And How Much Electricity Does It Use?!
I have been asking myself these questions for far too long. Have you? We’re constantly being told that dishwashers save water – it is often among the top ten ways to go green on various internet lists. But is it true?
Finally, I did some research. Here’s what I found…
The Waterwise Study
In 2006, Waterwise studied the amount of water used in hand washing versus machine washing. According to their website, “Waterwise is a UK NGO focused on decreasing water consumption in the UK by 2010 and building the evidence base for large scale water efficiency. We are the leading authority on water efficiency in the UK.”
I could not find the study online, but according to several places it was quoted, the study found the average dishwasher uses between 12 and 16 litres (3-4.25 gallons) of water, while washing dishes by hand uses as much as 63 litres (16.5 gallons).
But the study failed to address the energy efficiency between the two methods…
The University of Bonn Study
The University of Bonn also conducted a study of 113 people from 7 European countries (PDF), comparing their water usage with a dishwasher and without.
Interestingly, they categorized the different hand washers as “super dishwashers” (people who preclean, soap clean, and rinse), “dishwashing economizers” (people who squirt detergent on the sponge and try to use as little water as possible), and “care-free dishwashers” (people who used as much water and detergent as they wanted to without thinking about it). But noted that economizers didn’t always end up using less than the other two groups.
Each person washed 12 place settings. On average, hand washing used 27 gallons (103 liters) of water, and 2.5 kWh of water-heating energy. The human time it took to wash, rinse, dry, and put away was approximately 80 minutes.
The dishwasher used approximately 4 gallons (15 liters) of water, consumed 1-2 kWh of total energy, and required 15 minutes of human time (loading and unloading the dishwasher). All in all, the dishwashers got the dishes cleaner, in less human time, using less water and energy.
The study recommended these tips for maximum efficiency:
- Remove large food scraps from the dishes with a spoon or fork.
- If hand washing, wash right away before the food sticks. If washing by machine, the machines can clean dishes that have been stored without cleaning or rinsing for several days, so no need to pre-rinse.
- Do not pre-rinse the dishes under running tap water, whether washing them by hand or in a machine.
- Manual dishwashing is best in two sinks: one with hot water and detergent, the other with cold water for rinse. (The study noted that those who were “economizers” tended to use a LOT more detergent, which counteracted their low water usage.)
- Use the amount of detergent recommended by the manufacturer.
- If you can afford a machine, use one – preferably a new one that is the most energy-efficient.
The Environmental Protection Agency
The information provided by the EPA is in line with the previous study’s findings. It is unclear where the EPA received their information, but I do hope they’ve done their own research! Here’s what they say:
Washing dishes by hand uses much more water than using a dishwasher. Using an ENERGY STAR qualified dishwasher instead of hand washing will save you annually 5,000 gallons of water, $40 in utility costs, and 230 hours of your time.
Note also that the average Energy Star dishwasher uses 4 gallons of water, where the average non-Energy Star dishwasher uses 6 gallons.
The EPA recommends these tips to maximize your dishwasher’s energy efficiency:
- Run your dishwasher with a full load. Most of the energy used by a dishwasher goes to heat water. Since you can’t decrease the amount of water used per cycle, fill your dishwasher to get the most from the energy used to run it.
- Avoid using the heat-dry, rinse-hold and pre-rinse features. Instead use your dishwasher’s air-dry option.
- Scrape don’t rinse: just scrape food off the dishes and load. ENERGY STAR qualified dishwashers and today’s detergents are designed to do the cleaning so you don’t have to pre-rinse. If your dirty dishes are going to sit overnight, use your dishwasher’s rinse feature. **Pre-rinsing dishes before loading the dishwasher can use up to 20 gallons of water, where the pre-rinse cycle on a machine uses just 1-2 gallons.**
- Using a dishwasher that is made in the past few years (or at least after 1994), and one that uses a booster heater (ie, it heats the water on demand vs using your water heater), you can save even more water and energy.
This is great, and certainly caters to my want to be told that it’s more energy efficient to be lazy. I’m convinced that using a dishwasher is more water efficient than hand washing, unless you are hand washing and then trapping your water for use in watering plants or flushing the toilet (which a few people I know do). Of course, best of all would be to trap the water from the dishwasher and use the grey water on plants or to flush the toilet.
But… I’m not convinced about energy yet.
Problem #1: Have you ever bought an Energy Star compliant appliance and looked at one of those yellow cards that tells you its efficiency? If you haven’t yet done so, turn over the card to see the European standards for the same appliance. European standards are much higher. Many of our Energy Star appliances wouldn’t pass their energy inspections.
In other words, the two studies were done in Europe, where appliances are smaller and more efficient than in the U.S. This article tackles this problem more in depth. Make sure to buy the most energy efficient appliance you can afford.
Problem #2: These studies haven’t taken into account the energy and water used to manufacture a dishwasher. And once they are no longer wanted, how much energy is used to get rid of them?
Turns out I’m not the only one asking these questions. According to Smart Planet, up to 80% of the dishwasher can be recycled, if done properly. But the question about manufacturing energy still remains.
No studies have been done about this, at least no studies I could find on the internet. So until there is a study, we’ll have to do our best with the information we have:
*Rules Of Thumb For Machine Washing*
1. Run the dishwasher only when it’s full (but not over-full),
2. Scrape dishes clean but don’t pre-rinse under the tap,
3. Always air dry,
4. Follow the manufacturer’s advice for getting the most efficiency out of the machine.
*Rules Of Thumb For Hand Washing*
1. Wash right away, before the food sticks,
2. Scrape dishes clean but don’t pre-rinse under the tap,
3. Fill 2 sinks with water: one for soapy warm, one for cold rinse,
4. Capture the water and use as grey water for watering plants or toilet flushing.
What Do You Think?
Are you convinced? How do you wash your dishes? Has this changed your mind at all? Have you read any other studies that can help us all decide what to do?