The amount of water required to wash by hand is exorbitant compared to the efficiency of a dishwasher.
Last time I checked, handwashing is 3 times less efficient in water usage than in the best case scenario (for handwashing), and 40 times less efficient in water usage in the worst case scenario.
Plus, even if we were to assume that somehow a human manages to use the same amount of water, then you’re still warming up the water somehow. It doesn’t become magically hot by exiting the tap, so the vast majority of the dishwasher’s electric running cost would now be shouldered by the water heating unit. Except it’s now 3-40x more water that needs to be warmed up.
I measured my water usage washing by hand many years ago and can easily match the machine's usage.
The trick is to reuse the water for scrubbing/soaping and "flash rinse" by quickly turning off the water when rinsing.
Also, that's not how I wash dishes by hand any more; I splurge with the warm water used per cleaned item. But I still don't let it run for more than a few seconds. It just feels wasteful.
> you’re still warming up the water somehow
This is somewhat a sunk cost for most people: Unless you turn off the warm water in your pipes, constantly keeping warm water available costs something and using a bit of it costs very little in addition. We really should be thinking of turning off water heating at times when we know for certain it's not being used, and only heat up water anticipating usage. There's a lot of energy to be saved for modern smart homes (or anyone who cares to manually regulate, or simply has to because they're using wood burners and such).
That is assuming you don't use highly isolated water bodies as thermal batteries.
I don’t refute your claim, but I think on average people are unable to match a dishwasher.
12 litres for 150 items is ridiculously good. The university of Bonn indicates that the equivalent handwash uses 100L, and in some cases 300-400L. Pre-rinsing uses 100L alone. These are averages. Some people are extremely good, others are terrible. In some countries it’s customary to let the tap run while washing dishes, etc, so that’s why the extremes vary so wildly.
I also think that sink setup has a massive impact. In my current flat (which came with a dishwasher) I only have a single sink. It’s complicated to be efficient with water, unless I use a secondary container where I can store soapy dishes together until I can rinse everything. Years ago when I lived in a flat without a dishwasher, it had “three sinks” (two large and deep ones, and a smaller area in the middle). It was trivial to have a little soapy water on one side, wash everything, move it to the other side, and literally rinse and repeat. Water usage was greatly reduced compared to my current setup.
The only thing I don’t run in the dishwasher are wooden items and the couple of non-stick items I still have left.
> We really should be thinking of turning off water heating at times when we know for certain it's not being used, and only heat up water anticipating usage.
That’s what tankless water heaters are for. I’ve had one for a decade, it’s lovely.
> We really should be thinking of turning off water heating at times when we know for certain it's not being used, and only heat up water anticipating usage. There's a lot of energy to be saved for modern smart homes (or anyone who cares to manually regulate, or simply has to because they're using wood burners and such).
> That is assuming you don't use highly isolated water bodies as thermal batteries.
First paragraph describes a tankless water heater. Second paragraph describes a classic water heater, with a tank. That's practically all houses.
Last time I checked, handwashing is 3 times less efficient in water usage than in the best case scenario (for handwashing), and 40 times less efficient in water usage in the worst case scenario.
Plus, even if we were to assume that somehow a human manages to use the same amount of water, then you’re still warming up the water somehow. It doesn’t become magically hot by exiting the tap, so the vast majority of the dishwasher’s electric running cost would now be shouldered by the water heating unit. Except it’s now 3-40x more water that needs to be warmed up.