The internet provides plenty of answers to this question, and there is always a clear winner, with regard to water consumption and energy, the dishwasher. However, the source of the data used in online studies is rarely revealed.
The aim of this article is to provide a comparative analysis of the water and energy consumption for both ways of washing the dishes. It will be shown that water consumption greatly depends on how thrifty or wasteful each of us are, while energy use is affected by what type of boiler we use.
The conclusion drawn is that when washing by hand we have to be very skilful to rival the dishwasher with regard to water consumption, while when it comes to energy use, if we choose an efficient boiler we can indeed match the dishwasher’s consumption, even when taking into account the manufacturing and disposal phases of the appliance’s life cycle.
With regard to the liquid detergent used, it is clear that from an environmental point of view the best option is to use an ecological product in both cases, which as will be shown does not release ecotoxic substances and allows the wastewater to be used for irrigation. The Biobel range includes a washing-up liquid for washing the dishes by hand, while for dishwashers we make all-in-one tablets, salt and rinser.
Another parameter to be taken into account is the time spent cleaning the dishes by hand or with a dishwasher. It would seem that using a dishwasher saves time compared to doing this task by hand, but on occasions loading the machine properly is time consuming. There are also those who value the time spent washing up as time for relaxation, listening to music or meditation.
A dishwasher has three types of energy consumption:
- Heating the water for washing. If we do not use the appliance’s hot water supply (not all models have one), the water will be heated by a 100% efficient heating element: each kWh of electricity consumed is converted into a thermal kWh. More energy is required the hotter the water needs to be (and depending on how cold the water supply is).
- Drying the dishes. The majority of dishwashers dry the dishes by heating the water of the final rinse (up to 70 ºC or more) and allowing the vapour to condense on the dishwasher’s interior walls. With Bosch dishwashers, for example, around 20% of the energy consumption is related to this phase. Technological advances to reduce this aspect of energy consumption include the use of zeolite, or else the automatic opening of the door once the wash cycle ends.
- Hidden consumption while the dishwasher switched off. As is the case for all electrical products equipped with a transformer, they continue to consume electricity while not in use. The only way to avoid this usage is by unplugging these appliances. The power used while the appliance is not in use is often stated in the technical data provided.
The energy label displayed on dishwashers when purchased (which can be consulted online whenever needed) states the annual water and energy consumption (WC and EC annual respectively). In accordance with European Community regulations, it is calculated as follows:
WCannual = WCcycle·280
ECannual = ECcycle·280 + ECstandby
WC cycle and EC cycle are, respectively, the water and energy consumption for a normal wash cycle (manufacturers tend to run tests using an Eco programme, that operates at about 50ºC), EC standby is the energy consumption while the dishwasher is not in use, and 280 is a standard figure for the number of days per year that the dishwasher is used.
We have used a sample of 21 models of dishwasher from the leading brands on the market classified as A+, A++ o A+++ (the only models currently permitted; in March 2021 a new classification scale was introduced). We have assumed that the appliance’s hot water supply (in the event there is one) is not used, as this is the most common practice. The average usage for each time we use the dishwasher – including a proportion of the hidden consumption for each wash – is as follows:
WCperwash = 9,6 litres of water
ECperwash = 0,86 kWh of electric energy
Using these figures, we can obtain the average energy consumption used to heat a litre of water during the wash cycle, which is useful when it comes to drawing comparisons with washing the dishes by hand. EC per litre = 0.073 kWh.
It must be added that if you are accustomed to rinse the plates and utensils before putting them in the machine, the water consumption will increase significantly.
From the perspective of sustainability energy consumption is much more important than water consumption due to its close link to climate change.
Now let us consider how significant a difference there is between the energy and water consumption required when using a dishwasher and when cleaning the dishes by hand. For a start it is clear that the energy used for drying the dishes, and while the device is on standby ceases to be a concern.
Consumption when washing the dishes by hand
This depends on the type of boiler used to heat the water. Firstly, we will consider the two most commonly used, albeit not very sustainable, boilers, followed by a more modern, far more efficient, model.
- Electric storage water heater. The water is heated using a heating element, just like a dishwasher, whereby the energy used per litre of water heated in both cases would be the same (washing at the same temperature), with the difference being that when washing by hand one has to add the boiler’s energy loss:
- Some energy is lost while the water flows to the tap. The heater and kitchen sink are often close to one another, which means this tends to be insignificant.
- The water in the storage heater is always cooling (more so when the thermostat is higher and there is insufficient insulation), and this form of energy loss tends to be more significant than the previous issue. If we want to make an estimate, you can consult your storage heater’s manufacturer about its heat dispersion.
However, only on rare occasions would we do the washing-up using the same water temperature as a dishwasher: the latter almost always exceeds 45 ºC, and we would burn ourselves at that temperature. Furthermore, one is likely to rinse the dishes using cold water. Given that the programmes used to test dishwashers tend to work at temperatures of around 50 ºC, if we suppose that the average temperature for washing by hand is 40 ºC, and that water is supplied at 12 ºC (an average for the range os temperatures found in Spain), the energy consumption falls by 74%. Thus,
ECwashing by hand_storage heater = WCwashing by hand·Elitre·0,74 + losses
(remember that Elitre is the energy used by a dishwasher to heat one litre of water).
