Saving resources without sacrificing results: an empirical investigation of the dishwashing reality of British consumers in an international comparison

1 Introduction

The continuing rise in greenhouse gas emissions will lead to considerable damage in many parts of the world in the coming decades. ¹ The goal of reducing greenhouse gas emissions can only be achieved if all sectors that contribute to these emissions act responsibly. Households have an important role to play, as they consume a relevant proportion of global energy and water resources. ^{2 , 3} They form socio-technical systems that involve a complex interplay between a variety of factors and stakeholders, including manufacturers, technologies, regulators, consumer practices, cultural norms and infrastructures. ⁴ Routine household tasks, such as doing the laundry or washing the dishes, are all components of such a socio-technical system. ⁵ It is important to understand how social processes influence the growth and demand for natural and non-renewable resources. There is an urgent need for research to focus more on the way people act while consuming water and energy in their private environments and the reasons and conditions that influence their habits. Although this is a global task, it is recognised that consumer habits and practices vary in different parts of the world, between countries and even within countries. In this paper, we will focus on the dishwashing infrastructure available and the dishwashing behaviour of consumers from the United Kingdom (UK) and try to shed some light on the subject. At best, we will look for ways in which consumers can reduce the energy and water consumption of dishwashing with the existing infrastructure (i.e. the base of dishwashers installed and dishwashing detergents available) without compromising the performance achieved.

Early studies in the 1990s reported that dishwashers were used on average 4.9 times per week and consumed an average of 1.96 kWh for a 65 °C cleaning cycle, compared to 1.34 kWh for a 50 °C cycle. ⁶ At that time, 17 % of the households in the UK were reported to own an automatic dishwasher. This has increased drastically with a representative survey of 250 respondents in 2007 reporting that 75 % of households had an automatic dishwasher. ⁷ According to the data, 80 % of these dishwashers are less than six years old. Of the households that do not own a dishwasher, 61.9 % cite a lack of space in the kitchen as the main reason. More than half (56 %) of the consumers who own a dishwasher rate its influence on household energy consumption as high or very high. This proportion is significantly higher than the proportion of households without a dishwasher that estimate this influence (42 %). An average of 4.5 cycles per week are performed in the dishwasher, of which 12 % are done at a cleaning temperature of 70 °C, 39 % at 65 °C, 29.5 % at (50–55) °C and 19.5 % at (35–45) °C. The average temperature is calculated as 58.6 °C.

This data is roughly confirmed by the fact that in a consumer observational study of dishwasher users, an average temperature of 60.1 °C was recorded in a diary of each wash cycle performed by the 40 consumers over a two-week observation period. ⁸ This study also showed that in the UK about 9 % of the total number of dishes cleaned in a dishwasher have been pre-rinsed. This pre-rinsing takes an average about 4.4 L of water per dishwasher cycle for the UK (compared to 19.7 L for Italy)! This shows that it is not only important that the infrastructure is optimised, but also the performance must be provided to avoid such dispensable additional resource use.

It was shown in an empirical study with a representative sample of 150 consumers from the UK in 2010 that those who washed their dishes by hand used an average of 49 L of water and 1.7 kWh of energy, while the dishwasher at that time used an average of 13 L of water and 1.3 kWh of energy for the same amount of soiled dishes under the conditions tested. ⁹

In 2016, an online consumer behaviour study with 600 representative consumers from the UK revealed that 32 % of UK consumers normally pre-rinse their dish items under the tap. ¹⁰ The reasons given by 53 % of consumers for pre-rinsing were to avoid bad odours, 43 % were concerned that the dishwasher could not cope with the amount of soil on the dishes and 37 % thought that the dishes might not come out clean in the dishwasher without pre-rinsing (multiple answers were possible). ¹¹ Overall, the average number of dishwasher cycles in this study was reported to be 5.3 per week and the programme use is divided between eco programme: 19 %; (45–55) °C: 24 %; (60–65) °C: 17 %; (70–75) °C: 8 %; auto/sensor: 8 %; gentle: 5 %; short/quick: 12 % and rinse and hold: 12 %. This shows that there is a vast diversity of programmes offered by newer dishwashers, and that consumers seem to be overstrained to select the most appropriate programme, especially when combining the requirements of achieving the necessary performance at the lowest resource used. These requirements were identified as the number 1 (performance) and number 3 (low energy and water consumption) of the most important features a dishwasher should have out of a list of 19 different given features (number 2 was low purchase price). In this study, the consumers ’ reasons for using short/quick programmes are also investigated in more detail. Nearly two-thirds (64 %) of the consumers in the UK claim they do it to save energy and water, 40 % because the dishes are only slightly dirty, 16 % because they do not have time and 5 % out of habit (multiple answers allowed). It is very surprising that the time argument only plays a minor rule, while it is fully understandable that slightly soiled dishes do not need a long-running normal programme. The assumption that short or fast programmes are used to save energy and water is mostly incorrect. ¹²

