Dishwasher

Dear Al,
I have head that dishwashers use less water than washing dishes by hand? I assume this is only true if the dishwasher is full or almost full. Could you tell me how full the dishwasher needs to be? Also, since the dishwasher uses extra electricity does that negate the water saving effect? What if I turned out the kitchen lights, which would have to be on while I washed dishes by hand, and then subtracted the energy for the light bulb? I think my kitchen lights are florescent. (Not the fancy new kind but the kind they had in my elementary school.)

Since I don't actually have a dishwasher right now (but I like to think about getting one) I would also like to know how dish-washing-by-hand methods compare. For example, my Dad washes dishes by filling the sink with warm water and a small amount of soap and letting some dishes soak while he washes others. (Glasses and not-very-dirty things first!) Then he rinses them in the other side of the sink. I wash my dishes by putting some soap and warm water on a sponge and letting the wash and rinse water fall allover the dishes waiting to be washed in the sink. I think I use more soap this way than he does, but less water since the rinse water has to be used anyway. Except sometimes when I need to use more rinse water to get the soap off the glasses if it's the first thing I wash and I didn't let the soap sink into the sponge enough. So here is my question: Is it better to use more water or more soap?
Thanks Al,
Julia