Re-visiting the “washing-up dilemma”

Friends, I can remember a time when the main question about washing up was: “Who is going to do it?”

But dishwashers changed all that. At first there were questions about whether dishwashers or regular washing-up was technically superior.

However nowadays, dishwashers do a very creditable job of cleaning crockery and cutlery – and in what was a devastating blow for the British Tea Towel industry – they also do a fine job of drying.

[Aside: If you would like to see how dishwashers work – check out these Technology Connections videos:

But this then raised the thorny question of whether hand-washing or using a dishwasher was cheaper.

And now of course, the BIG question is which is greener i.e. has the lowest associated carbon dioxide emissions. This article is an attempt to answer this question.

Energy and Carbon Emissions

In her bookEnergy and Carbon Emissions: the way we live today“, Nicola Terry does a fine job of answering such questions.

The book is a gold mine of information on all aspects of our current ‘carbon problem’.

And she looks at this problem in Chapter 7: Should I get a more efficient X?

She concludes that:

“Washing dishes by hand probably generates less carbon [dioxide] emissions if you heat water by gas – even less if you have a solar panel for hot water

And in characteristic and admirable detail, she explains the assumptions underlying her conclusion.

But as His Bobness might have commented… things have changed.

2011 versus 2021

The energy landscape has changed significantly since 2011 when Nicola Terry published her book.

  • Carbon dioxide emissions from electricity are now 235 gCO2/kWh rather than 545 gCO2/kWh in 2011.
  • Carbon dioxide emissions from gas are still roughly 200 gCO2/kWh assuming ~90% boiler efficiency: the same as 2011.
  • In our house,
    • Hot water comes from a heat pump which produces hot water around 3 times more efficiently than a gas boiler.
    • In summer, practically all our electricity comes from solar panels with no proximate emissions: 0 gCO2/kWh
    • In winter, we set the dishwasher to run on cheap-rate electricity which typically has 10% less CO2 emissions than daytime electricity.
  • In other houses,
    • Hot water might come from solar thermal panels.

So given this complexity, can we definitively compare the environmental impact of using a dishwasher and washing up by hand?

Well I have made an attempt – and below I will run through some example calculations and then I’ll collate a more extensive set of calculations at the end.

Dishwasher

Taking our recent Bosch model as typical, a dishwashing cycle seems to typically consume around 12 litres of water and around 1 kWh of electricity.

Comparing the consumption for different modes of operation, it is clear that nearly all this energy is used to heat the water. At 70 °C – way more than your hands could stand – fats within food residues soften considerably making them easier to remove in the water spray.

In the winter,

  • Carbon dioxide emissions associated with running a cycle in 2021 on average would have been 235 gCO2 using electricity direct from the grid.
  • At peak demand (4 pm to 7 pm), the emissions might be higher – but if run in the early hours of the morning – emissions might be ~10% lower.

In the summer,

  • Carbon dioxide emissions associated with running the dishwasher are around zero because the electricity is derived from the solar panels.

So assuming a year is 50% summer and 50% winter, carbon dioxide emissions associated with running a single cycle in 2021 on average would be 0.5 x 235 = 118 gCO2 per cycle.

So running the dishwasher 3 times per week would result in emission of around 354 gCO2 per week.

Dishwashers do offer ‘eco’ modes using cooler water which could reduce this.

Washing-up by hand

In general, hand washing probably consumes a bit more water than a dishwasher, but let us assume in the first instance that the same 12 litres of water will suffice, and that the dishes are rinsed in cold water.

Heating 12 litres of water by 40 °C from (say) 10 °C to 50 °C – roughly the hottest water one might reasonably use – requires:

12 litres x 10 °C  x 4200 J/°C/litre = 2,016,000 joules

Or in more familiar units, 0.56 kWh. If this water were heated with a heat pump with a COP of 2.5, this would require 0.224 kWh of electrical energy.

