2.
Emissions from transportation are hard to calculate due to the many unknown factors: distances traveled by component materials as well as the final product, transportation modes, weight of load on each trip, fuel efficiency, etc.
According to my research, it is highly likely for a US dishwasher to be produced domestically in the US using a majority of US-made components. This is pleasantly surprising in light of the rise of the global economy pushing manufacturing overseas, but there are still carbon emissions involved.
Since all houses have a sink no matter what, I decided its production was negligible (versus a dishwasher which is an optional appliance that some homes do not have). We can still look at the carbon emissions from the water.
Water and Heating Water
I couldn’t find something specific to the US, but I doubt there’s much difference. This article states 0.59 grams of CO2 are required to produce one liter of tap water in the UK. This converts to 0.005 lbs of CO2/gallon.
This worksheet on the cost of heating water can be used to calculate that a gas tank heater at only 59% efficiency requires 847 BTUs to heat a gallon of water from 60°F to 120°F. This converts to 0.25 kWh/heated gallon or 0.34 lbs of CO2/heated gallon.
This worksheet does conflict, however, with Treehugger’s calculations for heating from 60°F to 120°F that show “heating the water with gas for each 2-gallon load emits about .17 pounds of carbon dioxide.” They use a higher efficiency and higher kWh to CO2 conversion, but that would result in 0.38 pounds per gallon using the worksheet so I’m not too sure who to believe. The Treehugger article also says heating with a tankless water heater results in 0.07 lbs of CO2/gallon.
The Guardian put out an article listing the carbon footprints of various washing methods:
- Almost zero CO2e: by hand in cold water
- 540g CO2e: by hand, using water sparingly and not too hot
- 770g CO2e: in a dishwasher at 55°C (131°F)
- 990g CO2e: in a dishwasher at 65°C (149°F)
- 8000g CO2e: by hand, with extravagant use of water
In this case, handwashing is the greener choice so long as you are careful about how you wash (not leaving the water running, mindful of temperature, etc.). The Guardian does make a case that water must be very hot or else bacteria persists. They provide a statistic but have no source. I searched around and this statistic popped up on many other sites, but not one linked a source so I have no idea where it came from.
Note: CO2e is a carbon dioxide equivalent which converts the effects of other greenhouse gases like methane into the equivalent amount of CO2.
Dishwashers are made using several types of materials, the main component being a type of plastic called polyvinyl chloride (PVC). Plastics are made from oil, a non-renewable resource with harmful extraction processes.
Due to the chlorine used to create PVC, PVC releases harmful chemicals called dioxins into the air during production and incineration at end of life. Dioxins disrupt biological processes (hormones, development, immune and endocrine systems) and are carcinogens.
There have been large reductions in dioxin emissions over the past few decades due to stricter standards in developed countries, but dioxins are still a large problem in other countries like China. A 2007 study in China showed people who lived near e-waste recycling facilities had much higher concentrations of dioxins in their bodies than other people.
Dioxins released into the environment are long-lasting and can enter the food chain. Since humans are at the top of the chain, they consume higher concentrations of dioxins.
PVC itself is a highly durable material that does not easily break down in the environment. Like all plastic, it will degrade over centuries into smaller and smaller pieces, but it will never truly return to organic components.
Incinerating PVC also releases hydrochloric acid, lead, and cadmium. Lead and cadmium are found in the ash after burning, which gets sent to landfill and could potentially leak into groundwater.
How important is hot water? Some websites will say hot water (such as that of a dishwasher) is the only way to properly clean plates. In fact, The Guardian’s article on the CO2 emissions of handwashing and using a dishwasher actually says “(but the plates aren’t clean)” if you use cold water to wash.
Most people can only tolerate 110°F for short periods of time, and the EPA recommends boiling water for 1 to 3 minutes to effectively purify water of bacteria. Although hot water helps activate soap/detergent to remove stuck on food, oils, and grease that can harbor bacteria, the temperature of water used either by the dishwasher or the sink itself does not make much of a difference in killing bacteria.
There’s also the practicality of needing an hour-or-more-long, superhot cycle to wash a dish. Most dishes, in my opinion, are not filthily dirty with bacteria and grime that they would require the dishwasher’s intensity (you did literally just eat off the thing, didn’t you?). A quick rinse to wash away some crumbs or scrubbing and soaping away food bits with warm water seems like a much better alternative than leaving the dish to wait around until you fill up the dishwasher and run a cycle.