All of this shows that there is still a great lack of understanding of the socio-technical systems of automatic dishwashing in sufficient detail for the UK, and especially to find the right recommendations for optimising this system to achieve relevant resource savings without having to ask for a complete refurbishment of the existing infrastructure. This task can only be fulfilled if, on the one hand, the base of dishwashers installed in the UK is known, including their resource use and, on the other hand, the behaviour of the consumer when using these dishwashers is known in a detailed and representative way. If both are available, it will be possible to model the average use of resources (e.g. per household and per week) and, in a next step, to simulate what a changed, optimised consumer use of the dishwasher base installed would look like and how many savings could be realised. All of this needs to be done under the overarching target of maintaining the performance of the dishwasher.

2 Materials and methods

3 Model of the resource use of dishwashers

By combining the consumption data of all 1704 datasets of programmes with the relative market share of these brands, a representative model of the average energy and water consumption and programme duration of the dishwashing programmes of dishwashers installed in UK households is built, called the “UK model”. Table 10 shows the average resource use and duration for each of the seven programmes with and without the use of the modifier “express/speed/quick/time-saving” for each programme. The ECO programme, which is one of the programmes recommended for cleaning normally soiled dishes (see Table 1), consumes an average of 0.87 kWh and 9.9 L. All the other programmes consume considerably more resources. However, the ECO programme also takes the longest duration of about 3½ h. This is explained by the fact that in the ECO programme the wash and drying temperatures are reduced to save energy, while the wash and drying times are extended to achieve the required performance. However, these dependencies are not well-known to consumers. ¹²

Combining this with the probability that a certain programme is selected (Table 2) and, eventually, the modifier “express/speed/quick/time-saving” is used (Table 4), it is possible to calculate the representative household average of the consumption values of the programmes of dishwashers installed in UK households (Table 10).

The average consumption of 1.16 kWh of energy and 12.7 L of water is considered the base case for this study as it best represents the average use of resources of dishwashers per cycle in UK households. The average energy and water consumption in the UK is 33 % higher for electricity and 29 % higher for water when using the ECO programme was used alone. Compared to automatic dishwashing in other European countries, ¹³ the consumers in the UK use the Eco programme and the Normal programme slightly more often. The average consumption data agree very well with recent data from another study, which found that the average energy consumption of the dishwashers on the UK market was 1.11 kWh of energy and 12.35 L of water for an average programme duration of 129 min. ¹³

From the consumer survey, it is known how consumers rate the degree of soiling of the dishes cleaned in the selected programmes. This makes it possible to present the average energy consumption of the programmes used for the levels of soiling encountered by the user (coding: light soiling = 1, normal = 2, heavy = 3). This shows (Figure 1), on the one hand, that the consumer makes a clear distinction when selecting a programme (especially for the intensive programme) and, on the other hand, that there is a wide range of programmes available for almost the same level of soiling between ∼1.6 and 2.0.

Figure 1:

Average energy consumption of the programme versus the average soil level of the dishes (coding: lightly soiled = 1, normal = 2, heavy = 3). For short names of programmes, see the caption of Tables 1 and 10.

4 Simulation of savings

The base case of the scenario simulation is the situation of the dishwasher as installed in UK households (via the “UK model”) and the consumer behaviour as assessed by the consumer questionnaire (representative consumer survey). Both together give an average consumption of 1.16 kWh of energy and 12.7 L of water per cycle.

Based on this, a scenario can be imagined where the intention is to use only those programmes for a given soil level that use the least amount of energy (Figure 1). Firstly, this means that the use of the modifier “express/speed/fast/time saving” should generally be avoided, as it significantly increases resource consumption. This would mean, for example, stopping the use of the “IntS” program and instead moving those program uses to the “Int” program. The same applies to all other uses of this modifier. Secondly, the use of the gentle ‘Gen’ programme should and could be shifted to the quick low temperature  ‘Qul’ programme, as the levels of soiling encountered by consumers are quite similar and both programmes are recommended for lightly soiled dishes. Thirdly, the use of the normal/regular/daily (Nor), automatic/sensor (Aut) and quick high temperature (Quh) programmes should be replaced by the ECO (Eco) programme (Figure 1), as all these programmes are defined to clean normally soiled dishes. This seems justified for the  ‘Nor’,  ‘Aut’ and  ‘Quh’ programmes, where consumers already associate a similar level of soiling, but not for the  ‘Eco’ programme.