- Gas fired water heater. Like a dishwasher this type of water heater heats the waster instantly, whereby the only loss occurs while the water flows to the tap: this is insignificant for the consumption as a whole. However, in this case the energy efficiency can range from 70%-80% in older water heaters, and to over 100% with the more modern condensation gas heaters.
However, it must be added that this option is less recommended from a sustainability point of view, as it depends upon an imported fossil fuel and gives rise to the emission of pollutants (these are minimized in models catalogued as “low NOx emissions”).
ECwashing by hand_gas = WCwashing by hand·Elitre·0,74 / efficiency
- Hybrid water heater. This type of water heater uses an aerothermal system to obtain heat with an efficiency of at least 200%, which can rise to at least 300%, and even more depending on the air temperature and how hot you want your water to be. Using the same principle as used in fridges (compression and decompression of a gas that releases/absorbs heat), it provides 2 or more thermal kWh (in the form of hot water) for each kWh of electricity that it consumes (for the compression/decompression and circulation of the gas through its circuit). As the energy efficiency is so high, the energy obtained from aerothermal systems is considered to be renewable.
Hybrid water heaters also use a heating element, which is used when water needs to be heated quickly. There are no statistics for the average efficiency achieved by aerothermal heat pumps in real scenarios, nor for how frequently they use their heating element, but it seems probable the latter would amount to at most half the amount of electricity; in other words, they would achieve the following:
ECwashing by hand_hybrid <= WCwashing by hand·Elitre·0,74 / 2 + losses
According to our calculations the two parts of this equation are equivalent, whereby the results obtained in the case of an aerothermal heat pump are in
Calculating a dishwasher’s complete life cycle
A dishwasher does not appear from nowhere, nor do they vanish by magic when scrapped. The production of this consumer appliance and the management of the waste derived from them incurs costs in terms of material and energy: design, sourcing raw materials, manufacturing and installation, distribution and transport, recycling and end . This cost does not exist when washing the dishes by hand, and we could not use a dishwasher without it. Therefore, to ensure this comparative study is fair, it has to be accounted for and added.
Accounting for these issues is achieved using a Life Cycle Assessment (LCA) for these appliances. In the case of dishwashers, an LCA study was undertaken by the European Commission’s Joint Research Centre in 2016, and it concluded that supposing a dishwasher is used for 12.5 years its manufacture and disposal would correspond approximately to 15% of the total energy consumption (the dishwasher’s complete lifespan), whereby its usage phase consumes around 85%. Adding this 15% to the usage phase means we must multiply the above formulas by 0.85.
Consumption comparison between washing the dishes by hand and using a dishwasher
In the table below we show how much energy consumption increases (with regard to using a dishwasher) when washing dishes by hand, for of water consumption is multiplied from between 1 and 5.
To estimate the loss in the case of a gas-fired water heater or hybrid water heater we have assumed the thermostat is set at 45ºC and we have used the generic data facilitated by this brief report: 0.699 kWh is lost per day (although for the more efficient hybrid water heater this would certainly be lower). On the other hand, we have assigned 25% of the water heater’s losses to washing the dishes by hand; the remainder would result from other hot water usage (showers, toilet use, kitchen).
It is clear that choosing more efficient appliances makes a significant difference in consumption (and thus also the household bills); the more efficient appliances reduce consumption practically by half compared to the less efficient ones. With more traditional water heaters there are significant increases in energy consumption with regard to using the dishwasher, while with a hybrid water heater hardly any more energy is used; however, water consumption is quadrupled, in other words, some 40 litres of water are used to wash one dishwasher load.
If you wish to use as little water as possible and at most consume twice as much energy compared to a dishwasher, with the more efficient water heaters you will use less energy than a dishwasher.
Maximizing efficient and renewable resources
If the dishwasher has a hot water supply and our water heater is more efficient than a heating element (for example if we have a hybrid water heater), the best thing to do is to connect the water heater to this supply, whereby the dishwasher will benefit from the same reduction in consumption as when washing by hand. In this case we cannot quantify the comparison of washing the dishes by hand and using a dishwasher because it would be too risky to estimate the proportion of the energy consumed devoted to the drying phase, but we know that both cases would be closer to what is shown in the table, because the appliance’s energy consumption would be closer to washing the dishes by hand.
Indeed, it is these more efficient options that make a dishwasher or washing machine’s hot water supply (not all models have one) so useful: if we are going to provide heated water less efficiently than that provided by the appliance itself, it is better it heats the water itself.
Another option for a water heater that maximizes savings and renewable energy is to use solar panels, which are included on many buildings built or restored in the last few years as this is stipulated by building regulations. However, in many cases they are never used, as they are not set up and used properly. It should be noted that if you have solar panels and want to use them, the hot water tank should also have a heating element as a backup during cloudy weather, and in the summer you will have to partially cover the panels so that the water does not end up boiling.
Renewable electricity too
The sustainability of your home is maximized if the electricity you use is also generated from renewable sources. You can achieve this by using a supplier that guarantees this (there are various in Spain), or else by installing solar panels in your house. This option of supplied renewable energy production is a key element for a sustainable future, it is already widely extended in many countries, and there are currently incentives for implementing it in our houses.
All of the above will assist when deciding whether to buy, not buy or renew a dishwasher or water heater. Over the course of this article we have brought together advice on sustainability regarding both the purchase and replacement of a dishwasher, as well as the everyday practice of cleaning the dishes.