The Bonn University study also specifically looks at how a group of random people do dishes instead of people who are trying to lower their environmental impacts. For the sake of showing in the average case what method is better, that’s fine. But could conscious consumers use less water and energy than a dishwasher?
Dishwashing v. Handwashing Experiment
It’s experiment time! Many articles and studies say people usually use way more water than a dishwasher does. They make it sound hard to come even close. So I wanted to see if I could wash a dishwasher’s worth of dishes by hand using less water than the dishwasher would. If I failed, I wanted to measure by how much. I also wanted to document my washing process to share how I limit my water usage. As a side note, I do not have a dishwasher in my apartment so I must wash all dishes by hand no matter the outcome of this experiment.
Thanks to AP Chemistry in high school, I still mostly remember how to fully write up an experiment. Enjoy the nerdiness!
To find out how many dishes you can wash with less water than a dishwasher uses.
If handwashing a set of dishes comparable to what a dishwasher is said to hold uses less than 6 gallons of water, then it is better than dishwashing.
- Scrub brush
- Dish soap
- Pot for holding water (and to wash, as shown below)
- Dirty dishes
- 1 pot (4 qt)
- 1 large plate
- 4 small plates
- 1 short glass
- 1 tall glass
- 2 metal straws
- 1 bread pan (9×5)
- 4 bowls
- 1 ice cream bowl
- 1 glass bowl
- 4 small containers (1.25 c)
- 1 tiny container (0.5 c)
- 1 sandwich container (3 c)
- 3 big containers (5 c)
- 1 tiny bowl
- 1 small lid
- 4 big lids
- 1 spatula
- 1 paring knife
- 7 butter knives
- 13 spoons
- 5 small forks
- 4 single serve yogurt cups
- 1 takeout container with lid (around 4 c)
- 1 big yogurt container (32 oz)
- Place pot under faucet
- Quickly wet any dry dishes allowing used water to be captured in pot
- Soap all the dishes with soap using the scrub brush
- Rinse dishes under faucet and let all water drain into pot
- Record how many times the pot fills with water before all dishes have been washed
- Calculate total amount of water used
- Compare to a dishwasher (scale to a similar number of dishes)
Total Water Usage
I filled up the big pot approximately 2.5 times, but I also need to add on additional water for all the water that the dishes were soaking in while sitting in the sink waiting for me to wash them. This would probably be another 1.5 gallons (pot-fulls), leaving me at 4 gallons.
Scaled water usage: I washed over the 54 individual pieces the EPA says a dishwasher holds. I washed a bunch of random stuff like recyclables and containers, but I did 25 pieces of silverware, 6 bowls, 5 plates, 2 glasses, and a serving utensil. I believe all the other stuff I washed definitely counts as at least the missing 2 bowls, 3 plates, 6 glasses, and 5 serving utensils.
So in total, I used around 4 gallons. This is an assumption though since I couldn’t/didn’t know how to measure the water used to rinse and soak the dishes each time we put more in the sink prior to washing. Despite this, it’s assuredly less than the 6 gallons an average dishwasher uses.
I couldn’t hold a constant temperature because I turn the water off and on repeatedly. It ranged from cool to hot, but I tried to avoid really hot as much as possible.
I don’t know how I would have measured soap usage since I don’t have a kitchen scale, but I believe I am more of a heavy soap user.
The total time I spent washing dishes was about 1 hour.
Conclusion – Who’s The Real Winner?
From water use to energy use to carbon emissions, I think it’s safe to say I beat the dishwasher. From the standpoint of convenience, the dishwasher wins. If you are very concerned about hygiene, the dishwasher may get your dishes cleaner.
If you’re stuck doing the dishes (and we all have to handwash some things like pots and recyclables), try either my method of soaping then rinsing by turning the faucet on and off with each dish, or filling up separate soapy water and rinsing water bins/sides of the sink to minimize water use. I personally don’t think the hot water makes much of a difference, but to each their own.
If you have a dishwasher and want to use it, go right ahead, especially if you don’t want to be constantly thinking about saving water and energy while still getting the dishes as clean as they can be. If you have a shorter or eco cycle, use that over the more intensive washes.
Although I believe I’ve found the winner in the dishwashing v. handwashing debate, make the best choice that works in your situation. What works for me may not work for you, and that’s ok!
Who do you think the winner is? Is there anything I’ve missed? Let me know in the comments!