In the winter,

  • Carbon dioxide emissions associated with 0.224 kWh of electricity drawn from the grid would have been 0.224 x 235 gCO2/kWh ~ 53 gCO2

In the summer,

  • Carbon dioxide emissions associated with 0.224 kWh of electricity from the solar PV panels would have been zero.

So assuming a year is 50% summer and 50% winter, then washing up by hand 3 times a week results in weekly emissions of 3 x 27 = 81 g CO2/week

If the water were heated by gas, the equivalent emissions would have very similar in winter, but would not be reduced in summer, resulting in emissions of about 150 gCO2/week.

And if the water were heated by direct electrical heating without a heat pump, three washes per week would result in emissions of about 158 gCO2/week.

Comparison

I have carried out these calculations for a number of situations using this spreadsheet (Dishwasher Calculations). I compared:

  • Hand washing with water from a heat pump powered by solar energy and a battery for half of the year.
  • Hand washing with water from a heat pump powered by grid electricity.
  • Hand washing with water from a gas boiler.
  • Hand washing with water heated by an immersion heater powered by solar energy and a battery for half of the year.
  • Hand washing with water heated by an immersion heater using grid electricity.
  • A dishwasher powered by solar energy and a battery for half of the year
  • A dishwasher powered by grid electricity.

The figure below summarises my calculations assuming that a family washes up 3 times per week either by hand or with a dishwasher.

Click on the image for a larger version. Summary of calculations showing estimated annual CO2 emissions for a variety of ways to do washing up. See text for details.

It is clear that wash-for-wash, washing-up by hand results in lower CO2 emissions. This is especially striking when using a heat pump to generate the hot water.

The lower emissions arise primarily because the water used for washing-up by hand is cooler than the water used in a dishwasher.

However different households behave in different ways, and wash-for-wash comparisons may be unrealistic.

For example, in a small household, a dishwasher can store soiled dishes out of sight for a day or two, and so operating a dishwasher (say) 3 times a week is practical while not leaving out unsightly piles of unwashed dishes. However, if a similar household were relying on hand-washing, they might wash up once each day. Let’s call this Option#2.

Or for larger households, running a dishwasher once a day is more typically part of a family routine, whereas hand-washing for a larger family would be more likely – in my experience – to use rather more water – perhaps as much as twice as much. Let’s call this Option#3.

The results for Options 2 & 3 are shown below. Note that the vertical scales are different on each of the graphs shown.

Click on the image for a larger version. Summary of calculations for Option#2 showing estimated annual CO2 emissions for a variety of ways to do washing up. See text for details. Note that the vertical is different from the other figures.

Click on the image for a larger version. Summary of calculations for Option#3 showing estimated annual CO2 emissions for a variety of ways to do washing up. See text for details. Note that the vertical is different from the other figures.

The different options have a range of CO2 emissions that vary from ‘a few kilograms per year’ to ‘a few tens of kilograms per year’.

But looking across all the options, hand-washing generally results in reduced emissions, just as Nicola Terry had concluded back in 2011. But now the minimum emissions come with water heated by a heat pump.

Click on the image for a larger version. Summary of calculations for Options #1, #2, and #3 showing estimated annual CO2 emissions for a variety of ways to do washing up. See text for details.

Conclusions

My first conclusion is this: How can anyone be expected to wade through such a calculation to determine how to do the washing up!

My second conclusion is not that “Dishwashers are bad”. Indeed my wife and I own a dishwasher and use it regularly.

Personally, I am convinced of the overwhelming need for society as a whole to reduce carbon dioxide emissions, and I also feel this as an intense personal responsibility too.

Nonetheless, no decision can be entirely one-dimensional. As with many similar decisions, there are other criteria that can also be significant – criteria that have nothing to do with carbon dioxide emissions.

Off the top of my head, it could be that some people just don’t like washing up! Or it could be that some people are busy and using a dishwasher is helpful. Or perhaps that someone’s partner might just want a dishwasher for some reason, and so a dishwasher could be part of a declaration of love – an important human dimension.

