Thirty degrees for nearly everything. Sixty for bedding and towels when somebody has been ill, and for anything that has to be genuinely hygienic. Forty is a habit rather than a decision.
- 30C cleans normally soiled clothing perfectly well with a modern detergent, and it is where nearly all the energy saving is. Colours last longer and there is less shrinkage.
- 40C is the default most people never change. It costs noticeably more than 30 because the machine is heating water, which is where nearly all of the electricity goes.
- 60C is what kills dust mites and most bacteria. Worth it for bedding after illness, for cloth nappies and for tea towels.
- 90C is almost never needed and is hard on both the clothes and the machine.
Washing everything cold forever lets a film of detergent and body oil build up in the drum and the seal, which is where that sour smell comes from. Run an empty 60C or 90C maintenance wash once a month.
Heating the water is the overwhelming majority of a washing machine's electricity use. The motor turning the drum is a small fraction of it, which is why cycle length matters far less to the bill than temperature does.
Household chemicals can be dangerous in combination. Never mix them, keep the room ventilated, and test anything on a hidden patch first. Full disclaimer.