Stop Hand Washing Your Dishes

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Great news for people who hate doing dishes: if you have a dishwasher, washing your dishes by hand is a colossal waste of water, energy, time, and money, and you can prove it with maths.

Most people drastically underestimate how much water their taps put out. Energy Star-certified dishwashers must use less than 270 kWh per year and 15l per cycle, but most kitchen faucets move one to 7.5 litres a minute. In other words, running the tap for just four or five minutes can use more water than an entire dishwasher cycle.

Older dishwashers do use more water, but it's still probably less than you think; unless yours is more than 10 years old (and somehow still working), there's a good chance that it's more efficient than hand-washing a sink full of dishes. The exact amount of water and energy you can save depends on a number of factors, so whoever pays the utilities should probably look up the usage stats for your dishwasher and faucet to see what they're working with.

What they find might just be the push they need to finally spring for a new dishwasher.

Switching over to a fully-automated dishwashing lifestyle may take a load off your energy bills — not to mention your feet, back, and cuticles — but it won't save the world. Certain items will always need a hand wash, and anyway, your shower and washing machine use more water than anything in the kitchen. Still, it's an easy way to save water with basically zero drawbacks. I think they call that a "win-win."


Comments

    Win-win? First you need to have enough dishes to fill a dishwasher, then you need space for a dishwasher, then you need to buy a dishwasher. Where's the savings?

    But if you're lucky to have a have a double sink in your small rented apartment kitchen, you can wash "Old school" i.e. fill both sinks - one for washing and one for rinsing. Does't use that much water either.

      So here are the dimensions of a sink with a smaller sink, I just picked the first one I found:
      FEATURES
      304 grade Stainless Steel
      0.8mm thickness
      Main Bowl Dimension: 450 (W) x 400 (D) x 200 (H)
      Secondary Bowl Dimension : 280 (W) x 400 (D) x 190 (H)

      That is 57L full or 40L 70% full.

      I took the first 3 dishwashers I found; a Dishlex, Fisher and Paykel and a Bosch. Water consumption (litres) – 11.6, 11.4 and13.4 respectively for 13, 15, 14 place settings.

      A 4 place setting dinner set from Kmart costs $7, and lasts indefinitely unless you drop it so the cost per use is close to zero.

      There are a lot of other variables, but on the face of it unless you do an average of at least 4 place settings each time you fill up the dishwasher will win out. Water use is a good proxy for energy consumption as most dishwasher can be fitted to the hot water tap at up to 60C to get the benefit of off-peak electricity.

      If you rinse things under the tap then it will swing more in favour of the dishwasher, on the other hand I am pretty sure dishwashers do not encourage people to use the same item multiple times and this is potentially a very big factor.

      It's fairly easy to make calculations in favour of the economy of dishwashers over hand washing, if want to justify a purchase. Ironically it probably is more relevant to single person households who wash up after every meal (esp. rinsing under the tap) and don't see it as a chore and the investment in the machine will take a lot longer to pay back (probably on financial terms it won't). It is however rather simplistic because the energy required to manufacture the dishwasher is usually ignored in energy calculations but accounted for in monetary terms by its payback period, but usage patterns make a massive difference.

      Last edited 21/11/18 2:45 pm

        You might want to brush up on your math. That larger bowl you mentioned, filled to a depth of 170 mm contains 30.6 litres, not 5.7 litres as you seem to to think. The smaller bowl, filled to the same depth, holds 19 litres. My dishwasher always runs full, and on the occasional times I do hand wash because I have too much to fit into the dishwasher, I fill both bowls multiple times because the water becomes so dirty I can't bear to put my hands into it any longer.

        Thanks, I edited the water figures from 5.7L and 4L to 57L and 40L respectively. A bit clumsy.

        So you can save approx. 30L per washing machine load which remember is a 13-15 place setting, but to be realistic is half cooking utensils and glasses. Would it be fair to say one load per day is sufficient for a household and equivalent to 2 washups with a 2-bowl sink filled to 70%. So the water saving per day would be 2*40 - 13(worst of the 3 models) = 67L.
        This 25,000L per year.

        My water usage costs $2.08 a kL, so the 67L of water saved saves about 14 cents per load, or over a year one load per day is 25kL of water saved worth about $51 a year.

        The large bowl is 25L 70% full, ignoring the small bowl which has cold water, for 2 washes that equate to one dishwasher load that is 50L that needs to be heated from say 20C to 50C ( a 30 degress Celsius rise) requires 50*4*30/3412 kWh = 1.76kWh
        ( volume in litres x 4 x temperature rise in degrees centigrade / 3412)

        A dishwasher heats only 12L which requires 0.56kWh (assuming 60C wash temperature).
        Dishwashers use approx. 1kWh for a load which includes the 0.56kWh to heat the water.
        So per dishwasher load you save 0.76kWh. But how much money do you save?
        The double sink will probably use off peak heated water @ 13c/kWh (23 cents) whereas a dishwasher may use 13c, 21c or 47c / kWh, so there may be no saving on electricity costs.

        You can connect most dishwashers to the hot water supply if it's less than 60C, but you will have to run the water before starting the washer (remember it only uses 11-13L) to get this offpeak heated water. So if the run is long from your hot water service connecting the dishwasher to the hot water supply is wasteful as it will probably not get any preheated water or you will waste so much cold water to make it a waste of time. With the double bowl the cold water is not wasted.

        So using all my dodgy assumptions I think you can save about $50 a year with a dishwasher used once a day compared to a two bowl sink used twice a day. You will also save about 25,000L of water. The 50kWh less electricity needed to be generated. So if your dishwasher lasts for 10-15 years it will not have cost you anything to own. If you are single and don't mind washing a few dishes it's probably not economic though.

    I find i need to rinse after using a dish washer anyway as you get a strange residue on everything, seems to be the case with every dishwasher I've used.

    Where as since i rinse as i wash i don't get this handwashing.

      Do you have 'hard' water with a high calcium content? You end up with a haze of chalk on glasses? Most European dishwashers have water softening systems which use salt to condition the water and it overcomes this problem. The so called 'rinse aid' is another way to deal with it, it's a wetting agent that helps the water run off along with the undissolved chalk in the water. Otherwise clean out the filters, they can get greasy, and run the machine hot and empty maybe even some dishwasher cleaning agents. If you don't want a commercial one there are plenty of suggestions on the web.

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