That's a good question -- I'm struggling to find a well researched summary of the situation. The biggest external cost with machine washing is the hot water, b

That's a good question -- I'm struggling to find a well researched summary of the situation. The biggest external cost with machine washing is the hot water, but most detergents now are optimised for cold / room temperature wash. The biggest actual cost for hand washing is the time consumed - you may call it opportunity cost. Historically the societal benefits of the shift from having someone (typically 'the woman' of the household) spending 10-20 hours a week attending to laundry, to requiring perhaps a half hour of someone's time, have been better studied.