25 machines at say 100W each is about 2.5KW. Can you even power them all at the same time at home without tripping circuit breakers? At your mentioned .12/KWH that is about 30 cents an hour, or over $200 to run them for a month, so that adds up too.
i5-4560S is 4597 passmark which isn't that great. 25 of them is 115k at best, so about like a big Ryzen server that you can rent for the same $200 or so. I can think of various computation projects that could use that, but I don't think I'd bother with a room full of crufty old PC's if I was pursuing something like that.