But… although any decision about how to live one’s life is multi-dimensional, one of those dimensions is very likely to be the associated carbon dioxide emissions, almost no matter what the actual decision concerns.

In this case, my calculations tell me that:

  • For washing up, the overwhelming contribution to the carbon emissions is caused by heating water – so using less hot water will reduce emissions no matter how the washing up is done.
  • If using a dishwasher, it is a good idea to try to ensure that it is more-or-less full, and to think about using ‘eco’ modes.
  • And when washing up is done by hand, it is a good idea to be mindful of the amount of hot water used – and possibly use cold water to rinse.

Finally, in future years, it is possible many of these emissions may well change again.

It is planned that the carbon intensity of UK electricity should fall to 100 gCO2/kWh by 2030 – and this will reduce the emissions of any appliances which use that electricity. Additionally, it could be that dishwashers will be built which can use hot water heated by a heat pump instead of electrically heating their own water.

Finally this calculation does not include the embodied carbon dioxide emissions from either a dishwasher, a heat pump, a battery or solar panels. My guess – for what it is worth – is that assuming reasonably long lifetimes for these items, the embodied carbon dioxide emissions will not significantly alter these conclusions.

 

8 Responses to “Re-visiting the “washing-up dilemma””

  1. David Edwards Says:

    We (a family of 4, 2 old and 2 young adults) use our dishwasher at least 5 times a week, when it’s full. What’s more we use a tumble-drier even on sunny days when we could hang washing outside.
    Washing up by hand and hanging wet laundry require *more effort*, but we’d rather do something more enjoyable, e.g. watching telly.
    It seems to me that to live a ‘greener’ life requires more manual effort and involves domestic jobs that take longer than using an electric machine.
    So, to put it bluntly, we’ve got to get off our fat arses and use some elbow-grease!

    • David Edwards Says:

      That’s low-carbon elbow-grease.

    • protonsforbreakfast Says:

      Life is a multi-dimensional challenge and is about more than just minimising carbon dioxide emissions.

      https://protonsforbreakfast.wordpress.com/2012/04/23/prosperity-without-growth-is-it-possible/

      I think you are right that greener lives require more effort – but most people (not me!) are busy doing important things and feel they don’t have time. So I don’t think you should feel bad about how you live – I feel sure your way of living will evolve as circumstances change.

      One possible technique is to offer to profit share with children if electrical/usage declines. I did it with my children and it worked for a couple of years and they made serious money!

      Almost 10 years ago I wrote an article about a book called “Prosperity without Growth” – what we would call “de-growth” now.

      In it the author called for a world in which ‘novelty’ was avoided and in which “The humble broom would be preferred to the diabolical ‘leaf blower’”.

      It sounded like North Korea to me.

  2. daveburton Says:

    Since you’re obviously interested in this topic, here’s a surprisingly interesting two-part youtube video series (though it’s better when sped up to 150% speed):

    • protonsforbreakfast Says:

      Dave

      Thank you – I enjoy the Technology Connections channel very much – and I had meant to put a link to those exact videos in the article – perhaps I’ll do it now!

      Best wishes

      M

  3. Ross Mason Says:

    It’s quite clear that you should do the dishes by hand. I am sure your ever suffering partner in life will appreciate that. She wins – dishes are done. You win – saving the planet.
    Sorry, life can be a bit hard.
    Now, is it my turn to empty the dish washer??? Dang, it is.

  4. Peter S Says:

    The ideal must surely be to use single-use cutlery made of paper and/or plastic sourced from biofuels, and then bury them permanently after use. Now you have negative CO2 emissions. 😉

  5. Clare Snowdon Says:

    Really informative – thank you! It occurred to me that there is also the small matter of what we do with any time the dishwasher saves – if it’s to watch streamed videos on a huge TV then probably not so great. If it’s to campaign for policies that will reduce our emissions, maybe that’s a good “offset!” So I am taking from this that it’s OK to use the dishwasher for a full load with water heated by renewables provided we use the time saved to change the world!

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