However, the “Eco” programme is the one where most stringent requirements on cleaning normally soiled dishes, for example, soiled with sauce residues or teacups, are requested. This programme is used as the basis for the Ecodesign requirements requested by Commission Regulation (EU) 2019/2022 of 1 October 2019 (and earlier versions) (valid also after the withdrawal of the UK from the European Union) and requests that “the cleaning performance index shall be >1.12”, i.e. the cleaning of dishes (including pots), soiled with seven types of food and dried in for 2 h at 80 °C, must be 12 % better than the reference programme on a Miele reference machine. This “Eco” programme is also the one used by Which? to assess the cleaning performance of dishwashers under test conditions. Thus, it is well-suited for cleaning normally soiled dishes.

Figure 2:

Average energy consumption of the programme versus the average soil level of the dishes. Coloured marks and circles show the most energy-saving strategy of shifting programmes.

Only three programmes remain (Figure 2) when this is done: eco, intensive and quick (low temperature) and the modifier “express/speed/quick/time-saving” is no longer used. This results in (Table 11) an average energy consumption of 0.91 kWh and water consumption of 10.4 L, corresponding to savings of 21.3 % in energy and 18 % in water consumption. This is at an expense of 60 min prolongation of the programme duration (from an average of 128 min to 188 min).

This can be identified as the most energy-saving scenario, maintaining the association to the soil level by the consumer and not requesting an unacceptable prolongation of the average programme duration.

Implementing this learning in consumer communication would require the following messages to be conveyed:

1. Do not use the programme modifier “express/speed/quick/time-saving” as it may require significant additional resources (energy, water).

2. Use the ECO programme for all normally soiled dish loads and accept the longer programme duration, as this will provide a good cleaning with a low amount of resources (energy, water).

3. Use the Quick/fast (45 °C, Jet, 30′, express, …) programme for lightly soiled dishes.

A comparison of these results with data from other European countries ¹⁴ (Figure 3), where similar observations and simulations have been carried out, shows that energy and water saving options are of a similar magnitude. This means that the saving options can be implemented at least on a European level.

Figure 3:

Comparison of the savings possible for UK with results of similar approaches in other European countries (HU = Hungary, PL = Poland, DE = Germany, IT = Italy, ES = Spain, TR = Turkey).

5 Discussion and conclusion

The research question was to understand the socio-technical systems of automatic dishwashing in sufficient detail for the UK and, in particular, to find the right recommendations for optimising this system in order to achieve relevant savings in resource use without requiring a complete refurbishment of the infrastructure installed. This is done especially by understanding that the consumer selects the programme on the dishwasher based mainly on the soil level the dishes have. Most of the selections are considered correct, with two exceptions. Firstly, the modifier “express/speed/quick/time-saving” is used without realising that this will cause a considerable increase in the resources used, which is in clear contrast to the third of the top priorities for consumers, namely saving energy and water. Secondly, the “Eco” programme is used at a rather low soil level of the dishes. This is in contrast to what this programme is designed for and verified by the government, as it is the basis for the energy label and consumer organisations, such as Which?.

Correcting these points in the consumer behaviour means giving the consumer three simple messages, that everyone can easily follow, regardless of the brand of dishwasher they have installed in their home. As the simulation shows, this will save, 21.3 % in energy and 18 % in water consumption. With the carbon intensity of electricity generation in the UK at 265 g of CO₂ per kWh, this will save 0.3 Mt of CO₂ emissions per year. This saving requires no additional costs or investments and can be realised immediately. It only needs the understanding that long running dishwashing programmes at a low temperature (such as the “Eco” programme uses) provide sufficient time to clean dishes as well as short programmes at a higher temperature, but save a lot of electrical energy. Comparison with results from other European countries shows that this saving option also works well in other countries. The main advantage of the proposed recommendations – compared to many others – is that the consumer ’s expectations of a sufficiently good cleaning result are met, as the three programmes are clearly classified according to the degree of soiling of the dishes to be cleaned, as perceived by the consumer.

Despite all this, when using the dishwasher, it is still important to observe the well-known rules of avoiding pre-cleaning the dishes under running water, using the full load capacity and cleaning the filters regularly. Researchers should pay more attention to the aspect of resource consumption when using modifiers, and manufacturers should provide clearer information in their instructions for use on how these modifiers affect resource consumption, and should actively promote the use of the eco programme.