I often question the water use reported by many diswasher manufacturers, here is one by Bosch.
Bosch claims that a dishwasher can save up to 12000L per year (or about $30 in water) compared to using a diswasher 4 times per week. Now does this number doesn't stack up....
An efficient dishwasher will use about 12-15L per cycle, lets assume the 12L used which is also quoted on the Bosch website. This 12L is assumed for a light load (where plates etc are not heavily soiled). This corresponds say to about 50L per week or 7L per day. This 7L also assumes that plates are not pre-rinsed...something which may be an issue if not done and dishwaster only used very second day (we will ignore this 'fact').
Now, 12,000L corresponds to about 33L per day or 11L per hand wash.
Now, it is a saving or 12,000L, so one must also add the water used by a dishwasher. usage (around 2-3 L per meal equivalent) This means that dishwashing would use around 13L per event/after each meal. This is a 10L bucket and a half of water.
Our kitchen sink has a volume of about 13-15L...so the savings quoted by Bosch and many other manufacturer's assume that we fill the sink up to the brim in order to wash the dishes..which would make hand dish washing impossible unless one makes a mess in their kitchen.
Now, these marketing/advertising claims have been tested and can be found here. It was found that these claims were not misleading.
I wonder if they are misleading when the numbers are supported by a Colmar Brunton study (which is a marketing and market research company) in May 2009 also seem a little unrealistic. One such claim which seems unrealistic is ...On average, it took 75.11lL of water to hand-wash a 12 place dinner setting (132 individually soiled pieces). By comparison, the Products use 13.6L to wash the same 12 place dinner setting, equating to a saving of 61.51L per wash. For a 13L sink, this means one would need to empty and refill between 5-6 times ...I am yet to do such after any dinner party we have. we may do it twice if the food was a little oily/fatty.
I suppose over two days one might generate 12 place setting work of plates (assuming on two in the household), but again, a dishwasher wouldn't also be able to handle the glasses (12 at least), mugs (could be 4-6), saucepans, frying plans/wok (okay, woks don't go in dishwashers so one would heed to add the hand washing water to that of the dishwasher), cooking utensils, cutlery (12 place steeing worth (the test famiky must be wealth to have 12 cutlery setting lying ajd), sharp knives, cutting boards (whoops, again these don't in the diswasher) etc.
I wonder if many of the saving claims by the dishwashing manufacturers are realistic or based on a worst case, OCD type scenario. Maybe the modelling of water use was for a couple that has big dinner and cutlery sets, tends to buy precooked foods (either bought as hot or only requiring reheating in disposable packaging) for most meals. In such case, the use of mainly plates, cutlery and standard glassware, the dishwasher may be more efficient if the residence was connected to reticulated water and mains electricity.
I think that the efficiency of diswasher's is over stated, especially when one may use solar hot water for hand washing dishes and/or use tank water.
It is also over stated if one uses a range of kitchen items that it is not recommended to be dishwashered (sharp knives, zinc/silver/gold metal plated items, crystal etc) or it is impracticable to dishwash (e.g. large saucepans or large cutting boards)
The claims seem a little dubious to say the least.
I expect that every individual household would have different dirty kitchen items/dishes so it is very hard to say which method is more efficient.
My own observation possible indicate the following:
- if one mainly eats out, then possibly the dishwasher is more efficient as it would be used less, but hand washing would be still regular is one wishes to keep the kitchen looking clean and tidy.
- if one buys precooked meals (either hot or to be reheated), then possibly dishwasher as it would be similar to the first point.
- average household that eats out 2 or less times per week and prepares home cooked dinners, then hand washing is likely to be more efficient. If the house is on PV solar and tank water, and dishwasher is used when the sun shines, then it could be argued that the dishwasher may be more efficient (namely use less of resources one may pay for).
- If one only cooks ones own meals, see dot point 3.
It is also worth noting that for large households, any dishwasher advantage over hand washing would be less as one would need to run a dishwasher at least daily and the dishwasher running costs, water and energy use would be more than that stated for a light load/use scenarios.
I still plan to mainly hand wash and use the dishwasher if we have a large dinner party and one doesn't feel like washing everything up after the guests leave (even though some hand washing up is still required).