Off grid dishwasher?

Thinking of building a new house off off grid.  Planning on rainwater collection only.

I have a big family and like to cook a lot.  We make a LOT of dishes, two dishwasher loads a day plus a sink full of pots and pans are not uncommon.

I am trying to do the math to figure out if I am cheaper to increase cistern size to hand wash all the dishes or add solar to handle the dishwasher.  I know which one my wife wants, but I still want to wrap my head around the math to figure out our needs.

Anybody have a link to help?

What is your hot water source going to be powered by? Solar thermal hot water is much cheaper in the long run than electric or gas, but like all solar it takes an expensive initial investment. That being said, the payback time on solar thermal hot water is much short than solar photovoltaic. This is because using the heat of the sun is generally above 80% efficient while the best PV is around 25%, and more often closer to 15%. The amount of energy the dishwasher itself uses is probably minimal compared to the energy embodied in the hot water it uses.

I've never met a dishwashing machine that actually gets dishes acceptably clean by my standards. The saying "so clean you could eat off it" means it is just below surgically clean by my understanding. It isn't like taking a car through a car wash that still has mud in the undercarriage and trash on the floor and people still call it 'clean', but I guess that is just a personal quirk of mine. I do use my old dishwasher as a large drying rack, which is a good way to divert it from going to the dump.

I don't have exact numbers, but looking at the water heating costs should help point you in the right direction. My ballpark guess would be electric heat - expensive hot water, gas heat - not as expensive as electric, solar thermal (not photovoltaic) hot water would most likely be cheapest per gallon of hot water in the long term.

Another thing worth mentioning would be Paul's thread on washing dishes. In summary, it can be substantially cheaper for grid powered homes to wash by hand, so I would imagine it could be even cheaper for off grid. This would need to be weighed against how much value you place on having harmony with people you live with and how much grief could be involved if you don't have a dishwashing machine that other people have become accustomed to. For instance, an extra solar thermal hot water panel, PV panel, and battery or two might be cheaper than going to family counseling. It would probably be best to take everything into account and decide from there.