The science of dishwashers vs handwashing

September 15th, 2015

Appliance Talk Dishwashers Environment, Energy & Water Saving Small Appliances

You may have heard it said that washing dishes in a dishwasher is actually more water-efficient than washing by hand. This statement is fairly controversial, as anecdotal evidence tells us that washing by hand puts you in complete control over how much water you use. Plus, a dishwasher’s much bigger than a sink!

So let’s look at the science – in this case, a study undertaken by Germany’s Bonn University that put the dishwasher to the test against hand-washing:

Euromaid DR14W Dishwasher

“As result out of the investigation of 113 persons it is found that the average consumption to clean 12 place settings of dishes was measured to 103 litres of water, 2.5 kWh of energy and 79 minutes time.”

That figure of 103 litres sounds like a lot, and it is. To put it in context, the Grand Bowl of the premium Oliveri Professional Series Sink PR1112 has a maximum capacity of 46L. Unless you were to fill this sink right to the brim, you wouldn’t use this amount when doing the washing up.

So why the 103L figure? According to the study, this average was based on the dishwashing habits of 113 test subjects from 7 European countries in laboratory conditions. The study proved what we always suspected – that everyone washes up differently. While some test subjects used less than 20L of water to hand wash their dishes, others left the taps running and used more than 440L!

“When these data are compared with values of a new A/A/A dishwashing machine (best machines according to European Energy Labelling scheme taking about 15 litre of water and 1.05 kWh of energy for a full load), possible savings get evident.”

In the years since the release of this study, dishwasher technology has continued to improve beyond the findings of this study, especially in terms of water and energy efficiency. For example, selected Smeg dishwashers use as little as 9.8L of water per cycle:

Smeg DWA315X Dishwasher

Remember when buying a new dishwasher that the figure listed for the water usage per wash is usually for the average load only. Your results at home may vary, depending on the size of your dishwashing load and your choice of wash setting (e.g. an Eco wash will typically use less water than an Intensive wash).

“For a full load this results in 88l water per cycle or about 32 000 litre a year saved when the dishwasher is used once per day fully loaded. As a bath tub takes about 160 litres this can be translated in 200 filled tubs which may be saved per year by changing to a dishwasher!”

Remember that the kitchen isn’t the only place where you can make big water savings – there are plenty of changes you can make to the laundry too.

“Also time saving may be a big argument, as it takes only about 15 minutes to load and unload a dishwasher. Therefore a time bonus of 64 minutes per day may be accounted for spare time. 64 minutes saved per day are about 400 hours per year or equivalent to 16 full days as additional time for doing something better than washing dishes per hand!”

source: Ryan McFarland on Wikimedia Commons

While some people may argue that loading/unloading a dishwasher actually takes more time than washing by hand as you still need to pre-rinse the dishes and dry them afterwards, that’s not quite accurate. Pre-rinsing has been proven to be ultimately unnecessary (the dishwasher detergent needs some grot to stick to, so a rinsed plate will actually get a wordse clean than a dirty one), and many modern dishwashers include a drying cycle at the end of their wash – just remember to open the door to release the steam.

“Additionally, average cleaning results were significant better for washing dishes by machine rather than by hand.”

To expand on this last point a little, the study rated the cleanliness of the post-wash dishes from 0 to five, where 5 stands for perfect cleanliness. The study found that more than half ofthe test subjects did not reach the minimum level of cleanliness required for the dishes to be acceptable on the dinner table, that being 3.5.

By comparison, the dishwashers used in the test achieved cleanliness results of between 3.3 and 4, and when switched to the Intensive cycle, they reached 4.3 and 4.4 – nice work, dishwashers!