I like the think about the details of things. So my question is: when is the best time to do laundry? My conclusion is that it depends on the weather, the time of day, and the day of the week. Here I will tell you the variables I consider when doing laundry (or running the dishwasher, etc etc).
Energy Use and the Time of Day
If you're going to use electricity from the grid, it is always best to consume when the grid is powered by the most efficient power plants. Generally speaking, grid electricity is most efficiently produced when the grid is at low load. That's the time when the most efficient base load power plants are supplying most of the grid's power, and the power from less efficient variable-load power plants are minimized.
So the first answer of when to do laundry is "when fossil fuel power plants aren't being over-worked". This means moderate temperature nights and weekends, or on moderate sunny days when solar power generation is greatly displacing fossil fuel power generation.
Drying Laundry
Clothes dryers are far more efficient when the ambient air has low humidity. Since (most) clothes dryers blow humid waste air outside, they in turn induce outside air to enter the building. Therefore, it is best to run a clothes dryer on low humidity days with moderate outdoor temperatures.
Water Issues
Clothes washers generate wastewater, and clearly there are times when you want to minimize overworked wastewater treatment facilities. Waste water is likely most efficiently processed when there isn't a huge load on the sewage treatment plant. Therefore, I avoid doing laundry if it has been exceptionally rainy, as often times the rain inundates the sewage treatment plant. I don't need to add more wastewater to the sewer system when it is over-capacity.
Detergents
Minimizing detergent is also important, as it is both expensive and always ends up as a pollutant pushed down the drain to be processed at a sewerage treatment plant.
- Say "no" to packets or sheets. They are stupidly inefficient, expensive, and don't clean as well as traditional liquid detergent.
- Say "no" to claims of "green organic" soap products. That's mere marketing dribble. Secondly, they have been shown to be ineffective as detergents. Ineffective products simply turn into zero-value pollutants.
I always use a quality name-brand liquid detergent, and I use the minimum quantity required to get the job done perfectly.
I always wash in cold water, as hot water is a pollutant, and it provides no benefit when using a decent detergent.
Putting that all together
- I generally do laundry weekends, when grid power is running at its highest efficiency. That means, nights, weekends, or on sunny moderate days when solar power generation greatly displaces fossil fuel powered plants.
- I generally avoid laundry on very hot, very cold, or very wet days, as this just adds more load to the grid, to over-worked HVAC systems , and to the wastewater system.
- I wash using effective detergent in cold water.
Of course I make exceptions, because sometimes I can't get all the stars to align. But if I can reasonably change my schedule to conform to the above principles, then I will.
If a large subset of people acted just slightly more thoughtfully, there would be substantial savings.