Is he really saying that it costs 0.18 MJ to wash one cup? That's 1.7 litre of water raised 25 K or or 1 litre raised by 43 K. Washing my cup by hand I don't use anything like that amount of water.

I've just checked by washing my teacup.

I used 0.25 litre of water at 53 C. Incoming water temperature here is about 10 C so that means:

    0.25 kg * 4200 J.K-1.kg-1 * (53 - 10) K

  = 45 kJ = 0.045 MJ

So it sounds as though dishwashers aren't as efficient as people like to say.

Your calculation assumes that the production and distribution of 1MJ of electricity takes 1MJ of raw energy (e.g. in the form of Coal).

This is off by a factor of several. For example coal fired power stations are maybe 45% efficient. Then you have to mine the coal, transport it to the power station etc etc.

> Your calculation assumes that the production and distribution of 1MJ of electricity takes 1MJ of raw energy (e.g. in the form of Coal).

> Hocking assumed a new, commercial dishwasher running on Canadian electricity, requiring about 0.18 MJ/cup-wash.

Nowhere in the article does it say, or even imply as far as i can see, that the energy cost included cost of mining and transporting coal (or oil or gas or solar energy) to the generator.

If that was the intention then it should have been made much clearer because it dramatically affects the results. Where I live (98% hydro) the total energy cost of the energy I use from the grid will be lower than, say the UK (mix of oil, gas, nuclear) simply because the raw material cost is eliminated.

If these details are omitted from the article then the information cannot be effectively used except in that one location in the world where it was written making it rather difficult to draw useful conclusions for or against the use of disposable cups.

The footnote at the bottom says that 1 MJ will raise 3 quarts of water to boiling and that is clearly the energy delivered not the energy consumed in the total process.

So perhaps the only conclusion to be drawn here is that ILEA Leaf carries some rather sloppy articles.

Like I said.. it's hard to work this out, especially in any kind of generalized way.