BNW16: A comparison of washing up by hand with a domestic dishwasher
This brief and referenced information is a public consultation document and will be used to inform Government decisions. The information and analysis in the brief forms part of an integrated, public domain knowledge base that is managed and held by Defra’s Market Transformation Programme. The policy scenarios and action plans are illustrative, intended to stimulate discussion and do not imply commitment by Government nor by any other body.
1. Scope
Washing up is rarely examined scientifically. This briefing note compares the energy and water consumption for washing up by hand with a domestic dishwasher. The conclusions take into account real use of a dishwasher and that washing up habits vary widely.
2. Summary
§ Water consumption is much less in a dishwasher – 3 to 4 times less than by hand.
§ The energy consumption comparison depends on the precise washing up habits, loading of the dishwasher and how the hot tap water is heated.
§ Items cleaned in a dishwasher are cleaner than when washed by hand.
§ Barriers to UK ownership include lack of space and wrong perceptions.
§ Washing up habits vary widely.
3. Research
Some of the analysis in this Briefing Note is based on a study carried out by Bonn University [1]. This briefing note will concentrate on UK data; data from other countries is for interest. Dishwashing habits (manual or machine) vary widely and therefore make any conclusion open to debate. Actual energy and water savings clearly depend on individual habits.
The Bonn University team observed the habits of users in seven countries across Europe. Water consumption, energy used and the effectiveness of cleaning were studied. This also allowed for a comparison with an automatic full size (12 place setting) AAA dishwasher. (A – energy consumption, A – cleaning performance, A – drying performance).
The test was similar to the standard test normally carried out for the energy labelling of domestic dishwashers. 140 items with seven different soils - egg, spinach, oatmeal, milk, minced beef, tea and margarine were prepared and dried in air for two hours. Volunteers washed these items while their water consumption, energy, task duration and quantity of washing up liquid were measured. After washing, items were assessed for cleanliness.
The following graphic shows resource consumption for the European study. Average and range of results are indicated. For each country / region, the first bar is cleaning index, the second bar is water consumption and the third bar is energy consumption. The long range bars for each result show that washing up habits within one country are extremely variable. As with many consumer habits, no one is really “average”, people adopt different habits often without thinking or varying what they do. Habits across Europe vary widely too. Summary of Average Results 4. Cleaning The cleaning index is assessed in exactly the same way as for dishwashing appliances. Each item is given a score (0 to 5) according to the amount and size of soil in accordance with EN 50242, the domestic dishwasher standard. (5 means no residue of soil, 0 means total soil residue > 200 mm2).
For the UK the cleaning index is less than 3 and between 3 and 3.5 for other European countries. An A class cleaning performance dishwasher would have a cleaning index of approximately 4. From this study, we (the U.K.) are the dirtiest in Europe!
5. Water Savings
The average UK water consumption is 63 litres. The water consumption for a dishwasher is typically less than 20 litres and often less than 15 litres. So even a conservative estimate means that washing the same load by hand uses at least three times more water. Those who wash all or part under running water could be using more than 150 litres. This additional water is used for pre-soaking before washing and/or rinsing after washing. The considerably lower water consumption for a dishwashing appliance might infer a detergent residue problem. According to Unilever [2], detergent residues are either zero or too small to be measured owing to the high dilution.
WATER CONSUMPTION: DISHWASHER vs WASHING UP
The chart above compares water consumption for washing up by hand with a dishwashing appliance. A dishwasher uses the same amount of water no matter what loading whereas washing more items by hand demands more water depending on frequency of changing the bowl water. For this comparison it is assumed that the bowl water is changed 9 times to wash 140 items.
Even washing 4 place settings by hand is more wasteful than a dishwasher. The case for using a domestic dishwasher to save water is very strong.
6. Energy Savings
For the Bonn study the water was heated via an instantaneous electric water heater. This Briefing Note assumes a more typical system via a standard gas boiler. 75% of UK homes are heated via a gas boiler with a typical 70% efficiency. The energy required to heat 63 litres of washing up water from 15°C to 55°C is 4.2 kWh [3]. For comparison, the energy consumption of a C-class dishwasher on a 65°C wash is assumed to be 1.3 kWh [4]. For comparison with gas water heating, this is multiplied by a “primary energy factor” of 2.5 [5].
ENERGY CONSUMPTION: DISHWASHER vs WASHING UP
The chart above compares energy consumption for washing up by hand with a dishwashing appliance. A dishwasher uses the same energy no matter what loading whereas washing more items by hand demands more energy depending on the amount of hot water actually used.
The value for water heating does not take into account inefficiencies (eg. heat losses) other than the gas boiler because this is so dependent on individual home installation. At home, users will not ensure their water is at 60°C. After running off cold water, whatever available hot water will be used.
From the energy consumption graph above the cross-over between manual washing up and dishwasher appliance is nine place settings. The argument for using an appliance to save energy is therefore weaker especially when considering that most dishwashers are only loaded to about 7 place settings [6]. A dishwasher either looks full from not being loaded efficiently or has a few larger “cookware” items which take space or the dishwasher is run part full simply because the user would like to use some items fairly soon.
However, more efficient dishwashers and lower temperature washes proffer a potential saving. For an A energy appliance, the cross-over occurs at 7 or 8 place settings.
Slimline dishwashers (8 or 9 place settings) use nearly the same energy so there is no energy advantage if they are similarly under loaded.
Other Factors
The real energy cost to consume a quantity of water has not yet been considered. For a complete argument, additional energy cost for supply and disposal of water should be included. This benefits the argument to use a dishwasher appliance.
A further issue sometimes not considered from a pure resource argument. Washing by machine is generally more effective and more hygienic than by hand. The hot water can be consistently around 60°C. For washing up, UK users tend to run the hot tap only but this quickly cools.
7. Barriers to Dishwasher ownership
Current UK ownership is around 27% of homes [7]; the remaining 73% may not have a dishwasher for the following reasons:
§ Cost, not all families are going to have a dishwasher high on their priority list albeit that cost of some appliances begins around £200.
§ Some perceive these appliances are wasteful with energy and water and not good at cleaning.
§ Space, many UK homes do not have sufficient kitchen or utility space for another appliance.
For comparison, dishwasher ownership in Germany and the Netherlands is around 50%.
8. Recommendations / Conclusions
There is no significant reason not to encourage dishwasher ownership on the grounds of water and energy use or performance.
To save energy, users should load their dishwasher appliance more fully and use the energy label programme which usually washes at a lower temperature. Since these may be up to 3 hours long, some consumers may prefer a shorter programme which is hotter to clean effectively. Other users run their appliance overnight and programme duration is not an issue. (Running any appliance overnight can be cheaper and creates a more even energy demand. However some consumers are concerned about noise and safety when running appliances unattended).
With feedback, questions or comments please contact [email protected] call the MTP enquiry line on +44(0)845 600 8951, quoting the document reference.
[1] “A European Comparison of Cleaning Dishes by Hand” Rainer Stamminger et al.
[2] Discussion with Willem Oldenburg, Unilever
[3] From Energy = heat capacity x temp rise / (3.6 x 106 x 70%)
[4] MTP Briefing Note BNW07 Assumptions underlying the energy projections for dishwashers
[5] Discussion with Bruce Young, BRE
[6] “Decade 2 MtC” page 63
[7] MTP Briefing note BNW07 Assumptions underlying the energy projections for dishwashers.
