Are dishwashers more efficient than my hand washing?

We often hear that dishwashers, counterintuitively, use less energy and water than washing the dishes by hand. No matter how many times I'm told this, I find it hard to believe, so when I finished a load of washing up today, I took a few measurements so I could compare my washing up with those that have been reported. Firstly, I bailed the water out of the bowl with a measuring jug to see how much there was, then added a bit to allow for water that had sloshed over the side. I hadn't rinsed under a running tap without catching that water in the bowl, so I didn't have to add any for that. The total amount of water was about 5.5 litres.

I know, because I've measured it before, that water that I find hot to the touch is 43°C. It's pretty chilly out at the moment, so let's say the incoming main is 5°C and round it up to 45° for washing-up temperature (all approximations here err on the side of increasing energy usage in my test). Since water is very reliable in how much energy it requires to heat up, I can calculate that this would take 256 Watt hours of energy to heat, or just over a quarter of a kWh.

I then needed to know how much washing up I'd achieved. One load for me is until either the draining rack is full or the water is too dirty to wash any more. This tends to occur at roughly the same time, though I do more rinsing under the tap towards the end of the load, as the water gets dirtier. I noted down what I'd washed:-

• 4 glasses
• 2 mugs
• 1 tin can and 1 jam jar
• 1 measuring jug (not the one I used for bailing)
• 5 plates
• 3 bowls and a basin
• pestle and mortar
• 2 saucepans
• 2 mixing bowls
• 2 plastic trays that fruit came in, for recycling
• 15 items of cutlery

One load of washing up

See those pans on the windowsill behind? I used the dirty water from this load to soak them so they'll be easier to wash when I get round to them. I do that.

Having noted down the details of my own washing up, I then needed to find appropriate figures for comparison with dishwashers. The fact that dishwashers are more efficient seems to come mainly from a study conducted at Bonn University, comparing many people actually washing up with dishwashers doing the same work. When I looked for this study I was pleased to find that they've done a follow-up with smaller loads (the original study was a full twelve place settings in one go): Our intention was to simulate the dishwashing situation in a two-person household, excellent! ... in which two place settings are used and cleaned three times per day ah, I'm not sure I wash up that frequently, ... and four additional items are soiled heavily in the food preparation processes and are washed separately. Do you wash up the pans separately? Still, we should be getting close to comparable figures.

I looked at the details of the study. One place setting consists of:-

• a soup plate
• a dinner plate
• a dessert dish
• a cup
• a saucer
• a serving dish
• a glass
• a fork, a knife, a soup spoon, a teaspoon, a dessert spoon

So that would be a three course meal with coffee to follow, three times a day?? OK, never mind how unrealistic this is. I now have some details to work with. How does my load of washing up map onto this test load?

two place settings plus pans	my washing up
2 soup plates; 2 dessert dishes	3 bowls and a basin
2 dinner plates; 2 saucers; 2 serving dishes	5 plates
2 cups	2 mugs
2 glasses	4 glasses
10 items of cutlery	15 items of cutlery
4 pans/casserole dishes	2 pans; 2 mixing bowls
not included	pestle and mortar
not included	measuring jug
not included	tin can; jam jar; plastic trays

It looks like my load of washing up was slightly more than one of their two-place-setting loads plus their food preparation load. Let's say they're roughly the same - how do the figures compare with their hand washers?

The water and energy measurements show a very wide distribution of consumption values, ranging from four to 90 l and from 0.03 kWh up to 2.6 kWh for washing a pair of place settings. Washing four heavily soiled cooking items required slightly more energy and water than two place settings.

I used 5.5 l and 0.26 kWh for a pair of place settings plus four cooking items. For water consumption I am off their scale, and I'm way down there for energy consumption too (apparently someone was happy to wash up in cold water).

How about the machines? We need to take account of the fact that you'd put more in a machine than my one load done by hand. I estimate that you'd get two to three times as much in a dishwasher, and their test used six place settings (three times as many as one of their hand-wash loads) plus the pots and pans. I think multiplying my usage by 2.5 gives a fair basis for comparison, so that's about 14 litres and 0.64 kWh. Their machines used 83 litres. With regard to energy consumption (Fig. 8), the tendencies are similar, but the difference between manual and automatic dishwashing is not as pronounced. They don't even report the figures, but examination of their Figure 8 shows almost identical energy consumption for the 6 place setting plus pots and pans; approx. 1.8 kWh.

As I'd suspected, the current emphasis on low water consumption serves to distract from high power usage. When compared with my own washing up, as best I can, I am far more efficient than a machine in both water and energy consumption.

I finish by noting a sentence from their acknowledgements section: Thanks are due to our four European manufacturers of dishwashing machines (Bosch und Siemens Hausgeräte, Electrolux, Merloni Elettrodomestici and Arcelik) for supporting this study.