A few weeks ago I tried to wonder why I run my dishwasher overnight. I set it to run at about 4 AM or 5 AM, and it finishes up by 6 AM or 7 AM or so. Here's a conclusion of my analysis.
Electricity is more efficient overnight, and therefore costs less and emits less carbon.
Power companies produce less electricity at night due to lower demand. During the day, they produce more electricity.
As for-profit entities, power companies want to use as little fuel as possible - after all, their biggest expense is fuel. So the plants that they run all the time are their most efficient plants, and the plants that they add during peak times - during the day - are their least efficient plants.
By running my dishwasher over night, I am guaranteed to be using the power by the most energy efficient power plants.
Winter Heat / Summer Heat
My dishwasher releases quite a bit of waste heat and moisture. Most of that heat from the dishwasher stays in my house. By running my dishwasher at the the early morning, all the heat released pretty much stays in my house, to keep me incrementally warmer right when I awake.
Similarly, I don't want that extra heat in my house in the summer during the daytime peak temperatures. By running it in the early morning, the dishwasher waste heat doesn't add to an already super-hot house - which then would need to be expelled by my hard-working air conditioner.
The waste water produced by my dishwasher gets dumped into the sewer earlier in the morning. This avoiding the peak time of wastewater production.
Hot Water Heater Cycling
By running the dishwasher in the early morning, the hot water heater gets busy heating up replacement water right before I get up to take a shower. This puts less thermal cycling on the hot water heater.
I don't want to hear my dishwasher run. I don't hear it while I sleep in the bedroom.
I like to open the dishwasher right after its cycle so its contents - particularly plastics - dry quickly. Since I get up as it is finishing its cycle, I am often there to open the door and let the ambient environment quickly dry its contents. By the time I'm ready for work, I can empty the cool and dry contents of the dishwasher.
There we have it! That's why I run my dishwasher overnight. Perhaps each line item has an extremely tiny impact, but perhaps when taken in their totality they are a little less than tiny. Let me know which ones you think are bull - and why.