Many people believe the way they wash dishes uses far less energy and water than using a dishwasher. Others believe we are no match and should let the machine do the work. So, which is it?
Let’s start with hand washing. The amount of water you use while hand washing dishes will depend on how much or how little you open the valve as well as how much time you leave the valve open for. The longer it takes you to wash dishes, the more water you will need. On the other hand, a dishwasher uses a predetermined amount of water based on the cycle you select. An older, less water efficient dishwasher will use about 7.2 gallons to wash a full load of dishes. This means to answer the question of “which uses less water?” we need to see if we can wash a full load of dishes by hand with less than 7.2 gallons of water.
To figure this out, we need to know which types of dishes we will be washing, how many of each dish we will be washing, the amount of time it takes to wash each of those dishes, and we also need to know the flow rate of our kitchen sink. To start, I looked inside my own dishwasher when it was full and recorded what was inside. So, the next thing to do is to find the flow rate of my kitchen sink. To find the flow rate of your kitchen sink grab a container you know the volume of or that has measurement markers on it. Then, grab a stopwatch and record how long it takes to fill the container or to reach a certain volume. Convert the amount of water you collected into gallons and the amount of time you recorded into minutes. Last, divide gallons by minutes to find the flow rate. If you’d like a bit more help finding the flow rate of your kitchen sink, check out my article on finding flow rate.
As it turns out, the flow rate of my kitchen sink is 1.8 gallons per minute. Next, I washed each dish and recorded how long it took me to wash it. Using all this information, I calculated that my sink used nearly 50 gallons of water to wash one full load of dishes which is much higher than the 7.2 gallons my dishwasher would have used. However, the times you see listed here are the total time it takes to wash a dish not the amount of time I actually needed to leave the sink on. By going back into the video, I can see the amount of time that I actually needed the water to figure out what the impact would be if i had shut the water off while washing dishes and by doing this, I found that my sink would still use over 17 gallons of water.
Even if I lowered the flow rate to just one gallon per minute, the sink still uses more water. There is a way to use less water when washing by hand and it’s known as the dual sink method. The dual sink method takes advantage of a sink that has two basins. By filling one basin with water and using it to scrub dishes and then using the other basin to rinse dishes you can use even less water. I went back to the video once more to see how much water this would save me, and there you have it. We found a way to use less water when washing by hand, but this is only if you are mindful about the way that you use water. For most people, this is going to take too much effort and so as a rule of thumb a dishwasher will almost always use less water.
But of course when it comes to sustainability we have to talk about the energy required to heat the water for the dishwasher and kitchen sink. Lust like before how a dishwasher uses a predetermined amount of water, it will also use a predetermined amount of energy. So, we need to see if we can wash enough dishes by hand using less than that amount of energy. If you go through the calculations you see that we can use about 26 gallons of water when washing by hand before it would become more energy efficient to use a dishwasher. If you remember from a moment ago shutting the water off when washing dishes uses about 17 gallons. So it is possible that washing by hand could be more energy efficient, however today’s dishwashers are much more energy efficient than the one used in this study and often have features to save energy such as turning the drying cycle off. With these energy saving features, an energy efficient dishwasher and the dual sink method will use roughly the same amount of energy.
To summarize, anything is better than leaving the water running when washing dishes by hand. In almost all cases, a dishwasher especially today’s energy efficient ones, will use less water. If you do choose to wash dishes by hand it is possible to use less energy and water if you use the dual sink method, but only compared to older, less water efficient dishwashers.